Three-dimensional laminar flow over an inclined spinning disk is investigated at a Reynolds number of Re = 500 and an angle of attack of α = 25°, for tip-speed ratios up to 3. Numerical simulations are performed to investigate the effect of spin on the aerodynamics and characterise the instabilities that occur. Increasing tip-speed ratio significantly increases both lift and drag monotonically. Several distinct wake regimes are observed, including vortex shedding in the non-spinning case, vortex-shedding suppression at moderate tip-speed ratios and a distinct corkscrew-like short-wavelength instability in the advancing tip vortex at higher tip-speed ratios. Vorticity generated by the spinning disk strengthens the advancing tip vortex, inducing a spanwise stretching in the trailing-edge vortex sheet. This helps to dissipate the vorticity, which in turn prevents roll up and suppresses vortex shedding. The short-wavelength instability shows qualitative and quantitative matches to the (-2,0,1) principal mode of the elliptic instabilities seen in pairs of counter-rotating Batchelor vortices. The addition of vorticity from the disk rotation significantly alters the circulation and axial velocity in the tip vortices, giving rise to elliptic instability despite its absence in the non-spinning case. In select cases, lock-in between the frequency of the elliptic instability and twice the spin frequency is observed, indicating that disk rotation acts as an additional forcing for the elliptic instability. Additional simulations at different Reynolds numbers and angle of attacks are considered to examine the robustness of observed phenomena across different parameter combinations.
", "doi": "10.1017/jfm.2025.10943", "issn": "0022-1120", "publisher": "Cambridge University Press (CUP)", "publication": "Journal of Fluid Mechanics", "publication_date": "2025-12-25", "volume": "1025", "pages": "A48" }, { "id": "authors:ek2h9-7xr22", "collection": "authors", "collection_id": "ek2h9-7xr22", "cite_using_url": "https://authors.library.caltech.edu/records/ek2h9-7xr22", "type": "article", "title": "Bispectral decomposition and energy transfer in a turbulent jet", "author": [ { "family_name": "Nekkanti", "given_name": "Akhil", "orcid": "0000-0002-2173-8704", "clpid": "Nekkanti-Akhil" }, { "family_name": "Pickering", "given_name": "Ethan", "orcid": "0000-0002-4485-6359" }, { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The triadic interactions and nonlinear energy transfer are investigated in a subsonic turbulent jet at Re=450000. The primary focus is on the role of these interactions in the formation and attenuation of streaky structures. To this end, we employ bispectral mode decomposition, a technique that extracts coherent structures associated with dominant triadic interactions. A strong triadic correlation is identified between Kelvin–Helmholtz (KH) wavepackets and streaks: interactions between counter-rotating KH waves generates streamwise vortices, which subsequently give rise to streaks through the lift-up mechanism. The most energetic streaks occur at azimuthal wavenumber m=2, with the dominant contributing triad being [m1,m2,m3]=[1,1,2]. The spectral energy budget reveals that the net effect of nonlinear triadic interactions is an energy loss from the streaks. As these streaks convect downstream, they engage in further nonlinear interactions with other frequencies, which drain their energy and ultimately lead to their attenuation. Further analysis identifies the dominant scales and direction of energy transfer across different spatial regions of the jet. While the turbulent jet exhibits a forward energy cascade in a global sense, the direction of energy transfer varies locally: in the shear layer near the nozzle exit, triadic interactions among smaller scales dominate, resulting in an inverse energy cascade, whereas farther downstream, beyond the end of the potential core, interactions among larger scales prevail, leading to a forward cascade.
", "doi": "10.1017/jfm.2025.10922", "issn": "0022-1120", "publisher": "Cambridge University Press (CUP)", "publication": "Journal of Fluid Mechanics", "publication_date": "2025-12-25", "volume": "1025", "pages": "A35" }, { "id": "authors:q2err-6nh23", "collection": "authors", "collection_id": "q2err-6nh23", "cite_using_url": "https://authors.library.caltech.edu/records/q2err-6nh23", "type": "article", "title": "Spectral proper orthogonal decomposition of rapid snapshot pairs sampled at sub-Nyquist intervals", "author": [ { "family_name": "Cardinale", "given_name": "Caroline", "orcid": "0009-0007-6446-7047", "clpid": "Cardinale-Caroline" }, { "family_name": "Brunton", "given_name": "Steven L." }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Modal decomposition methods are important for characterizing the low-dimensional dynamics of complex systems, including turbulent flows. Different methods have varying data requirements and produce modes with different properties. Spectral proper orthogonal decomposition (SPOD) produces orthogonal, energy-ranked spatial modes at discrete temporal frequencies for statistically stationary flows. However, SPOD requires long stretches of sequential, uniformly sampled, time-resolved data. These data requirements limit SPOD's use in experimental settings where the maximum capture rate of a camera is often slower than the Nyquist sampling rate required to resolve the highest turbulent frequencies. However, if two particle image velocimetry systems operate in tandem, pairs of data can be acquired that are arbitrarily close in time. The dynamic mode decomposition (DMD) uses this pairwise data to resolve frequencies up to the Nyquist frequency associated with the small time step within a pair. However, these modes do not form a basis and have no set ranking. The present work attempts to compute SPOD modes from pairwise data with a small time step but with large gaps between pairs. We use DMD on pairwise data to estimate segmentwise, uniformly sampled series that can then be used to estimate the SPOD modes, intending to resolve frequencies between the gap and pair Nyquist limits. The method is tested on numerically obtained data of the linearized complex Ginzburg-Landau equation, as well as a Mach 0.4 isothermal turbulent jet. For the jet, pairwise SPOD can accurately dealias the SPOD spectrum and estimate mode shapes at frequencies up to St≈1.0 while using more than 90% fewer data.
", "doi": "10.1103/vxqz-8fl5", "issn": "2469-990X", "publisher": "American Physical Society", "publication": "Physical Review Fluids", "publication_date": "2025-12-24", "series_number": "12", "volume": "10", "issue": "12", "pages": "124904" }, { "id": "authors:58y0k-71223", "collection": "authors", "collection_id": "58y0k-71223", "cite_using_url": "https://authors.library.caltech.edu/records/58y0k-71223", "type": "article", "title": "Normality-based analysis of multiscale velocity gradients and energy transfer in direct and large-eddy simulations of isotropic turbulence", "author": [ { "family_name": "Arun", "given_name": "Rahul", "orcid": "0000-0002-5942-169X", "clpid": "Arun-Rahul" }, { "family_name": "Kamal", "given_name": "Mostafa", "orcid": "0009-0000-3892-4987" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Johnson", "given_name": "Perry L.", "orcid": "0000-0002-7929-9396" } ], "abstract": "Symmetry-based analyses of multiscale velocity gradients highlight that strain self-amplification (SS) and vortex stretching (VS) drive forward energy transfer in turbulent flows. By contrast, a strain–vorticity covariance mechanism produces backscatter that contributes to the bottleneck effect in the subinertial range of the energy cascade. We extend these analyses by using a normality-based decomposition of filtered velocity gradients in forced isotropic turbulence to distinguish contributions from normal straining, pure shearing and rigid rotation at a given scale. Our analysis of direct numerical simulation (DNS) data illuminates the importance of shear layers in the inertial range and (especially) the subinertial range of the cascade. Shear layers contribute significantly to SS and VS and play a dominant role in the backscatter mechanism responsible for the bottleneck effect. Our concurrent analysis of large-eddy simulation (LES) data characterizes how different closure models affect the flow structure and energy transfer throughout the resolved scales. We thoroughly demonstrate that the multiscale flow features produced by a mixed model closely resemble those in a filtered DNS, whereas the features produced by an eddy viscosity model resemble those in an unfiltered DNS at a lower Reynolds number. This analysis helps explain how small-scale shear layers, whose imprint is mitigated upon filtering, amplify the artificial bottleneck effect produced by the eddy viscosity model in the inertial range of the cascade. Altogether, the present results provide a refined interpretation of the flow structures and mechanisms underlying the energy cascade and insight for designing and evaluating LES closure models.
", "doi": "10.1017/jfm.2025.10748", "issn": "0022-1120", "publisher": "Cambridge University Press (CUP)", "publication": "Journal of Fluid Mechanics", "publication_date": "2025-10-25", "volume": "1021", "pages": "A47" }, { "id": "authors:18py7-q1531", "collection": "authors", "collection_id": "18py7-q1531", "cite_using_url": "https://authors.library.caltech.edu/records/18py7-q1531", "type": "article", "title": "Numerical methods for multiphase flows", "author": [ { "family_name": "Garcia-Villalba", "given_name": "Manuel", "orcid": "0000-0002-6953-2270" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Desjardins", "given_name": "Olivier", "orcid": "0000-0001-6477-6658" }, { "family_name": "Lucas", "given_name": "Dirk", "orcid": "0000-0003-0463-2278" }, { "family_name": "Mani", "given_name": "Ali" }, { "family_name": "Marchisio", "given_name": "Daniele", "orcid": "0000-0002-9104-0571" }, { "family_name": "Matar", "given_name": "Omar K.", "orcid": "0000-0002-0530-8317" }, { "family_name": "Picano", "given_name": "Francesco", "orcid": "0000-0002-3943-8187" }, { "family_name": "Zaleski", "given_name": "St\u00e9phane", "orcid": "0000-0003-2004-9090" } ], "abstract": "In this paper, we present resolvent4py, a parallel Python package for the analysis, model reduction and control of large-scale linear systems with millions or billions of degrees of freedom. This package provides the user with a friendly Python-like experience (akin to that of well-established libraries such as numpy and scipy), while enabling MPI-based parallelism through mpi4py, petsc4py and slepc4py. In turn, this allows for the development of streamlined and efficient Python code that can be used to solve several problems in fluid mechanics, solid mechanics, graph theory, molecular dynamics and several other fields.
", "doi": "10.1016/j.softx.2025.102286", "issn": "2352-7110", "publisher": "Elsevier", "publication": "SoftwareX", "publication_date": "2025-09", "volume": "31", "pages": "102286" }, { "id": "authors:jsmfb-ecf46", "collection": "authors", "collection_id": "jsmfb-ecf46", "cite_using_url": "https://authors.library.caltech.edu/records/jsmfb-ecf46", "type": "article", "title": "Boundary-Layer Stability Analysis Using the Nonlinear One-Way Navier\u2013Stokes Approach", "author": [ { "family_name": "Sleeman", "given_name": "Michael K.", "orcid": "0000-0001-5949-9289", "clpid": "Sleeman-Michael-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Lakebrink", "given_name": "Matthew T." } ], "abstract": "We extend the one-way Navier Stokes (OWNS) approach to support nonlinear interactions between waves of different frequencies, which will enable nonlinear analysis of instability and transition. In OWNS, the linearized Navier\u2013Stokes equations are parabolized and solved in the frequency domain as a spatial initial-value (marching) problem. OWNS yields a reduced computational cost compared to global solvers while also conferring numerous advantages over the parabolized stability equations (PSEs), despite its higher computational cost relative to PSE, that we seek to extend to nonlinear analysis. We validate the nonlinear OWNS (NOWNS) method by examining the nonlinear evolution of two- and three-dimensional disturbances in a low-speed Blasius boundary layer compared to nonlinear PSE (NPSE) and direct numerical simulation (DNS) results from the literature. We demonstrate that NOWNS can be used to simulate flows involving blowing/suction strips, is more robust to numerical noise, and converges for stronger nonlinearities, as compared to NPSE.", "doi": "10.2514/1.j064909", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2025-08", "series_number": "8", "volume": "63", "issue": "8", "pages": "3145\u20133159" }, { "id": "authors:99b3c-1bd71", "collection": "authors", "collection_id": "99b3c-1bd71", "cite_using_url": "https://authors.library.caltech.edu/records/99b3c-1bd71", "type": "article", "title": "Linear Analysis of Boundary-Layer Instabilities on a Finned Cone at Mach 6", "author": [ { "family_name": "Araya", "given_name": "Daniel B.", "orcid": "0000-0001-6833-6852" }, { "family_name": "Bitter", "given_name": "Neal P." }, { "family_name": "Wheaton", "given_name": "Bradley M.", "orcid": "0009-0005-5765-0316" }, { "family_name": "Kamal", "given_name": "Omar", "orcid": "0000-0002-3431-2964", "clpid": "Kamal-Omar" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Knutson", "given_name": "Anthony" }, { "family_name": "Johnson", "given_name": "Heath" }, { "family_name": "Nichols", "given_name": "Joseph" }, { "family_name": "Candler", "given_name": "Graham V." }, { "family_name": "Russo", "given_name": "Vincenzo" }, { "family_name": "Brehm", "given_name": "Christoph", "orcid": "0000-0002-9006-3587" } ], "abstract": "Boundary-layer instabilities for a finned cone at Mach=6, \ud835\udc45\u2062\ud835\udc52=8.4×106 [m−1], and zero incidence angle are examined using linear stability methods of varying fidelity and maturity. The geometry and laminar flow conditions correspond to experiments conducted at the Boeing Air Force Mach 6 Quiet Tunnel at Purdue University. Where possible, a common baseflow is utilized among the stability computations, and comparisons are made along the acreage of the cone where transition is first observed in the experiment. Stability results utilizing linear stability theory, planar parabolized stability equations, one-way Navier–Stokes, forced direct numerical simulation, and adaptive mesh refinement wavepacket tracking are presented. A dominant three-dimensional vortex instability occurring at ≈250 kHz is identified and correlates well with experimental measurements of transition onset. With the exception of linear stability theory, all of the higher-fidelity linear methods considered in this work were consistent in predicting the initial growth and general structure of the vortex instability as it evolved downstream. Some of the challenges, opportunities, and development needs of the stability methods considered are discussed.
", "doi": "10.2514/1.j064036", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2025-07", "series_number": "7", "volume": "63", "issue": "7", "pages": "2594-2614" }, { "id": "authors:tab55-87k30", "collection": "authors", "collection_id": "tab55-87k30", "cite_using_url": "https://authors.library.caltech.edu/records/tab55-87k30", "type": "article", "title": "Fast and Robust Method for Screened Poisson Lattice Green's Function Using Asymptotic Expansion and Fast Fourier Transform", "author": [ { "family_name": "Hou", "given_name": "Wei", "orcid": "0000-0001-8023-6395", "clpid": "Hou-Wei" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We study the lattice Green’s function (LGF) of the screened Poisson equation on a two-dimensional rectangular lattice. This LGF arises in numerical analysis, random walks, solid-state physics, and other fields. Its defining characteristic is the screening term, which defines different regimes. When its coefficient is large, we can accurately approximate the LGF with an exponentially converging asymptotic expansion, and its convergence rate monotonically increases with the coefficient of the screening term. To tabulate the LGF when the coefficient is not large, we derive a one-dimensional integral representation of the LGF. We show that the trapezoidal rule can approximate this integral with exponential convergence, and we propose an efficient algorithm for its evaluation via the fast Fourier transform. We discuss applications including computing the LGF of the three-dimensional Poisson equation with one periodic direction and the return probability of a two-dimensional random walk with killing.
", "doi": "10.1137/24m1646789", "issn": "1064-8275", "publisher": "Society for Industrial & Applied Mathematics (SIAM)", "publication": "SIAM Journal on Scientific Computing", "publication_date": "2025-04", "series_number": "2", "volume": "47", "issue": "2", "pages": "A1198-A1224" }, { "id": "authors:p8shq-rj740", "collection": "authors", "collection_id": "p8shq-rj740", "cite_using_url": "https://authors.library.caltech.edu/records/p8shq-rj740", "type": "article", "title": "An adaptive lattice Green's function method for external flows with two unbounded and one homogeneous directions", "author": [ { "family_name": "Hou", "given_name": "Wei", "orcid": "0000-0001-8023-6395", "clpid": "Hou-Wei" }, { "family_name": "Colonius", "given_name": "Timothy E.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We solve the incompressible Navier-Stokes equations using a lattice Green's function (LGF) approach, including immersed boundaries (IB) and adaptive mesh refinement (AMR), for external flows with one homogeneous direction (e.g. infinite cylinders of arbitrary cross-section). We hybridize a Fourier collocation (pseudo-spectral) method for the homogeneous direction with a specially designed, staggered-grid finite-volume scheme on an AMR grid. The Fourier series is also truncated variably according to the refinement level in the other directions. We derive new algorithms to tabulate the LGF of the screened Poisson operator and viscous integrating factor. After adapting other algorithmic details from the fully inhomogeneous case [1], we validate and demonstrate the new method with transitional and turbulent flows over a circular cylinder at Re=300 and Re=12,000, respectively.
", "doi": "10.1016/j.jcp.2024.113370", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2024-12-15", "volume": "519", "pages": "113370" }, { "id": "authors:vgb5s-pqs25", "collection": "authors", "collection_id": "vgb5s-pqs25", "cite_using_url": "https://authors.library.caltech.edu/records/vgb5s-pqs25", "type": "article", "title": "Velocity gradient partitioning in turbulent flows", "author": [ { "family_name": "Arun", "given_name": "Rahul", "orcid": "0000-0002-5942-169X", "clpid": "Arun-Rahul" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The velocity gradient tensor can be decomposed into normal straining, pure shearing and rigid rotation tensors, each with distinct symmetry and normality properties. We partition the strength of turbulent velocity gradients based on the relative contributions of these constituents in several canonical flows. These flows include forced isotropic turbulence, turbulent channels and turbulent boundary layers. For forced isotropic turbulence, the partitioning is in excellent agreement with previous results. For wall-bounded turbulence, the partitioning collapses onto the isotropic partitioning far from the wall, where the mean shearing is relatively weak. By contrast, the near-wall partitioning is dominated by shearing. Between these two regimes, the partitioning collapses well at sufficiently high friction Reynolds numbers and its variations in the buffer layer and the log-law region can be reasonably modelled as a function of the mean shearing strength. Altogether, our results highlight the expressivity and broad applicability of the velocity gradient partitioning as advantages for turbulence modelling.", "doi": "10.1017/jfm.2024.1021", "issn": "0022-1120", "publisher": "Cambridge University Press (CUP)", "publication": "Journal of Fluid Mechanics", "publication_date": "2024-12-10", "volume": "1000", "pages": "R5" }, { "id": "authors:qbwkc-xt420", "collection": "authors", "collection_id": "qbwkc-xt420", "cite_using_url": "https://authors.library.caltech.edu/records/qbwkc-xt420", "type": "article", "title": "Superresolution and analysis of three-dimensional velocity fields of underexpanded jets in different screech modes", "author": [ { "family_name": "Lee", "given_name": "Chungil", "orcid": "0000-0002-3629-427X" }, { "family_name": "Ozawa", "given_name": "Yuta", "orcid": "0000-0002-5895-0506" }, { "family_name": "Nagata", "given_name": "Takayuki", "orcid": "0000-0003-3644-4888" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Nonomura", "given_name": "Taku", "orcid": "0000-0001-7739-7104" } ], "abstract": "Time-resolved (TR), three-dimensional (3D) velocity fields of screeching, underexpanded jets are estimated using non-time-resolved particle image velocimetry and simultaneous TR microphone measurements. Specifically, we aim to reconstruct TR 3D velocity fluctuation fields associated with the A2, B, and C modes of a screeching jet using a linear regression model and to analyze screech dynamics of these modes. The linear regression model is constructed on the basis of a linear relationship between the velocity and acoustic fields. Three nozzle pressure ratios (NPRs) of 2.30, 2.97, and 3.40 are employed. The dominant azimuthal modes for three cases are investigated using azimuthal Fourier coefficients of the acoustic data obtained by the azimuthal array of eight microphones placed near the nozzle exit. The dominant azimuthal modes at NPRs of 2.30, 2.97, and 3.40 are m=0, 1, and 1, respectively. The first two proper orthogonal decomposition (POD) modes in these azimuthal modes are dominant at all NPRs and are associated with screech. 3D velocity fluctuation fields associated with screech are reconstructed from these leading POD modes of the acoustic data. The reconstructed 3D velocity fluctuation fields at NPRs of 2.97 and 3.40 exhibit two helical structures with opposite rotation directions. The present results demonstrate that, in the B mode, the flapping structure exhibits random clockwise and counterclockwise rotations over an extended time domain, while maintaining a consistent direction within short time domains. In addition, in the C mode, two helical structures with opposite rotation directions, as well as the flapping structure, are observed.
", "doi": "10.1103/physrevfluids.9.104604", "issn": "2469-990X", "publisher": "American Physical Society", "publication": "Physical Review Fluids", "publication_date": "2024-10-18", "series_number": "10", "volume": "9", "issue": "10", "pages": "104604" }, { "id": "authors:yhrx9-xg748", "collection": "authors", "collection_id": "yhrx9-xg748", "cite_using_url": "https://authors.library.caltech.edu/records/yhrx9-xg748", "type": "article", "title": "Method for scalable and performant GPU-accelerated simulation of multiphase compressible flow", "author": [ { "family_name": "Radhakrishnan", "given_name": "Anand", "clpid": "Radhakrishnan-Anand" }, { "family_name": "Le Berre", "given_name": "Henry", "orcid": "0000-0002-4781-9502", "clpid": "Le-Berre-Henry" }, { "family_name": "Wilfong", "given_name": "Benjamin", "clpid": "Wilfong-Benjamin" }, { "family_name": "Spratt", "given_name": "Jean-Sebastien", "orcid": "0000-0002-1962-4214", "clpid": "Spratt-Jean-Sebastien" }, { "family_name": "Rodriguez", "given_name": "Mauro", "orcid": "0000-0003-0545-0265", "clpid": "Rodriguez-Mauro" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265", "clpid": "Bryngelson-Spencer-H" } ], "abstract": "Multiphase compressible flows are often characterized by a broad range of space and time scales, entailing large grids and small time steps. Simulations of these flows on CPU-based clusters can thus take several wall-clock days. Offloading the compute kernels to GPUs appears attractive but is memory-bound for many finite-volume and -difference methods, damping speedups. Even when realized, GPU-based kernels lead to more intrusive communication and I/O times owing to lower computation costs. We present a strategy for GPU acceleration of multiphase compressible flow solvers that addresses these challenges and obtains large speedups at scale. We use OpenACC for directive-based offloading of all compute kernels while maintaining low-level control when needed. An established Fortran preprocessor and metaprogramming tool, Fypp, enables otherwise hidden compile-time optimizations. This strategy exposes compile-time optimizations and high memory reuse while retaining readable, maintainable, and compact code. Remote direct memory access realized via CUDA-aware MPI and GPUDirect reduces halo-exchange communication time. We implement this approach in the open-source solver MFC [1]. Metaprogramming results in an 8-times speedup of the most expensive kernels compared to a statically compiled program, reaching 46% of peak FLOPs on modern NVIDIA GPUs and high arithmetic intensity (about 10 FLOPs/byte). In representative simulations, a single NVIDIA A100 GPU is 7-times faster compared to an Intel Xeon Cascade Lake (6248) CPU die, or about 300-times faster compared to a single such CPU core. At the same time, near-ideal (97%) weak scaling is observed for at least 13824 GPUs on OLCF Summit. A strong scaling efficiency of 84% is retained for an 8-times increase in GPU count. Collective I/O, implemented via MPI3, helps ensure the negligible contribution of data transfers (<1% of the wall time for a typical, large simulation). Large many-GPU simulations of compressible (solid-)liquid-gas flows demonstrate the practical utility of this strategy.
", "doi": "10.1016/j.cpc.2024.109238", "issn": "0010-4655", "publisher": "Elsevier", "publication": "Computer Physics Communications", "publication_date": "2024-09", "volume": "302", "pages": "109238" }, { "id": "authors:akfsk-aex42", "collection": "authors", "collection_id": "akfsk-aex42", "cite_using_url": "https://authors.library.caltech.edu/records/akfsk-aex42", "type": "article", "title": "Spectral proper orthogonal decomposition of harmonically forced turbulent flows", "author": [ { "family_name": "Heidt", "given_name": "Liam", "orcid": "0000-0003-1967-6847", "clpid": "Heidt-Liam" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Many turbulent flows exhibit time-periodic statistics. These include turbomachinery flows, flows with external harmonic forcing and the wakes of bluff bodies. Many existing techniques for identifying turbulent coherent structures, however, assume the statistics are statistically stationary. In this paper, we leverage cyclostationary analysis, an extension of the statistically stationary framework to processes with periodically varying statistics, to generalize the spectral proper orthogonal decomposition (SPOD) to the cyclostationary case. The resulting properties of the cyclostationary SPOD (CS-SPOD for short) are explored, a theoretical connection between CS-SPOD and the harmonic resolvent analysis is provided, simplifications for the low and high forcing frequency limits are discussed, and an efficient algorithm to compute CS-SPOD with SPOD-like cost is presented. We illustrate the utility of CS-SPOD using two example problems: a modified complex linearized Ginzburg\u2013Landau model and a high-Reynolds-number turbulent jet.", "doi": "10.1017/jfm.2024.70", "issn": "0022-1120", "publisher": "Cambridge University Press (CUP)", "publication": "Journal of Fluid Mechanics", "publication_date": "2024-04-25", "volume": "985", "pages": "A42" }, { "id": "authors:ndmce-5at62", "collection": "authors", "collection_id": "ndmce-5at62", "cite_using_url": "https://authors.library.caltech.edu/records/ndmce-5at62", "type": "article", "title": "The effect of flight on a turbulent jet: coherent structure eduction and resolvent analysis", "author": [ { "family_name": "Maia", "given_name": "Igor A.", "orcid": "0000-0003-2530-0897" }, { "family_name": "Heidt", "given_name": "Liam", "orcid": "0000-0003-1967-6847", "clpid": "Heidt-Liam" }, { "family_name": "Pickering", "given_name": "Ethan", "orcid": "0000-0002-4485-6359" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659" } ], "abstract": "We study coherent structures in subsonic turbulent jets subject to a flight stream. A thorough characterisation of the effects of a flight stream on the turbulent field was recently performed by Maia et al. (Phys. Rev. Fluids, vol. 8, 2023, 063902) and fluctuation energy attenuations were observed over a broad range of frequencies and azimuthal wavenumbers. The Kelvin\u2013Helmholtz, Orr and lift-up mechanisms were all shown to be weakened by the flight stream. Here we expand upon that study and model the changes in the dynamics of jets in flight using global resolvent analysis. The resolvent model is found to correctly capture the main effects of the flight stream on the dynamics of coherent structures, which are educed from a large-eddy simulation database using spectral proper orthogonal decomposition. Three modifications of note are: the damping of low-frequency streaky/Orr structures that carry most of the fluctuation energy; a degradation of the low-rank behaviour of the jet in frequencies where modal instability mechanisms are dominant; and a rank decrease at very low Strouhal numbers. The latter effect is underpinned by larger gain separations predicted by the resolvent analysis, due to a reduction in the wavelength of associated flow structures. This leads to a clearer relative dominance of streaky structures generated by the lift-up mechanism, despite the fact that the lift-up mechanism has been weakened with respect to the static jet.", "doi": "10.1017/jfm.2024.301", "issn": "0022-1120", "publisher": "Cambridge University Press (CUP)", "publication": "Journal of Fluid Mechanics", "publication_date": "2024-04-25", "volume": "985", "pages": "A21" }, { "id": "authors:t0yjn-swh07", "collection": "authors", "collection_id": "t0yjn-swh07", "cite_using_url": "https://authors.library.caltech.edu/records/t0yjn-swh07", "type": "article", "title": "Velocity gradient analysis of a head-on vortex ring collision", "author": [ { "family_name": "Arun", "given_name": "Rahul", "orcid": "0000-0002-5942-169X", "clpid": "Arun-Rahul" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We simulate the head-on collision between vortex rings with circulation Reynolds numbers of 4000 using an adaptive, multiresolution solver based on the lattice Green's function. The simulation fidelity is established with integral metrics representing symmetries and discretization errors. Using the velocity gradient tensor and structural features of local streamlines, we characterize the evolution of the flow with a particular focus on its transition and turbulent decay. Transition is excited by the development of the elliptic instability, which grows during the mutual interaction of the rings as they expand radially at the collision plane. The development of antiparallel secondary vortex filaments along the circumference mediates the proliferation of small-scale turbulence. During turbulent decay, the partitioning of the velocity gradients approaches an equilibrium that is dominated by shearing and agrees well with previous results for forced isotropic turbulence. We also introduce new phase spaces for the velocity gradients that reflect the interplay between shearing and rigid rotation and highlight geometric features of local streamlines. In conjunction with our other analyses, these phase spaces suggest that, while the elliptic instability is the predominant mechanism driving the initial transition, its interplay with other mechanisms, e.g. the Crow instability, becomes more important during turbulent decay. Our analysis also suggests that the geometry-based phase space may be promising for identifying the effects of the elliptic instability and other mechanisms using the structure of local streamlines. Moving forward, characterizing the organization of these mechanisms within vortices and universal features of velocity gradients may aid in modelling turbulent flows.", "doi": "10.1017/jfm.2024.90", "issn": "0022-1120", "publisher": "Cambridge University Press (CUP)", "publication": "Journal of Fluid Mechanics", "publication_date": "2024-03-10", "volume": "982", "pages": "A16" }, { "id": "authors:ce1pw-npb18", "collection": "authors", "collection_id": "ce1pw-npb18", "cite_using_url": "https://authors.library.caltech.edu/records/ce1pw-npb18", "type": "article", "title": "Filtering dynamical systems using observations of statistics", "author": [ { "family_name": "Bach", "given_name": "Eviatar", "orcid": "0000-0002-9725-0203" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Scherl", "given_name": "Isabel", "orcid": "0000-0002-0781-8863" }, { "family_name": "Stuart", "given_name": "Andrew", "orcid": "0000-0001-9091-7266", "clpid": "Stuart-A-M" } ], "abstract": "We consider the problem of filtering dynamical systems, possibly stochastic, using observations of statistics. Thus, the computational task is to estimate a time-evolving density ρ(v,t) given noisy observations of the true density ρ†; this contrasts with the standard filtering problem based on observations of the state v. The task is naturally formulated as an infinite-dimensional filtering problem in the space of densities ρ. However, for the purposes of tractability, we seek algorithms in state space; specifically, we introduce a mean-field state-space model, and using interacting particle system approximations to this model, we propose an ensemble method. We refer to the resulting methodology as the ensemble Fokker–Planck filter (EnFPF). Under certain restrictive assumptions, we show that the EnFPF approximates the Kalman–Bucy filter for the Fokker–Planck equation, which is the exact solution to the infinite-dimensional filtering problem. Furthermore, our numerical experiments show that the methodology is useful beyond this restrictive setting. Specifically, the experiments show that the EnFPF is able to correct ensemble statistics, to accelerate convergence to the invariant density for autonomous systems, and to accelerate convergence to time-dependent invariant densities for non-autonomous systems. We discuss possible applications of the EnFPF to climate ensembles and to turbulence modeling.
", "doi": "10.1063/5.0171827", "issn": "1054-1500", "publisher": "American Institute of Physics", "publication": "Chaos: An Interdisciplinary Journal of Nonlinear Science", "publication_date": "2024-03", "series_number": "3", "volume": "34", "issue": "3", "pages": "033119" }, { "id": "authors:6xmqz-jdr98", "collection": "authors", "collection_id": "6xmqz-jdr98", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20230530-441187700.29", "type": "article", "title": "Conditional moment methods for polydisperse cavitating flows", "author": [ { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265", "clpid": "Bryngelson-Spencer-H" }, { "family_name": "Fox", "given_name": "Rodney O.", "orcid": "0000-0003-1944-1861", "clpid": "Fox-Rodney-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The dynamics of cavitation bubbles are important in many flows, but their small sizes and high number densities often preclude direct numerical simulation. We present a computational model that averages their effect on the flow over larger spatiotemporal scales. The model is based on solving a generalized population balance equation (PBE) for nonlinear bubble dynamics and explicitly represents the evolving probability density of bubble radii and radial velocities. Conditional quadrature-based moment methods (QBMMs) are adapted to solve this PBE. A one-way-coupled bubble dynamics problem demonstrates the efficacy of different QBMMs for the evolving bubble statistics. Results show that enforcing hyperbolicity during moment inversion (CHyQMOM) provides comparable model-form accuracy to the traditional conditional method of moments and decreases computational costs by about ten times for a broad range of test cases. The CHyQMOM-based computational model is implemented in MFC, an open-source multi-phase and high-order-accurate flow solver. We assess the effect of the model and its parameters on a two-way coupled bubble screen flow problem.", "doi": "10.1016/j.jcp.2023.111917", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2023-03-15", "volume": "477", "pages": "111917" }, { "id": "authors:mx52g-98593", "collection": "authors", "collection_id": "mx52g-98593", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20230227-87934600.8", "type": "article", "title": "Global receptivity analysis: physically realizable input-output analysis", "author": [ { "family_name": "Kamal", "given_name": "Omar", "orcid": "0000-0002-3431-2964", "clpid": "Kamal-Omar" }, { "family_name": "Lakebrink", "given_name": "Matthew T.", "clpid": "Lakebrink-Matthew-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "In the context of transition analysis, linear input\u2013output analysis determines the worst-case disturbances to a laminar base flow based on a generic right-hand-side volumetric/boundary forcing term. The worst-case forcing is not physically realizable, and, to our knowledge, a generic framework for posing physically realizable worst-case disturbance problems is lacking. In natural receptivity analysis, disturbances are forced by matching (typically local) solutions within the boundary layer to outer solutions consisting of free-stream vortical, entropic and acoustic disturbances. We pose a scattering formalism to restrict the input forcing to a set of realizable disturbances associated with plane-wave solutions of the outer problem. The formulation is validated by comparing with direct numerical simulations of a Mach 4.5 flat-plate boundary layer. We show that the method provides insight into transition mechanisms by identifying those linear combinations of plane-wave disturbances that maximize energy amplification over a range of frequencies. We also discuss how the framework can be extended to accommodate scattering from shocks and in shock layers for supersonic flow.", "doi": "10.1017/jfm.2023.48", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2023-02-10", "volume": "956", "pages": "Art. No. R5" }, { "id": "authors:s9t60-zgj64", "collection": "authors", "collection_id": "s9t60-zgj64", "cite_using_url": "https://authors.library.caltech.edu/records/s9t60-zgj64", "type": "article", "title": "An Adaptive, Scalable, and Robust Approach for Computing External Flows via the Immersed Boundary Method", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "While there are few reliable techniques for characterizing highly compliant and viscoelastic materials under large deformations, laser-induced Inertial Microcavitaton Rheometry (IMR) was recently developed to fill this void and to characterize soft materials at high to ultra-high strain rates (O(10³)∼O(10\u2078) s\u207b¹). Yet, one of the current limitations in IMR has been the dependence of the cavitation nucleation physics on the intrinsic material properties often generating extreme deformation levels and thus complicating material characterization procedures.
\nThe objective of this study was to develop an experimental approach for modulating laser-induced cavitation (LIC) bubble amplitudes and their resulting maximum material deformations. Lowering the material stretch ratios during inertial cavitation will provide an experimental platform of broad applicability to a large class of polymeric materials and environmental conditions.
\nExperimental methods include using three types of micron-sized nucleation seed particles and varying laser energies in polyacrylamide hydrogels of known concentration. Using a Quadratic law Kelvin-Voigt material model, we implemented ensemble-based data assimilation (DA) techniques to robustly quantify the nonlinear constitutive material parameters, up through the first, second, and third bubble collapse cycles. Fitted values were then used to simulate bubble dynamics to compute critical bubble collapse Mach numbers, and to assess time-varying uncertainties of the full cavitation dynamics with respect to the current state-of-the art theoretical model featured in the IMR model.
\nWhile varying laser energy modulated bubble amplitude, seed particles successfully expanded (more than doubled) the finite deformation regime (i.e., maximum material stretch, λ\u2098\u2090\u2093 ≈ 4 - 9). Comparing experimental data to IMR simulations, we found that fitting beyond the first bubble collapse, as well as increasing laser energy, increased the bubble radius fit error, and larger λ\u2098\u2090\u2093 values exhibited increasingly violent bubble behavior (marked by increasing collapse Mach numbers greater than 0.08). Additionally, time-varying analysis showed the greatest model uncertainty during initial bubble collapse, where bubbles nucleated at lower laser energies and resulting λ\u2098\u2090\u2093 had less uncertainty at collapse compared to higher laser energy and λ\u2098\u2090\u2093 cases.
\nThis study indicates IMR’s current theoretical framework might be lacking important additional cavitation and/or material physics. However, expanding the finite deformation regime of soft materials to attain lower stretch regimes enables broader applicability to a larger class of soft polymeric materials and will enable future, systematic development and incorporation of more complex physics and constitutive models including damage and failure mechanisms into the theoretical framework of IMR.
", "doi": "10.1007/s11340-022-00861-7", "issn": "0014-4851", "publisher": "Springer", "publication": "Experimental Mechanics", "publication_date": "2022-07", "series_number": "6", "volume": "62", "issue": "6", "pages": "1037-1050" }, { "id": "authors:a80y9-vv085", "collection": "authors", "collection_id": "a80y9-vv085", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220412-864423600", "type": "article", "title": "Improving burst wave lithotripsy effectiveness for small stones and fragments by increasing frequency: theoretical modeling and ex vivo study", "author": [ { "family_name": "Bailey", "given_name": "Michael R.", "clpid": "Bailey-Michael-R" }, { "family_name": "Maxwell", "given_name": "Adam D.", "clpid": "Maxwell-Adam-D" }, { "family_name": "Cao", "given_name": "Shunxiang", "clpid": "Cao-Shunxiang" }, { "family_name": "Ramesh", "given_name": "Shivani", "clpid": "Ramesh-Shivani" }, { "family_name": "Liu", "given_name": "Ziyue", "clpid": "Liu-Ziyue" }, { "family_name": "Williams", "given_name": "James Caldwell, Jr.", "clpid": "Williams-James-Caldwell-Jr" }, { "family_name": "Thiel", "given_name": "Jeff", "clpid": "Thiel-Jeff" }, { "family_name": "Dunmire", "given_name": "Barbrina", "clpid": "Dunmire-Barbrina" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Kuznetsova", "given_name": "Ekaterina", "clpid": "Kuznetsova-Ekaterina" }, { "family_name": "Kreider", "given_name": "Wayne", "clpid": "Kreider-Wayne" }, { "family_name": "Sorensen", "given_name": "Mathew D.", "clpid": "Sorensen-Mathew-D" }, { "family_name": "Lingeman", "given_name": "James E.", "clpid": "Lingeman-James-E" }, { "family_name": "Sapozhnikov", "given_name": "Oleg A.", "clpid": "Sapozhnikov-Oleg-A" } ], "abstract": "In clinical trial NCT03873259, a 2.6-mm lower pole stone was treated transcutaneously and ex vivo with 390-kHz burst wave lithotripsy (BWL) for 40 minutes and failed to break. The stone was subsequently fragmented with 650-kHz BWL after a 4-minute exposure. This study investigated how to fragment small stones and why varying the BWL frequency may more effectively fragment stones to dust.
\nA linear elastic theoretical model was used to calculate the stress created inside stones from shock wave lithotripsy (SWL) and different BWL frequencies mimicking the stone's size, shape, lamellar structure, and composition. To test model predictions about the impact of BWL frequency, matched pairs of stones (1–5 mm) were treated at (1) 390 kHz, (2) 830 kHz, and (3) 390 kHz followed by 830 kHz. The mass of fragments >1 and 2 mm was measured over 10 minutes of exposure.
\nThe linear elastic model predicts that the maximum principal stress inside a stone increases to more than 5.5 times the pressure applied by the ultrasound wave as frequency is increased, regardless of the composition tested. The threshold frequency for stress amplification is proportionate to the wave speed divided by the stone diameter. Thus, smaller stones may be likely to fragment at a higher frequency, but not at a lower frequency below a limit. Unlike with SWL, this amplification in BWL occurs consistently with spherical and irregularly shaped stones. In water tank experiments, stones smaller than the threshold size broke fastest at high frequency (p = 0.0003), whereas larger stones broke equally well to submillimeter dust at high, low, or mixed frequencies.
\nFor small stones and fragments, increasing frequency of BWL may produce amplified stress in the stone causing the stone to break. Using the strategies outlined here, stones of all sizes may be turned to dust efficiently with BWL.
\nThe dynamics of a cantilever plate clamped at its trailing edge and placed at a moderate angle ((\u03b1 \u2264 30\u00b0) to a uniform flow are investigated experimentally and numerically, and a large experimental data set is provided. The dynamics are shown to differ significantly from the zero-angle-of-attack case, commonly called the inverted-flag configuration. Four distinct dynamical regimes arise at non-zero angles: a small oscillation around a small-deflection equilibrium (deformed regime), a small-amplitude flapping motion, a large-amplitude flapping motion and a small oscillation around a large-deflection equilibrium (deflected regime). The small- and large-amplitude flapping motions are shown to be produced by different physical mechanisms. The small-amplitude flapping motion appears gradually as the flow speed is increased and is consistent with a limit-cycle oscillation caused by the quasi-steady fluid forcing. The large-amplitude flapping motion is observed to appear at a constant critical flow speed that is independent of angle of attack. Its characteristics match those of the large-amplitude vortex-induced vibration present at zero angle of attack. The flow speed at which the plate enters the deflected regime decreases linearly as the angle of attack is increased, causing the flapping motion to disappear for angles of attack greater than \u03b1 \u2248 28\u00b0. Finally, the effect of aspect ratio on the plate dynamics is considered, with a plate of reduced aspect ratio being shown to lack a sharp distinction between flapping regimes for \u03b1 > 8\u00b0.
", "doi": "10.1017/jfm.2020.922", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2021-02-25", "volume": "909", "pages": "A20" }, { "id": "authors:ywh7t-5ha37", "collection": "authors", "collection_id": "ywh7t-5ha37", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200624-090733478", "type": "article", "title": "Dynamics of an inverted cantilever plate at moderate angle of attack", "author": [ { "family_name": "Huertas-Cerdeira", "given_name": "Cecilia", "orcid": "0000-0003-4553-0470", "clpid": "Huertas-Cerdeira-Cecilia" }, { "family_name": "Goza", "given_name": "Andres", "orcid": "0000-0002-9372-7713", "clpid": "Goza-Andres" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-John-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "abstract": "The dynamics of a cantilever plate clamped at its trailing edge and placed at a moderate angle (\u03b1\u226430\u2218) to a uniform flow are investigated experimentally and numerically, and a large experimental data set is provided. The dynamics are shown to differ significantly from the zero-angle-of-attack case, commonly called the inverted-flag configuration. Four distinct dynamical regimes arise at non-zero angles: a small oscillation around a small-deflection equilibrium (deformed regime), a small-amplitude flapping motion, a large-amplitude flapping motion and a small oscillation around a large-deflection equilibrium (deflected regime). The small- and large-amplitude flapping motions are shown to be produced by different physical mechanisms. The small-amplitude flapping motion appears gradually as the flow speed is increased and is consistent with a limit-cycle oscillation caused by the quasi-steady fluid forcing. The large-amplitude flapping motion is observed to appear at a constant critical flow speed that is independent of angle of attack. Its characteristics match those of the large-amplitude vortex-induced vibration present at zero angle of attack. The flow speed at which the plate enters the deflected regime decreases linearly as the angle of attack is increased, causing the flapping motion to disappear for angles of attack greater than \u03b1\u224828\u2218. Finally, the effect of aspect ratio on the plate dynamics is considered, with a plate of reduced aspect ratio being shown to lack a sharp distinction between flapping regimes for \u03b1>8\u2218.", "doi": "10.1017/jfm.2020.922", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2021-02-25", "volume": "909", "pages": "Art. No. A20" }, { "id": "authors:077rb-xv855", "collection": "authors", "collection_id": "077rb-xv855", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20201208-105036247", "type": "article", "title": "Amplitude Scaling of Wave Packets in Turbulent Jets", "author": [ { "family_name": "Antonialli", "given_name": "Luigi A.", "clpid": "Antonialli-Luigi-A" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-Andr\u00e9-V-G" }, { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-Oliver-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-Peter" }, { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-Aaron" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-Guillaume-A" } ], "abstract": "This paper studies the amplitude of large-scale coherent wave-packet structures in jets, modeled by the parabolized stability equations (PSEs). Linear PSEs can retrieve the shape of the wave packets, but linearity leads to solutions with a free amplitude, which has traditionally been obtained in an ad hoc manner using limited data. We systematically determine the free amplitude as a function of frequency and azimuthal wave number by comparing the fluctuation fields retrieved from PSEs with coherent structures educed from large-eddy simulation data using spectral proper orthogonal decomposition. The wave-packet amplitude is shown to decay exponentially with the Strouhal number for axisymmetric and helical modes at both Mach numbers considered in the study: 0.4 and 0.9. Analytical fit functions are proposed, and the scaled wave packets provide reasonable reconstructions of pressure and velocity spectra on the jet centerline and lip line over a range of streamwise positions.", "doi": "10.2514/1.j059599", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2021-02", "series_number": "2", "volume": "59", "issue": "2", "pages": "559-568" }, { "id": "authors:8jfq2-gvg19", "collection": "authors", "collection_id": "8jfq2-gvg19", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20201012-140154360", "type": "article", "title": "Dynamics and decay of a spherical region of turbulence in free space", "author": [ { "family_name": "Yu", "given_name": "Ke", "orcid": "0000-0003-0157-4471", "clpid": "Yu-Ke" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Pullin", "given_name": "D. I.", "orcid": "0009-0007-5991-2863", "clpid": "Pullin-D-I" }, { "family_name": "Winckelmans", "given_name": "Gr\u00e9goire", "clpid": "Winckelmans-Gr\u00e9goire" } ], "abstract": "We perform direct numerical simulation and large-eddy simulation of an initially spherical region of turbulence evolving in free space. The computations are performed with a lattice Green's function method, which allows the exact free-space boundary conditions to be imposed on a compact vortical region. Large-eddy simulations are conducted with the stretched vortex subgrid stress model. The initial condition is spherically windowed, isotropic homogeneous incompressible turbulence. We study the spectrum and statistics of the decaying turbulence and compare the results with decaying isotropic turbulence, including cases representing different low-wavenumber behaviour of the energy spectrum (i.e. k\u00b2 versus k\u2074). At late times the turbulent sphere expands with both mean radius and integral scale showing similar timewise growth exponents. The low-wavenumber behaviour has little effect on the inertial scales, and we find that decay rates follow the predictions of Saffman (J. Fluid Mech., vol. 27, 1967, pp. 581\u2013593) in both cases, at least until approximately 400 initial eddy turnover times. The boundary of the spherical region develops intermittency and features ejections of vortex rings. These are shown to occur at the integral scale of the initial turbulence field and are hypothesized to occur due to a local imbalance of impulse on this scale.", "doi": "10.1017/jfm.2020.818", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2021-01-25", "volume": "907", "pages": "Art. No. A19" }, { "id": "authors:wpr22-d0e16", "collection": "authors", "collection_id": "wpr22-d0e16", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20210121-111627081", "type": "article", "title": "Lift coefficient estimation for a rapidly pitching airfoil", "author": [ { "family_name": "An", "given_name": "Xuanhong", "orcid": "0000-0001-6950-2867", "clpid": "An-Xuanhong" }, { "family_name": "Williams", "given_name": "David R.", "clpid": "Williams-David-R" }, { "family_name": "Eldredge", "given_name": "Jeff D.", "orcid": "0000-0002-2672-706X", "clpid": "Eldredge-Jeff-D" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We develop a method for estimating the instantaneous lift coefficient on a rapidly pitching airfoil that uses a small number of pressure sensors and a measurement of the angle of attack. The approach assimilates four surface pressure measurements with a modified nonlinear state space model (Goman\u2013Khrabrov model) through a Kalman filter. The error of lift coefficient estimates based only on a weighted-sum of the measured pressures are found to be noisy and biased, which leads to inaccurate estimates. The estimate is improved by including the predictive model in an conventional Kalman filter. The Goman\u2013Khrabrov model is shown to be a linear parameter-varying system and can therefore be used in the Kalman filter without the need for linearization. Additional improvement is realized by modifying the algorithm to provide more accurate estimate of the lift coefficient. The improved Kalman filtering approach results in a bias-free lift coefficient estimate that is more precise than either the pressure-based estimate or the Goman\u2013Khrabrov model on their own. The new method will enable performance enhancements in aerodynamic systems whose performance relies on lift.", "doi": "10.1007/s00348-020-03105-3", "issn": "0723-4864", "publisher": "Springer", "publication": "Experiments in Fluids", "publication_date": "2021-01", "series_number": "1", "volume": "62", "issue": "1", "pages": "Art. No. 11" }, { "id": "authors:f037h-p8n15", "collection": "authors", "collection_id": "f037h-p8n15", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200427-080644291", "type": "article", "title": "On the formation and recurrent shedding of ligaments in droplet aerobreakup", "author": [ { "family_name": "Dorschner", "given_name": "Benedikt", "orcid": "0000-0001-8926-7542", "clpid": "Dorschner-Benedikt" }, { "family_name": "Biasiori-Poulanges", "given_name": "Luc", "clpid": "Biasiori-Poulanges-Luc" }, { "family_name": "Schmidmayer", "given_name": "Kevin", "orcid": "0000-0003-0444-3098", "clpid": "Schmidmayer-Kevin" }, { "family_name": "El-Rabii", "given_name": "Hazem", "clpid": "El-Rabii-Hazem" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The breakup of water droplets when exposed to high-speed gas flows is investigated using both high-magnification shadowgraphy experiments as well as fully three-dimensional numerical simulations, which account for viscous as well as capillary effects. After thorough validation of the simulations with respect to the experiments, we elucidate the ligament formation process and the effect of surface tension. By Fourier decomposition of the flow field, we observe the development of specific azimuthal modes, which destabilize the liquid sheet surrounding the droplet. Eventually, the liquid sheet is ruptured, which leads to the formation of ligaments. We further observe the ligament formation and shedding to be a recurrent process. While the first ligament shedding weakly depends on the Weber number, subsequent shedding processes seem to be driven primarily by inertia and the vortex shedding in the wake of the deformed droplet.", "doi": "10.1017/jfm.2020.699", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2020-12-10", "volume": "904", "pages": "Art. No. A20" }, { "id": "authors:7tj86-asp68", "collection": "authors", "collection_id": "7tj86-asp68", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200824-094426276", "type": "article", "title": "Ambiguity in mean-flow-based linear analysis", "author": [ { "family_name": "Karban", "given_name": "U.", "orcid": "0000-0003-0803-0581", "clpid": "Karban-U" }, { "family_name": "Bugeat", "given_name": "B.", "clpid": "Bugeat-B" }, { "family_name": "Martini", "given_name": "E.", "orcid": "0000-0002-3144-5702", "clpid": "Martini-E" }, { "family_name": "Towne", "given_name": "A.", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Cavalieri", "given_name": "A. V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Lesshafft", "given_name": "L.", "orcid": "0000-0002-2513-4553", "clpid": "Lesshafft-L" }, { "family_name": "Agarwal", "given_name": "A.", "clpid": "Agarwal-A" }, { "family_name": "Jordan", "given_name": "P.", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Linearisation of the Navier\u2013Stokes equations about the mean of a turbulent flow forms the foundation of popular models for energy amplification and coherent structures, including resolvent analysis. While the Navier\u2013Stokes equations can be equivalently written using many different sets of dependent variables, we show that the properties of the linear operator obtained via linearisation about the mean depend on the variables in which the equations are written prior to linearisation, and can be modified under nonlinear transformation of variables. For example, we show that using primitive and conservative variables leads to differences in the singular values and modes of the resolvent operator for turbulent jets, and that the differences become more severe as variable-density effects increase. This lack of uniqueness of mean-flow-based linear analysis provides new opportunities for optimising models by specific choice of variables while also highlighting the importance of carefully accounting for the nonlinear terms that act as a forcing on the resolvent operator.", "doi": "10.1017/jfm.2020.566", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2020-10-10", "volume": "900", "pages": "Art. No. R5" }, { "id": "authors:dzmnz-gsg42", "collection": "authors", "collection_id": "dzmnz-gsg42", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200806-103840041", "type": "article", "title": "Near-surface dynamics of a gas bubble collapsing above a crevice", "author": [ { "family_name": "Trummler", "given_name": "Theresa", "orcid": "0000-0001-8869-5860", "clpid": "Trummler-Theresa" }, { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265", "clpid": "Bryngelson-S-H" }, { "family_name": "Schmidmayer", "given_name": "Kevin", "orcid": "0000-0003-0444-3098", "clpid": "Schmidmayer-Kevin" }, { "family_name": "Schmidt", "given_name": "Steffen J.", "orcid": "0000-0001-6661-4505", "clpid": "Schmidt-S-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Adams", "given_name": "Nikolaus A.", "orcid": "0000-0001-5048-8639", "clpid": "Adams-N-A" } ], "abstract": "The impact of a collapsing gas bubble above rigid, notched walls is considered. Such surface crevices and imperfections often function as bubble nucleation sites, and thus have a direct relation to cavitation-induced erosion and damage structures. A generic configuration is investigated numerically using a second-order accurate compressible multi-component flow solver in a two-dimensional axisymmetric coordinate system. Results show that the crevice geometry has a significant effect on the collapse dynamics, jet formation, subsequent wave dynamics and interactions. The wall-pressure distribution associated with erosion potential is a direct consequence of development and intensity of these flow phenomena.", "doi": "10.1017/jfm.2020.432", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2020-09-25", "volume": "899", "pages": "Art. No. A16" }, { "id": "authors:rmv2k-2nx60", "collection": "authors", "collection_id": "rmv2k-2nx60", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200122-091643948", "type": "article", "title": "Lift-up, Kelvin-Helmholtz and Orr mechanisms in turbulent jets", "author": [ { "family_name": "Pickering", "given_name": "Ethan", "orcid": "0000-0002-4485-6359", "clpid": "Pickering-E-M" }, { "family_name": "Rigas", "given_name": "Georgios", "orcid": "0000-0001-6692-6437", "clpid": "Rigas-G" }, { "family_name": "Nogueira", "given_name": "Petr\u00f4nio A. S.", "clpid": "Nogueira-P-A" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Three amplification mechanisms present in turbulent jets, namely lift-up, Kelvin\u2013Helmholtz and Orr, are characterized via global resolvent analysis and spectral proper orthogonal decomposition (SPOD) over a range of Mach numbers. The lift-up mechanism was recently identified in turbulent jets via local analysis by Nogueira et al. (J. Fluid Mech., vol. 873, 2019, pp. 211\u2013237) at low Strouhal number ( St ) and non-zero azimuthal wavenumbers ( m ). In these limits, a global SPOD analysis of data from high-fidelity simulations reveals streamwise vortices and streaks similar to those found in turbulent wall-bounded flows. These structures are in qualitative agreement with the global resolvent analysis, which shows that they are a response to upstream forcing of streamwise vorticity near the nozzle exit. Analysis of mode shapes, component-wise amplitudes and sensitivity analysis distinguishes the three mechanisms and the regions of frequency\u2013wavenumber space where each dominates, finding lift-up to be dominant as St/m\u21920 . Finally, SPOD and resolvent analyses of localized regions show that the lift-up mechanism is present throughout the jet, with a dominant azimuthal wavenumber inversely proportional to streamwise distance from the nozzle, with streaks of azimuthal wavenumber exceeding five near the nozzle, and wavenumbers one and two most energetic far downstream of the potential core.", "doi": "10.1017/jfm.2020.301", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2020-08-10", "volume": "896", "pages": "Art. No. A2" }, { "id": "authors:1xdr6-8n320", "collection": "authors", "collection_id": "1xdr6-8n320", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200526-131556031", "type": "article", "title": "Enhancement of shock-capturing methods via machine learning", "author": [ { "family_name": "Stevens", "given_name": "Ben", "orcid": "0000-0002-3410-5922", "clpid": "Stevens-Ben" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "In recent years, machine learning has been used to create data-driven solutions to problems for which an algorithmic solution is intractable, as well as fine-tuning existing algorithms. This research applies machine learning to the development of an improved finite-volume method for simulating PDEs with discontinuous solutions. Shock-capturing methods make use of nonlinear switching functions that are not guaranteed to be optimal. Because data can be used to learn nonlinear relationships, we train a neural network to improve the results of a fifth-order WENO method. We post-process the outputs of the neural network to guarantee that the method is consistent. The training data consist of the exact mapping between cell averages and interpolated values for a set of integrable functions that represent waveforms we would expect to see while simulating a PDE. We demonstrate our method on linear advection of a discontinuous function, the inviscid Burgers' equation, and the 1-D Euler equations. For the latter, we examine the Shu\u2013Osher model problem for turbulence\u2013shock wave interactions. We find that our method outperforms WENO in simulations where the numerical solution becomes overly diffused due to numerical viscosity.", "doi": "10.1007/s00162-020-00531-1", "issn": "0935-4964", "publisher": "Springer", "publication": "Theoretical and Computational Fluid Dynamics", "publication_date": "2020-08", "series_number": "4", "volume": "34", "issue": "4", "pages": "483-496" }, { "id": "authors:ewgen-srj21", "collection": "authors", "collection_id": "ewgen-srj21", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200918-102621101", "type": "article", "title": "QBMMlib: A library of quadrature-based moment methods", "author": [ { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265", "clpid": "Bryngelson-Spencer-H" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Fox", "given_name": "Rodney O.", "orcid": "0000-0003-1944-1861", "clpid": "Fox-Rodney-O" } ], "abstract": "QBMMlib is an open source package of quadrature-based moment methods and their algorithms. Such methods are commonly used to solve fully-coupled disperse flow and combustion problems, though formulating and closing the corresponding governing equations can be complex. QBMMlib aims to make analyzing these techniques simple and more accessible. Its routines use symbolic manipulation to formulate the moment transport equations for a population balance equation and a prescribed dynamical system. The resulting moment transport equations are closed by first trading the moments for a set of quadrature points and weights via an inversion algorithm, of which several are available. Quadratures then compute the moments required for closure. Embedded code snippets show how to use QBMMlib, with the algorithm initialization and solution spanning just 13 total lines of code. Examples are shown and analyzed for a linear harmonic oscillator and a bubble dynamics problem.", "doi": "10.1016/j.softx.2020.100615", "issn": "2352-7110", "publisher": "Elsevier", "publication": "SoftwareX", "publication_date": "2020-07", "volume": "12", "pages": "Art. No. 100615" }, { "id": "authors:semrs-rcz98", "collection": "authors", "collection_id": "semrs-rcz98", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200415-094204836", "type": "article", "title": "Role of Coherent Structures in Turbulent Premixed Flame Acoustics", "author": [ { "family_name": "Brouzet", "given_name": "D.", "clpid": "Brouzet-D" }, { "family_name": "Haghiri", "given_name": "A.", "clpid": "Haghiri-A" }, { "family_name": "Talei", "given_name": "M.", "clpid": "Talei-M" }, { "family_name": "Brear", "given_name": "M. J.", "clpid": "Brear-M-J" }, { "family_name": "Schmidt", "given_name": "O. T.", "clpid": "Schmidt-O-T" }, { "family_name": "Rigas", "given_name": "G.", "orcid": "0000-0001-6692-6437", "clpid": "Rigas-G" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Spectral proper orthogonal decomposition (SPOD) is applied to direct numerical simulation datasets of a lean and a stoichiometric methane\u2013air turbulent premixed jet flame. SPOD is used to extract the coherent structures that correlate with the radiated sound by using an inner product based on a linearized disturbance energy. Two types of structures are prominent in the data. The first type arises in the jet's shear layer and is linked to the Kelvin\u2013Helmholtz instability, which is an important mechanism of sound generation in nonreacting jets. These structures produce sound through deformation of the flame front in the shear layer. They contain most of the acoustic energy and are dominant at Strouhal numbers (defined based on the jet's diameter and the inlet mean velocity) less than unity. The second type of structures is found near the jet centerline, where large fluctuations of the flame surface are observed. The structures are linked to small nonlinear flame dynamics and to the Orr mechanism. They travel at a speed close to the inlet mean velocity and are important at higher Strouhal numbers. Regardless of their energy content, both types of structures have important contributions to the broadband nature of combustion noise.", "doi": "10.2514/1.j058480", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2020-06", "series_number": "6", "volume": "58", "issue": "6", "pages": "2635-2642" }, { "id": "authors:5cahp-55q16", "collection": "authors", "collection_id": "5cahp-55q16", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200309-084519190", "type": "article", "title": "A Gaussian moment method and its augmentation via LSTM recurrent neural networks for the statistics of cavitating bubble populations", "author": [ { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265" }, { "family_name": "Charalampopoulos", "given_name": "Alexis" }, { "family_name": "Sapsis", "given_name": "Themistoklis P." }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Phase-averaged dilute bubbly flow models require high-order statistical moments of the bubble population. The method of classes, which directly evolve bins of bubbles in the probability space, are accurate but computationally expensive. Moment-based methods based upon a Gaussian closure present an opportunity to accelerate this approach, particularly when the bubble size distributions are broad (polydisperse). For linear bubble dynamics a Gaussian closure is exact, but for bubbles undergoing large and nonlinear oscillations, it results in a large error from misrepresented higher-order moments. Long short-term memory recurrent neural networks, trained on Monte Carlo truth data, are proposed to improve these model predictions. The networks are used to correct the low-order moment evolution equations and improve prediction of higher-order moments based upon the low-order ones. Results show that the networks can reduce model errors to less than 1% of their unaugmented values.", "doi": "10.1016/j.ijmultiphaseflow.2020.103262", "issn": "0301-9322", "publisher": "Elsevier", "publication": "International Journal of Multiphase Flow", "publication_date": "2020-06", "volume": "127", "pages": "Art. No. 103262" }, { "id": "authors:ef77k-ja894", "collection": "authors", "collection_id": "ef77k-ja894", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200423-103521218", "type": "article", "title": "Flow state estimation in the presence of discretization errors", "author": [ { "family_name": "da\u00a0Silva", "given_name": "Andre F. C.", "orcid": "0000-0002-8125-6010", "clpid": "da-Silva-A-F-C" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Ensemble data assimilation methods integrate measurement data and computational flow models to estimate the state of fluid systems in a robust, scalable way. However, discretization errors in the dynamical and observation models lead to biased forecasts and poor estimator performance. We propose a low-rank representation for this bias, whose dynamics is modelled by data-informed, time-correlated processes. State and bias parameters are simultaneously corrected online with the ensemble Kalman filter. The proposed methodology is then applied to the problem of estimating the state of a two-dimensional flow at modest Reynolds number using an ensemble of coarse-mesh simulations and pressure measurements at the surface of an immersed body in a synthetic experiment framework. Using an ensemble size of 60, the bias-aware estimator is demonstrated to achieve at least 70 % error reduction when compared to its bias-blind counterpart. Strategies to determine the bias statistics and their impact on the estimator performance are discussed.", "doi": "10.1017/jfm.2020.103", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2020-05-10", "volume": "890", "pages": "Art. No. A10" }, { "id": "authors:zw96b-4v596", "collection": "authors", "collection_id": "zw96b-4v596", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20191223-155123107", "type": "article", "title": "A fast multi-resolution lattice Green's function method for elliptic difference equations", "author": [ { "family_name": "Dorschner", "given_name": "Benedikt", "orcid": "0000-0001-8926-7542", "clpid": "Dorschner-Benedikt" }, { "family_name": "Yu", "given_name": "Ke", "orcid": "0000-0003-0157-4471", "clpid": "Yu-Ke" }, { "family_name": "Mengaldo", "given_name": "Gianmarco", "orcid": "0000-0002-0157-5477", "clpid": "Mengaldo-Gianmarco" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We propose a mesh refinement technique for solving elliptic difference equations on unbounded domains based on the fast lattice Green's function (FLGF) method. The FLGF method exploits the regularity of the Cartesian mesh and uses the fast multipole method in conjunction with fast Fourier transforms to yield linear complexity and decrease time-to-solution. We extend this method to a multi-resolution scheme and allow for locally refined Cartesian blocks embedded in the computational domain. Appropriately chosen interpolation and regularization operators retain consistency between the discrete Laplace operator and its inverse on the unbounded domain. Second-order accuracy and linear complexity are maintained, while significantly reducing the number of degrees of freedom and hence the computational cost.", "doi": "10.1016/j.jcp.2020.109270", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2020-04-15", "volume": "407", "pages": "Art. No. 109270" }, { "id": "authors:gcvbg-5ks79", "collection": "authors", "collection_id": "gcvbg-5ks79", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20200306-133727805", "type": "article", "title": "Guide to Spectral Proper Orthogonal Decomposition", "author": [ { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "This paper discusses the spectral proper orthogonal decomposition and its use in identifying modes, or structures, in flow data. A specific algorithm based on estimating the cross-spectral density tensor with Welch's method is presented, and guidance is provided on selecting data sampling parameters and understanding tradeoffs among them in terms of bias, variability, aliasing, and leakage. Practical implementation issues, including dealing with large datasets, are discussed and illustrated with examples involving experimental and computational turbulent flow data.", "doi": "10.2514/1.j058809", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2020-03", "series_number": "3", "volume": "58", "issue": "3", "pages": "1023-1033" }, { "id": "authors:ns24x-nsz38", "collection": "authors", "collection_id": "ns24x-nsz38", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190513-102413759", "type": "article", "title": "An assessment of multicomponent flow models and interface capturing schemes for spherical bubble dynamics", "author": [ { "family_name": "Schmidmayer", "given_name": "Kevin", "orcid": "0000-0003-0444-3098", "clpid": "Schmidmayer-Kevin" }, { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265", "clpid": "Bryngelson-S-H" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Numerical simulation of bubble dynamics and cavitation is challenging; even the seemingly simple problem of a collapsing spherical bubble is difficult to compute accurately with a general, three-dimensional, compressible, multicomponent flow solver. Difficulties arise due to both the physical model and the numerical method chosen for its solution. We consider the 5-equation model of Allaire et al. [1], the 5-equation model of Kapila et al. [2], and the 6-equation model of Saurel et al. [3] as candidate approaches for spherical bubble dynamics, and both MUSCL and WENO interface-capturing methods are implemented and compared. We demonstrate the inadequacy of the traditional 5-equation model of Allaire et al. [1] for spherical bubble collapse problems and explain the corresponding advantages of the augmented model of Kapila et al. [2] for representing this phenomenon. Quantitative comparisons between the augmented 5-equation and 6-equation models for three-dimensional bubble collapse problems demonstrate the versatility of pressure-disequilibrium models. Lastly, the performance of pressure disequilibrium model for representing a three-dimensional spherical bubble collapse for different bubble interior/exterior pressure ratios is evaluated for different numerical methods. Pathologies associated with each factor and their origins are identified and discussed.", "doi": "10.1016/j.jcp.2019.109080", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2020-02-01", "volume": "402", "pages": "Art. No. 109080" }, { "id": "authors:98kt9-p2247", "collection": "authors", "collection_id": "98kt9-p2247", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20191216-160711298", "type": "article", "title": "Simulation of humpback whale bubble-net feeding models", "author": [ { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265", "clpid": "Bryngelson-S-H" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Humpback whales can generate intricate bubbly regions, called bubble nets, via blowholes. Humpback whales appear to exploit these bubble nets for feeding via loud vocalizations. A fully-coupled phase-averaging approach is used to model the flow, bubble dynamics, and corresponding acoustics. A previously hypothesized waveguiding mechanism is assessed for varying acoustic frequencies and net void fractions. Reflections within the bubbly region result in observable waveguiding for only a small range of flow parameters. A configuration of multiple whales surrounding and vocalizing towards an annular bubble net is also analyzed. For a range of flow parameters, the bubble net keeps its core region substantially quieter than the exterior. This approach appears more viable, though it relies upon the cooperation of multiple whales. A spiral bubble net configuration that circumvents this requirement is also investigated. The acoustic wave behaviors in the spiral interior vary qualitatively with the vocalization frequency and net void fraction. The competing effects of vocalization guiding and acoustic attenuation are quantified. Low void fraction cases allow low-frequency waves to partially escape the spiral region, with the remaining vocalizations still exciting the net interior. Higher void fraction nets appear preferable, guiding even low-frequency vocalizations while still maintaining a quiet net interior.", "doi": "10.1121/10.0000746", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2020-02", "series_number": "2", "volume": "147", "issue": "2", "pages": "1126-1135" }, { "id": "authors:wnwvz-bmy96", "collection": "authors", "collection_id": "wnwvz-bmy96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190823-093318240", "type": "article", "title": "Spatial stability analysis of subsonic corrugated\u00a0jets", "author": [ { "family_name": "Laj\u00fas", "given_name": "F. C., Jr.", "orcid": "0000-0002-0773-3595", "clpid": "Laj\u00fas-F-C-Jr" }, { "family_name": "Sinha", "given_name": "A.", "orcid": "0000-0002-7122-3549", "clpid": "Sinha-A" }, { "family_name": "Cavalieri", "given_name": "A. V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Deschamps", "given_name": "C. J.", "orcid": "0000-0001-8912-1295", "clpid": "Deschamps-C-J" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The linear stability of high-Reynolds-number corrugated jets is investigated by solving the compressible Rayleigh equation linearized about the time-averaged flow field. A Floquet ansatz is used to account for periodicity of this base flow in the azimuthal direction. The origin of multiple unstable solutions, which are known to appear in these non-circular configurations, is traced through gradual perturbations of a parametrized base-flow profile. It is shown that all unstable modes are corrugated jet continuations of the classical Kelvin\u2013Helmholtz modes of circular jets, highlighting that the same instability mechanism, modified by corrugations, leads to the growth of disturbances in such flows. It is found that under certain conditions the eigenvalues may form saddles in the complex plane and display axis switching in their eigenfunctions. A parametric study is also conducted to understand how penetration and number of corrugations impact stability. The effect of these geometric properties on growth rates and phase speeds of the multiple unstable modes is explored, and the results provide guidelines for the development of nozzle configurations that more effectively modify the Kelvin\u2013Helmholtz instability.", "doi": "10.1017/jfm.2019.573", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2019-10-10", "volume": "876", "pages": "766-791" }, { "id": "authors:p0k53-7kw05", "collection": "authors", "collection_id": "p0k53-7kw05", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20191119-113221067", "type": "article", "title": "Furthering resolvent-based jet noise models", "author": [ { "family_name": "Pickering", "given_name": "Ethan M.", "orcid": "0000-0002-4485-6359", "clpid": "Pickering-E-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Resolvent analysis continues to provide promising results for modeling the hydrodynamic near-field and acoustic far-field in turbulent jets, particularly when compared to modes deduced through spectral proper orthogonal decomposition (SPOD) of high-fidelity large eddy simulations (LES). Although previous studies have shown that the acoustic field for supersonic jets is of low-rank and primarily described by the Kelvin-Helmholtz instability, the agreement between the acoustically optimal resolvent mode (Kelvin-Helmholtz) and optimal SPOD mode is still lacking. The discrepancy is due to the spatial coloring of turbulent mechanisms in supersonic jets and presents a challenge for reconstructing the acoustic field, as well as the full field flow statistics. To account for coloring, additional (i.e., suboptimal) resolvent modes, associated with the Orr mechanism, must be included and their correlation to other modes determined. Here, we estimate the coloring between resolvent modes by projecting onto an ensemble of LES realizations and reconstructing the realizations in the resolvent basis. The associated projection coefficients provide an ensemble of observations which inherently possess the statistical information necessary to reconstruct the flow and to which we propose a reduced-order stochastic model. We find that the inclusion of a few resolvent suboptimal modes (i.e., Orr-type) allows for modeling of the acoustic field to within 2 dB and increases the region of acoustic agreement when compared to a single mechanism model.", "doi": "10.1121/1.5137546", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2019-10", "series_number": "4", "volume": "146", "issue": "4", "pages": "3043" }, { "id": "authors:tyze9-akd64", "collection": "authors", "collection_id": "tyze9-akd64", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20191119-101701319", "type": "article", "title": "Annular and spiral bubble nets: A simulation-focused analysis of humpback whale feeding strategies", "author": [ { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265", "clpid": "Bryngelson-S-H" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Humpback whales can generate bubbly regions (called bubble nets) via their blowholes, which they appear to exploit via loud vocalizations for feeding purposes. We model this phenomenon as the acoustic excitation of an dilute bubble net of radially varying void fraction. A fully coupled phase-averaging approach is used to compute the bubble response and corresponding acoustics. We first assess the possibility of a sophisticated wave-guidance behavior of high-frequency whale vocalizations within the bubble net. For a small range of flow parameters, the reflections associated with the bubbly region result in an observable wave-guidance behavior, though even then these reflections disperse rapidly. In light of this, we also consider multiple whales surrounding the bubble net, each vocalizing towards its center. We show that for physically realistic configurations, including variations in the bubble net void fraction and number of whales, the bubble net can keep its core region substantially quieter than the exterior. Finally, we investigate the ability of spiral, rather than annular, geometries for keeping the bubble-free region quiet.", "doi": "10.1121/1.5136599", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2019-10", "series_number": "4", "volume": "146", "issue": "4", "pages": "2771" }, { "id": "authors:d2rc4-0cw13", "collection": "authors", "collection_id": "d2rc4-0cw13", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190613-095543057", "type": "article", "title": "A critical assessment of the parabolized stability equations", "author": [ { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-Aaron" }, { "family_name": "Rigas", "given_name": "Georgios", "orcid": "0000-0001-6692-6437", "clpid": "Rigas-Georgios" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The parabolized stability equations (PSE) are a ubiquitous tool for studying the stability and evolution of disturbances in weakly nonparallel, convectively unstable flows. The PSE method was introduced as an alternative to asymptotic approaches to these problems. More recently, PSE has been applied with mixed results to a more diverse set of problems, often involving flows with multiple relevant instability modes. This paper investigates the limits of validity of PSE via a spectral analysis of the PSE operator. We show that PSE is capable of accurately capturing only disturbances with a single wavelength at each frequency and that other disturbances are not necessarily damped away or properly evolved, as often assumed. This limitation is the result of regularization techniques that are required to suppress instabilities arising from the ill-posedness of treating a boundary value problem as an initial value problem. These findings are valid for both incompressible and compressible formulations of PSE and are particularly relevant for applications involving multiple modes with different wavelengths and growth rates, such as problems involving multiple instability mechanisms, transient growth, and acoustics. Our theoretical results are illustrated using a generic problem from acoustics and a dual-stream jet, and the PSE solutions are compared to both global solutions of the linearized Navier\u2013Stokes equations and a recently developed alternative parabolization.", "doi": "10.1007/s00162-019-00498-8", "issn": "0935-4964", "publisher": "Springer", "publication": "Theoretical and Computational Fluid Dynamics", "publication_date": "2019-08", "series_number": "3-4", "volume": "33", "issue": "3-4", "pages": "359-382" }, { "id": "authors:zfads-fsn90", "collection": "authors", "collection_id": "zfads-fsn90", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190312-090052833", "type": "article", "title": "Modeling and simulation of a fluttering cantilever in channel flow", "author": [ { "family_name": "Tosi", "given_name": "Lu\u00eds Phillipe", "orcid": "0000-0002-0819-4765", "clpid": "Tosi-L-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Characterizing the dynamics of a cantilever in channel flow is relevant to applications ranging from snoring to energy harvesting. Aeroelastic flutter induces large oscillating amplitudes and sharp changes with frequency that impact the operation of these systems. The fluid\u2013structure mechanisms that drive flutter can vary as the system parameters change, with the stability boundary becoming especially sensitive to the channel height and Reynolds number, especially when either or both are small. In this paper, we develop a coupled fluid\u2013structure model for viscous, two-dimensional channel flow of arbitrary shape. Its flutter boundary is then compared to results of two-dimensional direct numerical simulations to explore the model's validity. Provided the non-dimensional channel height remains small, the analysis shows that the model is not only able to replicate DNS results within the parametric limits that ensure the underlying assumptions are met, but also over a wider range of Reynolds numbers and fluid\u2013structure mass ratios. Model predictions also converge toward an inviscid model for the same geometry as Reynolds number increases.", "doi": "10.1016/j.jfluidstructs.2019.02.021", "issn": "0889-9746", "publisher": "Elsevier", "publication": "Journal of Fluids and Structures", "publication_date": "2019-08", "volume": "89", "pages": "174-190" }, { "id": "authors:85yhg-ccv47", "collection": "authors", "collection_id": "85yhg-ccv47", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190729-100946483", "type": "article", "title": "High-speed video microscopy and numerical modeling of bubble dynamics near a surface of urinary stone", "author": [ { "family_name": "Pishchalnikov", "given_name": "Yuri A.", "clpid": "Pishchalnikov-Y-A" }, { "family_name": "Behnke-Parks", "given_name": "William M.", "clpid": "Behnke-Parks-W-M" }, { "family_name": "Schmidmayer", "given_name": "Kevin", "orcid": "0000-0003-0444-3098", "clpid": "Schmidmayer-Kevin" }, { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Kenny", "given_name": "Thomas W.", "clpid": "Kenny-T-W" }, { "family_name": "Laser", "given_name": "Daniel J.", "clpid": "Laser-D-J" } ], "abstract": "Ultra-high-speed video microscopy and numerical modeling were used to assess the dynamics of microbubbles at the surface of urinary stones. Lipid-shell microbubbles designed to accumulate on stone surfaces were driven by bursts of ultrasound in the sub-MHz range with pressure amplitudes on the order of 1\u2009MPa. Microbubbles were observed to undergo repeated cycles of expansion and violent collapse. At maximum expansion, the microbubbles' cross-section resembled an ellipse truncated by the stone. Approximating the bubble shape as an oblate spheroid, this study modeled the collapse by solving the multicomponent Euler equations with a two-dimensional-axisymmetric code with adaptive mesh refinement for fine resolution of the gas-liquid interface. Modeled bubble collapse and high-speed video microscopy showed a distinctive circumferential pinching during the collapse. In the numerical model, this pinching was associated with bidirectional microjetting normal to the rigid surface and toroidal collapse of the bubble. Modeled pressure spikes had amplitudes two-to-three orders of magnitude greater than that of the driving wave. Micro-computed tomography was used to study surface erosion and formation of microcracks from the action of microbubbles. This study suggests that engineered microbubbles enable stone-treatment modalities with driving pressures significantly lower than those required without the microbubbles.", "doi": "10.1121/1.5116693", "pmcid": "PMC6660306", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2019-07", "series_number": "1", "volume": "146", "issue": "1", "pages": "516-531" }, { "id": "authors:wheah-f7357", "collection": "authors", "collection_id": "wheah-f7357", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190403-101200730", "type": "article", "title": "A quantitative comparison of phase-averaged models for bubbly, cavitating flows", "author": [ { "family_name": "Bryngelson", "given_name": "Spencer H.", "orcid": "0000-0003-1750-7265", "clpid": "Bryngelson-S-H" }, { "family_name": "Schmidmayer", "given_name": "Kevin", "orcid": "0000-0003-0444-3098", "clpid": "Schmidmayer-Kevin" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We compare the computational performance of two modeling approaches for the flow of dilute cavitation bubbles in a liquid. The first approach is a deterministic model, for which bubbles are represented in a Lagrangian framework as advected features, each sampled from a distribution of equilibrium bubble sizes. The dynamic coupling to the liquid phase is modeled through local volume averaging. The second approach is stochastic; ensemble-phase averaging is used to derive mixture-averaged equations and field equations for the associated bubble properties are evolved in an Eulerian reference frame. For polydisperse mixtures, the probability density function of the equilibrium bubble radii is discretized and bubble properties are solved for each representative bin. In both cases, the equations are closed by solving Rayleigh\u2013Plesset-like equations for the bubble dynamics as forced by the local or mixture-averaged pressure, respectively. An acoustically excited dilute bubble screen is used as a case study for comparisons. We show that observables of ensemble- and volume-averaged simulations match closely and that their convergence is first order under grid refinement. Guidelines are established for phase-averaged simulations by comparing the computational costs of methods. The primary costs are shown to be associated with stochastic closure; polydisperse ensemble-averaging requires many samples of the underlying PDF and volume-averaging requires repeated, randomized simulations to accurately represent a homogeneous bubble population. The relative sensitivities of these costs to spatial resolution and bubble void fraction are presented.", "doi": "10.1016/j.ijmultiphaseflow.2019.03.028", "issn": "0301-9322", "publisher": "Elsevier", "publication": "International Journal of Multiphase Flow", "publication_date": "2019-06", "volume": "115", "pages": "137-143" }, { "id": "authors:jhb95-fxs05", "collection": "authors", "collection_id": "jhb95-fxs05", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190410-092338329", "type": "article", "title": "Comparative study of the dynamics of laser and acoustically generated bubbles in viscoelastic media", "author": [ { "family_name": "Wilson", "given_name": "Chad T.", "clpid": "Wilson-C-T" }, { "family_name": "Hall", "given_name": "Timothy L.", "clpid": "Hall-T-L" }, { "family_name": "Johnsen", "given_name": "Eric", "clpid": "Johnsen-E" }, { "family_name": "Mancia", "given_name": "Lauren", "orcid": "0000-0003-4366-1944", "clpid": "Mancia-L" }, { "family_name": "Rodriguez", "given_name": "Mauro", "clpid": "Rodriguez-M" }, { "family_name": "Lundt", "given_name": "Jonathan E.", "clpid": "Lundt-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Henann", "given_name": "David L.", "clpid": "Henann-D-L" }, { "family_name": "Franck", "given_name": "Christian", "clpid": "Franck-C" }, { "family_name": "Xu", "given_name": "Zhen", "clpid": "Xu-Zhen" }, { "family_name": "Sukovich", "given_name": "Jonathan R.", "clpid": "Sukovich-J-R" } ], "abstract": "Experimental observations of the growth and collapse of acoustically and laser-nucleated single bubbles in water and agarose gels of varying stiffness are presented. The maximum radii of generated bubbles decreased as the stiffness of the media increased for both nucleation modalities, but the maximum radii of laser-nucleated bubbles decreased more rapidly than acoustically nucleated bubbles as the gel stiffness increased. For water and low stiffness gels, the collapse times were well predicted by a Rayleigh cavity, but bubbles collapsed faster than predicted in the higher stiffness gels. The growth and collapse phases occurred symmetrically (in time) about the maximum radius in water but not in gels, where the duration of the growth phase decreased more than the collapse phase as gel stiffness increased. Numerical simulations of the bubble dynamics in viscoelastic media showed varying degrees of success in accurately predicting the observations.", "doi": "10.1103/physreve.99.043103", "issn": "2470-0045", "publisher": "American Physical Society", "publication": "Physical Review E", "publication_date": "2019-04", "series_number": "4", "volume": "99", "issue": "4", "pages": "Art. No. 043103" }, { "id": "authors:dwj3s-xp206", "collection": "authors", "collection_id": "dwj3s-xp206", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180828-125637273", "type": "article", "title": "Bubble cloud dynamics in an ultrasound field", "author": [ { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The dynamics of bubble clouds induced by high-intensity focused ultrasound is investigated in a regime where the cloud size is similar to the ultrasound wavelength. High-speed images show that the cloud is asymmetric; the bubbles nearest the source grow to a larger radius than the distal ones. Similar structures of bubble clouds are observed in numerical simulations that mimic the laboratory experiment. To elucidate the structure, a parametric study is conducted for plane ultrasound waves with various amplitudes and diffuse clouds with different initial void fractions. Based on an analysis of the kinetic energy of liquid induced by bubble oscillations, a new scaling parameter is introduced to characterize the dynamics. The new parameter generalizes the cloud interaction parameter originally introduced by d'Agostino & Brennen (J. Fluid Mech., vol. 199, 1989, pp. 155\u2013176). The dynamic interaction parameter controls the energy localization and consequent anisotropy of the cloud. Moreover, the amplitude of the far-field, bubble-scattered acoustics is likewise correlated with the proposed parameter. Findings of the present study not only shed light on the physics of cloud cavitation, but may also be of use for the quantification of the effects of cavitation on outcomes of ultrasound therapies including high-intensity focused ultrasound-based lithotripsy.", "doi": "10.1017/jfm.2018.968", "pmcid": "PMC6761994", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2019-03-10", "volume": "862", "pages": "1105-1134" }, { "id": "authors:qsf67-tpx91", "collection": "authors", "collection_id": "qsf67-tpx91", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180122-091425148", "type": "article", "title": "Global modes and nonlinear analysis of inverted-flag flapping", "author": [ { "family_name": "Goza", "given_name": "Andres", "orcid": "0000-0002-9372-7713", "clpid": "Goza-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" } ], "abstract": "An inverted flag has its trailing edge clamped and exhibits dynamics distinct from that of a conventional flag, whose leading edge is restrained. We perform nonlinear simulations and a global stability analysis of the inverted-flag system for a range of Reynolds numbers, flag masses and stiffnesses. Our global stability analysis is based on a linearisation of the fully coupled fluid\u2013structure system of equations. The calculated equilibria are steady-state solutions of the fully coupled nonlinear equations. By implementing this approach, we (i) explore the mechanisms that initiate flapping, (ii) study the role of vorticity generation and vortex-induced vibration (VIV) in large-amplitude flapping and (iii) characterise the chaotic flapping regime. For point (i), we identify a deformed-equilibrium state and show through a global stability analysis that the onset of small-deflection flapping \u2013 where the oscillation amplitude is significantly smaller than in large-amplitude flapping \u2013 is due to a supercritical Hopf bifurcation. For large-amplitude flapping, point (ii), we confirm the arguments of Sader et al. (J. Fluid Mech., vol. 793, 2016a) that classical VIV exists when the flag is sufficiently light with respect to the fluid. We also show that for heavier flags, large-amplitude flapping persists (even for Reynolds numbers < 50) and is not classical VIV. Finally, with respect to point (iii), chaotic flapping has been observed experimentally for Reynolds numbers of O(10^4) , and here we show that chaos also persists at a moderate Reynolds number of 200. We characterise this chaotic regime and calculate its strange attractor, whose structure is controlled by the above-mentioned deformed equilibria and is similar to a Lorenz attractor.", "doi": "10.1017/jfm.2018.728", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2018-12-25", "volume": "857", "pages": "312-344" }, { "id": "authors:nq2j6-93t97", "collection": "authors", "collection_id": "nq2j6-93t97", "cite_using_url": "https://authors.library.caltech.edu/records/nq2j6-93t97", "type": "article", "title": "Global modes and nonlinear analysis of inverted-flag flapping", "author": [ { "family_name": "Goza", "given_name": "Andres", "orcid": "0000-0002-9372-7713", "clpid": "Goza-Andres" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" } ], "abstract": "An inverted flag has its trailing edge clamped and exhibits dynamics distinct from that of a conventional flag, whose leading edge is restrained. We perform nonlinear simulations and a global stability analysis of the inverted-flag system for a range of Reynolds numbers, flag masses and stiffnesses. Our global stability analysis is based on a linearisation of the fully coupled fluid\u2013structure system of equations. The calculated equilibria are steady-state solutions of the fully coupled nonlinear equations. By implementing this approach, we (i) explore the mechanisms that initiate flapping, (ii) study the role of vorticity generation and vortex-induced vibration (VIV) in large-amplitude flapping and (iii) characterise the chaotic flapping regime. For point (i), we identify a deformed-equilibrium state and show through a global stability analysis that the onset of small-deflection flapping \u2013 where the oscillation amplitude is significantly smaller than in large-amplitude flapping \u2013 is due to a supercritical Hopf bifurcation. For large-amplitude flapping, point (ii), we confirm the arguments of Sader et al. (J. Fluid Mech., vol. 793, 2016a) that classical VIV exists when the flag is sufficiently light with respect to the fluid. We also show that for heavier flags, large-amplitude flapping persists (even for Reynolds numbers <50 ) and is not classical VIV. Finally, with respect to point (iii), chaotic flapping has been observed experimentally for Reynolds numbers of O(10\u2074) , and here we show that chaos also persists at a moderate Reynolds number of 200. We characterise this chaotic regime and calculate its strange attractor, whose structure is controlled by the above-mentioned deformed equilibria and is similar to a Lorenz attractor.
", "doi": "10.1017/jfm.2018.728", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2018-12-25", "volume": "857", "pages": "312-344" }, { "id": "authors:txhh8-1d492", "collection": "authors", "collection_id": "txhh8-1d492", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20181213-103728580", "type": "article", "title": "Data-assimilated low-order vortex modeling of separated flows", "author": [ { "family_name": "Darakananda", "given_name": "Darwin", "clpid": "Darakananda-D" }, { "family_name": "de Castro da Silva", "given_name": "Andr\u00e9 Fernando", "clpid": "de-Castro-da-Silva-A-F" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Eldredge", "given_name": "Jeff D.", "clpid": "Eldredge-J-D" } ], "abstract": "Vortex models have been used for decades as computationally efficient tools to investigate unsteady aerodynamics. However, their utility for separated flows\u2014particularly when such flows are subjected to incident disturbances\u2014has been hindered by the tradeoff between the model's physical fidelity and its expectation for fast prediction (e.g., relative to computational fluid dynamics). In this work, it is shown that physical fidelity and speed can be simultaneously achieved by assimilating measurement data into the model to compensate for unrepresented physics. The underlying inviscid vortex model captures the transport of vortex structures with a standard collection of regularized vortex elements that interact mutually and with an infinitely thin flat plate. In order to maintain a low-dimensional representation, with fewer than O(100) degrees of freedom, an aggregation procedure is developed and utilized in which vortex elements are coalesced at each time step. A flow state vector, composed of vortex element properties as well as the critical leading-edge suction parameter, is advanced within an ensemble Kalman filter (EnKF) framework. In this framework, surface pressure is used to correct the states of an ensemble of randomly initiated vortex models. The overall algorithm is applied to several scenarios of an impulsively started flat plate, in which data from a high-fidelity Navier-Stokes simulation at Reynolds number 500 are used as a surrogate for the measurements. The assimilated vortex model efficiently and accurately predicts the evolving flow as well as the normal force in both the undisturbed case (a separated flow) as well as in the presence of one or more incident gusts, despite lack of a priori knowledge of the gust's characteristics.", "doi": "10.1103/physrevfluids.3.124701", "issn": "2469-990X", "publisher": "American Physical Society", "publication": "Physical Review Fluids", "publication_date": "2018-12", "series_number": "12", "volume": "3", "issue": "12", "pages": "Art. No. 124701" }, { "id": "authors:8af3f-64f78", "collection": "authors", "collection_id": "8af3f-64f78", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180618-080041218", "type": "article", "title": "Spectral analysis of jet turbulence", "author": [ { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Rigas", "given_name": "Georgios", "orcid": "0000-0001-6692-6437", "clpid": "Rigas-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" } ], "abstract": "Informed by large-eddy simulation (LES) data and resolvent analysis of the mean flow, we examine the structure of turbulence in jets in the subsonic, transonic and supersonic regimes. Spectral (frequency-space) proper orthogonal decomposition is used to extract energy spectra and decompose the flow into energy-ranked coherent structures. The educed structures are generally well predicted by the resolvent analysis. Over a range of low frequencies and the first few azimuthal mode numbers, these jets exhibit a low-rank response characterized by Kelvin\u2013Helmholtz (KH) type wavepackets associated with the annular shear layer up to the end of the potential core and that are excited by forcing in the very-near-nozzle shear layer. These modes too have been experimentally observed before and predicted by quasi-parallel stability theory and other approximations \u2013 they comprise a considerable portion of the total turbulent energy. At still lower frequencies, particularly for the axisymmetric mode, and again at high frequencies for all azimuthal wavenumbers, the response is not low-rank, but consists of a family of similarly amplified modes. These modes, which are primarily active downstream of the potential core, are associated with the Orr mechanism. They occur also as subdominant modes in the range of frequencies dominated by the KH response. Our global analysis helps tie together previous observations based on local spatial stability theory, and explains why quasi-parallel predictions were successful at some frequencies and azimuthal wavenumbers, but failed at others.", "doi": "10.1017/jfm.2018.675", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2018-11-25", "volume": "855", "pages": "953-982" }, { "id": "authors:yv875-3s862", "collection": "authors", "collection_id": "yv875-3s862", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190123-101938742", "type": "article", "title": "Experimental observations and numerical modeling of lipid-shell microbubbles with calcium-adhering moieties for minimally-invasive treatment of urinary stones", "author": [ { "family_name": "Pishchalnikov", "given_name": "Yuri A.", "clpid": "Pishchalnikov-Y-A" }, { "family_name": "Behnke-Parks", "given_name": "William", "clpid": "Behnke-Parks-W" }, { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Mellema", "given_name": "Matthew", "clpid": "Mellema-M" }, { "family_name": "Hopcroft", "given_name": "Matthew", "clpid": "Hopcroft-M" }, { "family_name": "Luong", "given_name": "Alice", "clpid": "Luong-Alice" }, { "family_name": "Wiener", "given_name": "Scott", "clpid": "Wiener-S" }, { "family_name": "Stoller", "given_name": "Marshall L.", "clpid": "Stoller-M-L" }, { "family_name": "Kenny", "given_name": "Thomas", "clpid": "Kenny-T" }, { "family_name": "Laser", "given_name": "Daniel J.", "clpid": "Laser-D-J" } ], "abstract": "A novel treatment modality incorporating calcium-adhering microbubbles has recently entered human clinical trials as a new minimally-invasive approach to treat urinary stones. In this treatment method, lipid-shell gas-core microbubbles can be introduced into the urinary tract through a catheter. Lipid moities with calcium-adherance properties incorporated into the lipid shell facilitate binding to stones. The microbubbles can be excited by an extracorporeal source of quasi-collimated ultrasound. Alternatively, the microbubbles can be excited by an intraluminal source, such as a fiber-optic laser. With either excitation technique, calcium-adhering microbubbles can significantly increase rates of erosion, pitting, and fragmentation of stones. We report here on new experiments using high-speed photography to characterize microbubble expansion and collapse. The bubble geometry observed in the experiments was used as one of the initial shapes for the numerical modeling. The modeling showed that the bubble dynamics strongly depends on bubble shape and stand-off distance. For the experimentally observed shape of microbubbles, the numerical modeling showed that the collapse of the microbubbles was associated with pressure increases of some two-to-three orders of magnitude compared to the excitation source pressures. This in-vitro study provides key insights into the use of microbubbles with calcium-adhering moieties in treatment of urinary stones.", "doi": "10.1121/2.0000958", "pmcid": "PMC7241592", "issn": "1939-800X", "publisher": "Acoustical Society of America", "publication": "Proceedings of Meetings on Acoustics", "publication_date": "2018-11-05", "series_number": "1", "volume": "35", "issue": "1", "pages": "Art. No. 020008" }, { "id": "authors:3xr1b-1ar53", "collection": "authors", "collection_id": "3xr1b-1ar53", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190123-080437707", "type": "article", "title": "Modeling and numerical simulation of the bubble cloud dynamics in an ultrasound field for burst wave lithotripsy", "author": [ { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Maxwell", "given_name": "Adam", "clpid": "Maxwell-A-D" }, { "family_name": "Kreider", "given_name": "Wayne", "clpid": "Kreider-W" }, { "family_name": "Bailey", "given_name": "Michael", "clpid": "Bailey-M-R" } ], "abstract": "Modeling and numerical simulation of bubble clouds induced by intense ultrasound waves are conducted to quantify the effect of cloud cavitation on burst wave lithotripsy, a proposed non-invasive alternative to shock wave lithotripsy that uses pulses of ultrasound with an amplitude of O(1) MPa and a frequency of O(100) kHz. A unidirectional acoustic source model and an Eulerian-Lagrangian method are developed for simulation of ultrasound generation from a multi-element array transducer and cavitation bubbles, respectively. Parametric simulations of the spherical bubble cloud dynamics reveal a new scaling parameter that dictates both the structure of the bubble cloud and the amplitude of the far-field, bubble-scattered acoustics. The simulation further shows that a thin layer of bubble clouds nucleated near a kidney stone model can shield up to 90% of the incoming wave energy, indicating a potential loss of efficacy during the treatment due to cavitation. Strong correlations are identified between the far-field, bubble-scattered acoustics and the magnitude of the shielding, which could be used for ultrasound monitoring of cavitation during treatments. The simulations are validated by companion experiments in vitro.", "doi": "10.1121/2.0000946", "issn": "1939-800X", "publisher": "Acoustical Society of America", "publication": "Proceedings of Meetings on Acoustics", "publication_date": "2018-11-05", "series_number": "1", "volume": "35", "issue": "1", "pages": "Art. No. 020006" }, { "id": "authors:ywdh7-6f031", "collection": "authors", "collection_id": "ywdh7-6f031", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180827-090424631", "type": "article", "title": "Energy shielding by cavitation bubble clouds in burst wave lithotripsy", "author": [ { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Maxwell", "given_name": "Adam D.", "clpid": "Maxwell-Adam-D" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Kreider", "given_name": "Wayne", "clpid": "Kreider-Wayne" }, { "family_name": "Bailey", "given_name": "Michael R.", "clpid": "Bailey-Michael-R" } ], "abstract": "Combined laboratory experiment and numerical simulation are conducted on bubble clouds nucleated on the surface of a model kidney stone to quantify the energy shielding of the stone caused by cavitation during burst wave lithotripsy (BWL). In the experiment, the bubble clouds are visualized and bubble-scattered acoustics are measured. In the simulation, a compressible, multi-component flow solver is used to capture complex interactions among cavitation bubbles, the stone, and the burst wave. Quantitative agreement is confirmed between results of the experiment and the simulation. In the simulation, a significant shielding of incident wave energy by the bubble clouds is quantified. The magnitude of shielding can reach up to 90% of the energy of the incoming burst wave that otherwise would be transmitted into the stone, suggesting a potential loss of efficacy of stone comminution. There is a strong correlation between the magnitude of the energy shielding and the amplitude of the bubble-scattered acoustics, independent of the initial size and the void fraction of the bubble cloud within a range addressed in the simulation. This correlation could provide for real-time monitoring of cavitation activity in BWL.", "doi": "10.1121/1.5079641", "pmcid": "PMC6258362", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2018-11", "series_number": "5", "volume": "144", "issue": "5", "pages": "2952-2961" }, { "id": "authors:sfm72-ba678", "collection": "authors", "collection_id": "sfm72-ba678", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180828-133412782", "type": "article", "title": "Jet-edge interaction tones", "author": [ { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Jaunet", "given_name": "Vincent", "clpid": "Jaunet-V" }, { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Schmidt", "given_name": "Oliver", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Agarwal", "given_name": "Anurag", "clpid": "Agarwal-A" } ], "abstract": "Motivated by the problem of jet\u2013flap interaction noise, we study the tonal dynamics that occurs when an isothermal turbulent jet grazes a sharp edge. We perform hydrodynamic and acoustic pressure measurements to characterise the tones as a function of Mach number and streamwise edge position. The observed distribution of spectral peaks cannot be explained using the usual edge-tone model, in which resonance is underpinned by coupling between downstream-travelling Kelvin\u2013Helmholtz wavepackets and upstream-travelling sound waves. We show, rather, that the strongest tones are due to coupling between Kelvin\u2013Helmholtz wavepackets and a family of trapped, upstream-travelling acoustic modes in the potential core, recently studied by Towne et al. (J. Fluid Mech. vol. 825, 2017) and Schmidt et al. (J. Fluid Mech. vol. 825, 2017). We also study the band-limited nature of the resonance, showing the high-frequency cutoff to be due to the frequency dependence of the upstream-travelling waves. Specifically, at high Mach number, these modes become evanescent above a certain frequency, whereas at low Mach number they become progressively trapped with increasing frequency, which inhibits their reflection in the nozzle plane.", "doi": "10.1017/jfm.2018.566", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2018-10-25", "volume": "853", "pages": "333-358" }, { "id": "authors:rc87r-jvn05", "collection": "authors", "collection_id": "rc87r-jvn05", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190709-092102441", "type": "article", "title": "Experimental observations and numerical modeling of lipid-shell microbubbles with stone targeting moieties for minimally-invasive treatment of urinary stones", "author": [ { "family_name": "Pishchalnikov", "given_name": "Yuri A.", "clpid": "Pishchalnikov-Y-A" }, { "family_name": "Behnke-Parks", "given_name": "William", "clpid": "Behnke-Parks-W" }, { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Mellema", "given_name": "Matt", "clpid": "Mellema-M" }, { "family_name": "Hopcroft", "given_name": "Matt", "clpid": "Hopcroft-M" }, { "family_name": "Luong", "given_name": "Alice", "clpid": "Luong-Alice" }, { "family_name": "Wiener", "given_name": "Scott", "clpid": "Wiener-S" }, { "family_name": "Stoller", "given_name": "Marshall", "clpid": "Stoller-M-L" }, { "family_name": "Kenny", "given_name": "Thomas", "clpid": "Kenny-T" }, { "family_name": "Laser", "given_name": "Daniel", "clpid": "Laser-D" } ], "abstract": "Products incorporating stone-targeting microbubbles have recently entered human clinical trials as a new minimally-invasive approach to treat urinary stones. Lipid-shell, gas-core microbubbles can be introduced into the urinary tract through a catheter. Calcium-binding moieties incorporated into the lipid shell can facilitate binding to stones. The microbubbles can be excited by an extracorporeal source of low-intensity ultrasound. Alternatively, the microbubbles can be excited by an intraluminal source, such as a fiber-optic laser. With either excitation technique, stone-targeting microbubbles can significantly increase rates of erosion, pitting, and fragmentation of stones, as has recently been reported for in-vitro experiments with synthetic stones [Wiener et al., J. Urology, v.199, no.4S, e322 (2018)]. We report here on new experiments using high-speed photography to characterize microbubbles expansion of cracks within a stone and resultant breaking-off of stone fragments. Numerical modeling shows that the direction of microjets produced by collapsing stone-bound microbubbles depends strongly on bubble shape and stand-off distance. For a wide range of stand-off distances and bubble shapes, microbubble collapse is associated with pressure increases of some two orders of magnitude compared to the excitation source pressures. This in-vitro study provides key insights into the use of stone-targeting microbubbles in treatment of urinary stones.", "doi": "10.1121/1.5067871", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2018-10-19", "series_number": "3", "volume": "144", "issue": "3", "pages": "1781" }, { "id": "authors:ypvz9-xqg97", "collection": "authors", "collection_id": "ypvz9-xqg97", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180518-133233019", "type": "article", "title": "Eulerian-Lagrangian method for simulation of cloud cavitation", "author": [ { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We present a coupled Eulerian\u2013Lagrangian method to simulate cloud cavitation in a compressible liquid. The method is designed to capture the strong, volumetric oscillations of each bubble and the bubble-scattered acoustics. The dynamics of the bubbly mixture is formulated using volume-averaged equations of motion. The continuous phase is discretized on an Eulerian grid and integrated using a high-order, finite-volume weighted essentially non-oscillatory (WENO) scheme, while the gas phase is modeled as spherical, Lagrangian point-bubbles at the sub-grid scale, each of whose radial evolution is tracked by solving the Keller\u2013Miksis equation. The volume of bubbles is mapped onto the Eulerian grid as the void fraction by using a regularization (smearing) kernel. In the most general case, where the bubble distribution is arbitrary, three-dimensional Cartesian grids are used for spatial discretization. In order to reduce the computational cost for problems possessing translational or rotational homogeneities, we spatially average the governing equations along the direction of symmetry and discretize the continuous phase on two-dimensional or axi-symmetric grids, respectively. We specify a regularization kernel that maps the three-dimensional distribution of bubbles onto the field of an averaged two-dimensional or axi-symmetric void fraction. A closure is developed to model the pressure fluctuations at the sub-grid scale as synthetic noise. For the examples considered here, modeling the sub-grid pressure fluctuations as white noise agrees a priori with computed distributions from three-dimensional simulations, and suffices, a posteriori, to accurately reproduce the statistics of the bubble dynamics. The numerical method and its verification are described by considering test cases of the dynamics of a single bubble and cloud cavitation induced by ultrasound fields.", "doi": "10.1016/j.jcp.2018.05.029", "pmcid": "PMC6364854", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2018-10-15", "volume": "371", "pages": "994-1017" }, { "id": "authors:r63dp-njj61", "collection": "authors", "collection_id": "r63dp-njj61", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180801-142726732", "type": "article", "title": "Importance of the nozzle-exit boundary-layer state in subsonic turbulent jets", "author": [ { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Jaunet", "given_name": "Vincent", "clpid": "Jaunet-V" }, { "family_name": "Le Rallic", "given_name": "Maxime", "clpid": "Le-Rallic-M" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Lele", "given_name": "Sanjiva K.", "clpid": "Lele-S-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" } ], "abstract": "To investigate the effects of the nozzle-exit conditions on jet flow and sound fields, large-eddy simulations of an isothermal Mach 0.9 jet issued from a convergent-straight nozzle are performed at a diameter-based Reynolds number of 1 x 10^6. The simulations feature near-wall adaptive mesh refinement, synthetic turbulence and wall modelling inside the nozzle. This leads to fully turbulent nozzle-exit boundary layers and results in significant improvements for the flow field and sound predictions compared with those obtained from the typical approach based on laminar flow in the nozzle. The far-field pressure spectra for the turbulent jet match companion experimental measurements, which use a boundary-layer trip to ensure a turbulent nozzle-exit boundary layer to within 0.5 dB for all relevant angles and frequencies. By contrast, the initially laminar jet results in greater high-frequency noise. For both initially laminar and turbulent jets, decomposition of the radiated noise into azimuthal Fourier modes is performed, and the results show similar azimuthal characteristics for the two jets. The axisymmetric mode is the dominant source of sound at the peak radiation angles and frequencies. The first three azimuthal modes recover more than 97 % of the total acoustic energy at these angles and more than 65 % (i.e. error less than 2 dB) for all angles. For the main azimuthal modes, linear stability analysis of the near-nozzle mean-velocity profiles is conducted in both jets. The analysis suggests that the differences in radiated noise between the initially laminar and turbulent jets are related to the differences in growth rate of the Kelvin\u2013Helmholtz mode in the near-nozzle region.", "doi": "10.1017/jfm.2018.476", "issn": "0022-1120", "publisher": "Journal of Fluid Mechanics", "publication": "Journal of Fluid Mechanics", "publication_date": "2018-09-25", "volume": "851", "pages": "83-124" }, { "id": "authors:ynke9-q5h47", "collection": "authors", "collection_id": "ynke9-q5h47", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20181107-131334414", "type": "article", "title": "Spectral analysis of jet turbulence and radiated sound", "author": [ { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Rigas", "given_name": "Georgios", "orcid": "0000-0001-6692-6437", "clpid": "Rigas-G" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" } ], "abstract": "Informed by LES data and resolvent analysis of the mean flow, we examine the structure of turbulence in jets in the subsonic, transonic, and supersonic regimes. Spectral (frequency-space) proper orthogonal decomposition is used to extract energy spectra and decompose the flow into energy-ranked coherent structures. We demonstrate that two distinct mechanisms, which can be distinguished by their characteristic frequency scaling and spatial support, lead to the formation of wavepackets\u2014coherent structures that are known for their acoustic importance in the aft-angle radiation of high subsonic and supersonic jets. We compare these characteristics to acoustic source features extracted from hologram sound pressure measurements in a recent publication. The evidence strongly suggests that both mechanisms are active in full-scale jets and comprise the experimentally educed sources of sound.", "doi": "10.1121/1.5067451", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2018-09", "series_number": "3", "volume": "144", "issue": "3", "pages": "Art. No. 1673" }, { "id": "authors:mwxkf-7ac87", "collection": "authors", "collection_id": "mwxkf-7ac87", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20181102-084214143", "type": "article", "title": "Resolvent analysis for jet noise source identification", "author": [ { "family_name": "Pickering", "given_name": "Ethan M.", "orcid": "0000-0002-4485-6359", "clpid": "Pickering-E-M" }, { "family_name": "Rigas", "given_name": "Georgios", "orcid": "0000-0001-6692-6437", "clpid": "Rigas-G" }, { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We use resolvent analysis and spectral proper orthogonal decomposition (SPOD) to deduce the acoustic sources for an isothermal Mach 1.5 round jet. Both physics-based resolvent analysis and data-driven SPOD (using a high-fidelity, experimentally-verified, large-eddy simulation (LES) database) provide a basis for predicting the perturbation field. Singular value decomposition of the resolvent operator based upon the LES baseflow provides optimal volumetric forcing modes, or sources, and their associated linear responses. To identify physically relevant resolvent modes, comparisons are made between the highest gain responses and the highest energy SPOD modes computed directly from LES realizations. The prevalence of the associated resolvent forcing modes in the data are then assessed by projecting them onto the full LES nonlinear terms. The resulting distributions are presented and a jet noise model leveraging these forcing statistics is discussed.", "doi": "10.1121/1.5067573", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2018-09", "series_number": "3", "volume": "144", "issue": "3", "pages": "Art. No. 1706" }, { "id": "authors:z4n4s-qge26", "collection": "authors", "collection_id": "z4n4s-qge26", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20181022-152225740", "type": "article", "title": "Modeling and numerical simulation of the bubble cloud dynamics in an ultrasound field for burst wave lithotripsy", "author": [ { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Maxwell", "given_name": "Adam D.", "clpid": "Maxwell-A-D" }, { "family_name": "Kreider", "given_name": "Wayne", "clpid": "Kreider-W" }, { "family_name": "Bailey", "given_name": "Michael R.", "clpid": "Bailey-M-R" } ], "abstract": "Modeling and numerical simulation of bubble clouds induced by intense ultrasound waves are conducted to quantify the effect of cloud cavitation on burst wave lithotripsy, a proposed non-invasive alternative to shock wave lithotripsy that uses pulses of ultrasound with an amplitude of O(1) MPa and a frequency of O(100) kHz. A unidirectional acoustic source model and an Eulerian-Lagrangian method are developed for simulation of ultrasound generation from a multi-element array transducer and cavitation bubbles, respectively. Parametric simulations of the spherical bubble cloud dynamics reveal a new scaling parameter that dictates both the structure of the bubble cloud and the amplitude of the far-field, bubble-scattered acoustics. The simulation further shows that a thin layer of bubble clouds nucleated near a kidney stone model can shield up to 90% of the incoming wave energy, indicating a potential loss of efficacy during the treatment due to cavitation. Strong correlations are identified between the far-field, bubble-scattered acoustics and the magnitude of the shielding, which could be used for ultrasound monitoring of cavitation during treatments. The simulations are validated by companion experiments in vitro.", "doi": "10.1121/1.5067866", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2018-09", "series_number": "3", "volume": "144", "issue": "3", "pages": "Art. No. 1780" }, { "id": "authors:erqg4-57g18", "collection": "authors", "collection_id": "erqg4-57g18", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180529-085928867", "type": "article", "title": "Modal decomposition of fluid-structure interaction with application to flag flapping", "author": [ { "family_name": "Goza", "given_name": "Andres", "orcid": "0000-0002-9372-7713", "clpid": "Goza-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Modal decompositions such as proper orthogonal decomposition (POD), dynamic mode decomposition (DMD) and their variants are regularly used to educe physical mechanisms of nonlinear flow phenomena that cannot be easily understood through direct inspection. In fluid\u2013structure interaction (FSI) systems, fluid motion is coupled to vibration and/or deformation of an immersed structure. Despite this coupling, data analysis is often performed using only fluid or structure variables, rather than incorporating both. This approach does not provide information about the manner in which fluid and structure modes are correlated. We present a framework for performing POD and DMD where the fluid and structure are treated together. As part of this framework, we introduce a physically meaningful norm for FSI systems. We first use this combined fluid\u2013structure formulation to identify correlated flow features and structural motions in limit-cycle flag flapping. We then investigate the transition from limit-cycle flapping to chaotic flapping, which can be initiated by increasing the flag mass. Our modal decomposition reveals that at the onset of chaos, the dominant flapping motion increases in amplitude and leads to a bluff-body wake instability. This new bluff-body mode interacts triadically with the dominant flapping motion to produce flapping at the non-integer harmonic frequencies previously reported by Connell and Yue (2007). While our formulation is presented for POD and DMD, there are natural extensions to other data-analysis techniques.", "doi": "10.1016/j.jfluidstructs.2018.06.014", "issn": "0889-9746", "publisher": "Elsevier", "publication": "Journal of Fluids and Structures", "publication_date": "2018-08", "volume": "81", "pages": "728-737" }, { "id": "authors:bpq3j-0f809", "collection": "authors", "collection_id": "bpq3j-0f809", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180627-121240173", "type": "article", "title": "Spectral proper orthogonal decomposition and its relationship to dynamic mode decomposition and resolvent analysis", "author": [ { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We consider the frequency domain form of proper orthogonal decomposition (POD), called spectral proper orthogonal decomposition (SPOD). Spectral POD is derived from a space\u2013time POD problem for statistically stationary flows and leads to modes that each oscillate at a single frequency. This form of POD goes back to the original work of Lumley (Stochastic Tools in Turbulence, Academic Press, 1970), but has been overshadowed by a space-only form of POD since the 1990s. We clarify the relationship between these two forms of POD and show that SPOD modes represent structures that evolve coherently in space and time, while space-only POD modes in general do not. We also establish a relationship between SPOD and dynamic mode decomposition (DMD); we show that SPOD modes are in fact optimally averaged DMD modes obtained from an ensemble DMD problem for stationary flows. Accordingly, SPOD modes represent structures that are dynamic in the same sense as DMD modes but also optimally account for the statistical variability of turbulent flows. Finally, we establish a connection between SPOD and resolvent analysis. The key observation is that the resolvent-mode expansion coefficients must be regarded as statistical quantities to ensure convergent approximations of the flow statistics. When the expansion coefficients are uncorrelated, we show that SPOD and resolvent modes are identical. Our theoretical results and the overall utility of SPOD are demonstrated using two example problems: the complex Ginzburg\u2013Landau equation and a turbulent jet.", "doi": "10.1017/jfm.2018.283", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2018-07-25", "volume": "847", "pages": "821-867" }, { "id": "authors:5tbgj-79030", "collection": "authors", "collection_id": "5tbgj-79030", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180711-155550672", "type": "article", "title": "Ensemble-Based State Estimator for Aerodynamic Flows", "author": [ { "family_name": "da Silva", "given_name": "Andre F. C.", "clpid": "da-Silva-A-F-C" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Regardless of the plant model, robust flow estimation based on limited measurements remains a major challenge in successful flow control applications. Aiming to combine the robustness of a high-dimensional representation of the dynamics with the cost efficiency of a low-order approximation of the state covariance matrix, a flow state estimator based on the ensemble Kalman filter is applied to two-dimensional flow past a cylinder and an airfoil at high angle of attack and low Reynolds number. For development purposes, the numerical algorithm is used as both the estimator and as a surrogate for the measurements. Estimation is successful using a reduced number of either pressure sensors on the surface of the body or sparsely placed velocity probes in the wake. Because the most relevant features of these flows are restricted to a low-dimensional manifold of the state space, asymptotic behavior of the estimator is shown to be achieved with a small ensemble size. The relative importance of each sensor location is evaluated by analyzing how they influence the estimated flowfield, and optimal locations for pressure sensors are determined. Covariance inflation is used to enhance the estimator performance in the presence of unmodeled freestream perturbations. A combination of parametric modeling and augmented state methodology is used to successfully estimate the forces on immersed bodies subjected to deterministic and random gusts.", "doi": "10.2514/1.J056743", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2018-07", "series_number": "7", "volume": "56", "issue": "7", "pages": "2568-2578" }, { "id": "authors:2be44-74404", "collection": "authors", "collection_id": "2be44-74404", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180626-102029765", "type": "article", "title": "On the lift-optimal aspect ratio of a revolving wing at low Reynolds number", "author": [ { "family_name": "Jardin", "given_name": "T.", "orcid": "0000-0001-9704-2984", "clpid": "Jardin-Thierry" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Lentink & Dickinson (2009 J. Exp. Biol. 212, 2705\u20132719. (doi:10.1242/jeb.022269)) showed that rotational acceleration stabilized the leading-edge vortex on revolving, low aspect ratio (AR) wings and hypothesized that a Rossby number of around 3, which is achieved during each half-stroke for a variety of hovering insects, seeds and birds, represents a convergent high-lift solution across a range of scales in nature. Subsequent work has verified that, in particular, the Coriolis acceleration plays a key role in LEV stabilization. Implicit in these results is that there exists an optimal AR for wings revolving about their root, because it is otherwise unclear why, apart from possible morphological reasons, the convergent solution would not occur for an even lower Rossby number. We perform direct numerical simulations of the flow past revolving wings where we vary the AR and Rossby numbers independently by displacing the wing root from the axis of rotation. We show that the optimal lift coefficient represents a compromise between competing trends with competing time scales where the coefficient of lift increases monotonically with AR, holding Rossby number constant, but decreases monotonically with Rossby number, when holding AR constant. For wings revolving about their root, this favours wings of AR between 3 and 4.", "doi": "10.1098/rsif.2017.0933", "pmcid": "PMC6030621", "issn": "1742-5689", "publisher": "Royal Society", "publication": "Journal of the Royal Society Interface", "publication_date": "2018-06", "series_number": "143", "volume": "15", "issue": "143", "pages": "Art. No. 20170933" }, { "id": "authors:scr2s-cvy59", "collection": "authors", "collection_id": "scr2s-cvy59", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180329-105253706", "type": "article", "title": "Active Control of Noise from Hot Supersonic Jets", "author": [ { "family_name": "Sinha", "given_name": "Aniruddha", "orcid": "0000-0002-7122-3549", "clpid": "Sinha-A" }, { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Schlinker", "given_name": "Robert H.", "clpid": "Schlinker-R-H" }, { "family_name": "Reba", "given_name": "Ramons", "clpid": "Reba-R" }, { "family_name": "Simonich", "given_name": "John C.", "clpid": "Simonich-J-C" }, { "family_name": "Shannon", "given_name": "Daniel W.", "clpid": "Shannon-D-W" } ], "abstract": "This paper presents diagnostic experiments aimed at understanding and mitigating supersonic jet noise from the coherent wave-packet structures that are the source of peak aft-angle mixing noise. Both isothermal and heated, nearly perfectly expanded, Mach 1.5 jets were forced in the near-nozzle region with air injection generated by a spinning-valve device designed to excite the jet at frequencies approaching those of the dominant turbulent structures. Substantial reductions in the peak aft-angle radiation were achieved with steady blowing at amplitudes corresponding to 2\u20136% of the mass flow rate of the primary jet. The noise benefit saturated at mass flow rates above 4%, with as much as a 6 dB reduction in overall sound pressure level at aft angles. Increasing the mass flow rates yielded a monotonically increasing high-frequency noise penalty at the sideline, where noise levels in the natural jet were already 15 dB lower than the aft-angle peak, so that the penalty due to actuation was minor. Although both steady and periodic unsteady mass injections were produced by the spinning valve when it rotated, it was calibrated to hold the steady mass flow rate constant as the frequency of unsteady blowing was changed. In this way, the effect of steady and unsteady blowings on the acoustic field could be decoupled. It is shown that the noise benefit was uniquely associated with the steady component of blowing, whereas the unsteady component resulted in additive tones in the spectra. This implied linearity is consistent with theory and experiments showing that the wave-packet structures, which give rise to the dominant aft-angle radiation, evolve in the turbulent mean flowfield in a nearly linear fashion from their origin in the near-nozzle region. The interpretation of noise reduction is that the steady component of blowing spreads the mean flow more rapidly, resulting in weaker wave packets. Periodic unsteady blowing forces coherent wave packets that are largely uncorrelated from the random natural ones, which then leads to the observed additive tones.", "doi": "10.2514/1.J056159", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2018-03", "series_number": "3", "volume": "56", "issue": "3", "pages": "933-948" }, { "id": "authors:trxaf-xhg87", "collection": "authors", "collection_id": "trxaf-xhg87", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20171220-135648871", "type": "article", "title": "Numerical simulation of the aerobreakup of a water droplet", "author": [ { "family_name": "Meng", "given_name": "Jomela C.", "orcid": "0000-0002-8966-2291", "clpid": "Meng-Jomela-Chen-Chen" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We present a three-dimensional numerical simulation of the aerobreakup of a spherical water droplet in the flow behind a normal shock wave. The droplet and surrounding gas flow are simulated using the compressible multicomponent Euler equations in a finite-volume scheme with shock and interface capturing. The aerobreakup process is compared with available experimental visualizations. Features of the droplet deformation and breakup in the stripping breakup regime, as well as descriptions of the surrounding gas flow, are discussed. Analyses of observed surface instabilities and a Fourier decomposition of the flow field reveal asymmetrical azimuthal modulations and broadband instability growth that result in chaotic flow within the wake region.", "doi": "10.1017/jfm.2017.804", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2018-01-25", "volume": "835", "pages": "1108-1135" }, { "id": "authors:1jcgf-xyj41", "collection": "authors", "collection_id": "1jcgf-xyj41", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170912-144135521", "type": "article", "title": "A source term approach for generation of one-way acoustic waves in the Euler and Navier\u2013Stokes equations", "author": [ { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We derive a volumetric source term for the Euler and Navier\u2013Stokes equations that mimics the generation of unidirectional acoustic waves from an arbitrary smooth surface in three-dimensional space. The model is constructed as a linear combination of monopole and dipole sources in the mass, momentum, and energy equations. The singular source distribution on the surface is regularized on a computational grid by convolution with a smeared Dirac delta function. The source is implemented in the Euler equations using a Cartesian-grid finite-volume WENO scheme, and validated by comparing with analytical solution for unidirectional planar and spherical acoustic waves. Using the scheme, we emulate a spherical piezoelectric transducer and a multi-array transducer to simulate focused ultrasound fields in water. The simulated ultrasound fields show favorable agreement with previous experiments.", "doi": "10.1016/j.wavemoti.2017.08.004", "pmcid": "PMC6159925", "issn": "0165-2125", "publisher": "Elsevier", "publication": "Wave Motion", "publication_date": "2017-12", "volume": "75", "pages": "36-49" }, { "id": "authors:x9qxf-tja52", "collection": "authors", "collection_id": "x9qxf-tja52", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20171201-152324924", "type": "article", "title": "Experimental study of turbulent-jet wave packets and their acoustic efficiency", "author": [ { "family_name": "Breakey", "given_name": "David E. S.", "clpid": "Breakey-D-E-S" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Nogueira", "given_name": "Petr\u00f4nio A.", "clpid": "Nogueira-P-A" }, { "family_name": "L\u00e9on", "given_name": "Olivier", "clpid": "L\u00e9on-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Rodr\u00edguez", "given_name": "Daniel", "clpid": "Rodr\u00edguez-D" } ], "abstract": "This paper details the statistical and time-resolved analysis of the relationship between the near-field pressure fluctuations of unforced, subsonic free jets (0.4 \u2264 M \u2264 0.6) and their far-field sound emissions. Near-field and far-field microphone measurements were taken on a conical array close to the jets and an azimuthal ring at 20\u2218 to the jet axis, respectively. Recent velocity and pressure measurements indicate the presence of linear wave packets in the near field by closely matching predictions from the linear homogenous parabolized stability equations, but the agreement breaks down both beyond the end of the potential core and when considering higher order statistical moments, such as the two-point coherence. Proper orthogonal decomposition (POD), interpreted in terms of inhomogeneous linear models using the resolvent framework allows us to understand these discrepancies. A new technique is developed for projecting time-domain pressure measurements onto a statistically obtained POD basis, yielding the time-resolved activity of each POD mode and its correlation with the far field. A single POD mode, interpreted as an optimal high-gain structure that arises due to turbulent forcing, captures the salient near-field\u2013far-field correlation signature; further, the signatures of the next two modes, understood as suboptimally forced structures, suggest that these POD modes represent higher order, acoustically important near-field behavior. An existing Green's-function-based technique is used to make far-field predictions, and results are interpreted in terms of POD/resolvent modes, indicating the acoustic importance of this higher order behavior. The technique is extended to provide time-domain far-field predictions.", "doi": "10.1103/PhysRevFluids.2.124601", "issn": "2469-990X", "publisher": "American Physical Society", "publication": "Physical Review Fluids", "publication_date": "2017-12", "series_number": "12", "volume": "2", "issue": "12", "pages": "Art. No. 124601" }, { "id": "authors:xs8a9-qk853", "collection": "authors", "collection_id": "xs8a9-qk853", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20171006-075054128", "type": "article", "title": "Special issue on global flow instability and control", "author": [ { "family_name": "Sharma", "given_name": "Ati", "orcid": "0000-0002-7170-1627", "clpid": "Sharma-A-S" }, { "family_name": "Theofilis", "given_name": "Vassilis", "clpid": "Theofilis-V" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "This special issue is the second on the topic of \"Global Flow Instability and Control,\" following the first in 2011. As with the previous special issue, the participants of the last two symposia on Global Flow Instability and Control, held in Crete, Greece, were invited to submit publications. These papers were peer reviewed according to the standards of the journal, and this issue represents a snapshot of the progress since 2011. In this preface, a sampling of important developments in the field since the first issue is discussed. A synopsis of the papers in this issue is given in that context.", "doi": "10.1007/s00162-017-0444-y", "issn": "0935-4964", "publisher": "Springer", "publication": "Theoretical and Computational Fluid Dynamics", "publication_date": "2017-12", "series_number": "5-6", "volume": "31", "issue": "5-6", "pages": "471-474" }, { "id": "authors:q7fn7-mdy41", "collection": "authors", "collection_id": "q7fn7-mdy41", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20171213-075938896", "type": "article", "title": "Modal Analysis of Fluid Flows: An Overview", "author": [ { "family_name": "Taira", "given_name": "Kunihiko", "orcid": "0000-0002-3762-8075", "clpid": "Taira-Kunihiko" }, { "family_name": "Brunton", "given_name": "Steven L.", "clpid": "Brunton-S-L" }, { "family_name": "Dawson", "given_name": "Scott T. M.", "orcid": "0000-0002-0020-2097", "clpid": "Dawson-S-T-M" }, { "family_name": "Rowley", "given_name": "Clarence W.", "orcid": "0000-0002-9099-5739", "clpid": "Rowley-C-W" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Gordeyev", "given_name": "Stanislav", "clpid": "Gordeyev-S" }, { "family_name": "Theofilis", "given_name": "Vassilios", "clpid": "Theofilis-V" }, { "family_name": "Ukeiley", "given_name": "Lawrence S.", "clpid": "Ukeiley-L-S" } ], "abstract": "Simple aerodynamic configurations under even modest conditions can exhibit complex flows with a wide range of temporal and spatial features. It has become common practice in the analysis of these flows to look for and extract physically important features, or modes, as a first step in the analysis. This step typically starts with a modal decomposition of an experimental or numerical dataset of the flowfield, or of an operator relevant to the system. We describe herein some of the dominant techniques for accomplishing these modal decompositions and analyses that have seen a surge of activity in recent decades [1\u20138]. For a nonexpert, keeping track of recent developments can be daunting, and the intent of this document is to provide an introduction to modal analysis that is accessible to the larger fluid dynamics community. In particular, we present a brief overview of several of the well-established techniques and clearly lay the framework of these methods using familiar linear algebra. The modal analysis techniques covered in this paper include the proper orthogonal decomposition (POD), balanced proper orthogonal decomposition (balanced POD), dynamic mode decomposition (DMD), Koopman analysis, global linear stability analysis, and resolvent analysis.", "doi": "10.2514/1.J056060", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2017-12", "series_number": "12", "volume": "55", "issue": "12", "pages": "4013-4041" }, { "id": "authors:sp6dy-37s98", "collection": "authors", "collection_id": "sp6dy-37s98", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20171020-092303673", "type": "article", "title": "High-frequency wavepackets in turbulent jets", "author": [ { "family_name": "Sasaki", "given_name": "Kenzo", "orcid": "0000-0002-3347-4996", "clpid": "Sasaki-Kenzo" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" } ], "abstract": "Wavepackets obtained as solutions of the flow equations linearised around the mean flow have been shown in recent work to yield good agreement, in terms of amplitude and phase, with those educed from turbulent jets. Compelling agreement has been demonstrated, for the axisymmetric and first helical mode, up to Strouhal numbers close to unity. We here extend the range of validity of wavepacket models to Strouhal number St = 4.0 and azimuthal wavenumber m = 4 by comparing solutions of the parabolised stability equations with a well-validated large-eddy simulation of a Mach 0.9 turbulent jet. The results show that the near-nozzle dynamics can be correctly described by the homogeneous linear model, the initial growth rates being accurately predicted for the entire range of frequencies and azimuthal wavenumbers considered. Similarly to the lower-frequency wavepackets reported prior to this work, the high-frequency linear waves deviate from the data downstream of their stabilisation locations, which move progressively upstream as the frequency increases.", "doi": "10.1017/jfm.2017.659", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2017-11-10", "volume": "830", "pages": "Art. No. R2" }, { "id": "authors:hkfxm-jrc87", "collection": "authors", "collection_id": "hkfxm-jrc87", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170818-081556658", "type": "article", "title": "Acoustic resonance in the potential core of subsonic jets", "author": [ { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Schmidt", "given_name": "Oliver", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Jaunet", "given_name": "Vincent", "clpid": "Jaunet-V" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" } ], "abstract": "The purpose of this paper is to characterize and model waves that are observed within the potential core of subsonic jets and relate them to previously observed tones in the near-nozzle region. The waves are detected in data from a large-eddy simulation of a Mach 0.9 isothermal jet and modelled using parallel and weakly non-parallel linear modal analysis of the Euler equations linearized about the turbulent mean flow, as well as simplified models based on a cylindrical vortex sheet and the acoustic modes of a cylindrical soft duct. In addition to the Kelvin\u2013Helmholtz instability waves, three types of waves with negative phase velocities are identified in the potential core: upstream- and downstream-propagating duct-like acoustic modes that experience the shear layer as a pressure-release surface and are therefore radially confined to the potential core, and upstream-propagating acoustic modes that represent a weak coupling between the jet core and the free stream. The slow streamwise contraction of the potential core imposes a frequency-dependent end condition on the waves that is modelled as the turning points of a weakly non-parallel approximation of the waves. These turning points provide a mechanism by which the upstream- and downstream-travelling waves can interact and exchange energy through reflection and transmission processes. Paired with a second end condition provided by the nozzle, this leads to the possibility of resonance in limited frequency bands that are bound by two saddle points in the complex wavenumber plane. The predicted frequencies closely match the observed tones detected outside of the jet. The vortex-sheet model is then used to systematically explore the Mach number and temperature ratio dependence of the phenomenon. For isothermal jets, the model suggests that resonance is likely to occur in a narrow range of Mach number, 0.82 < M < 1.", "doi": "10.1017/jfm.2017.346", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2017-08-25", "volume": "825", "pages": "1113-1152" }, { "id": "authors:drycm-54814", "collection": "authors", "collection_id": "drycm-54814", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170818-083639884", "type": "article", "title": "Wavepackets and trapped acoustic modes in a turbulent jet: coherent structure eduction and global stability", "author": [ { "family_name": "Schmidt", "given_name": "Oliver T.", "orcid": "0000-0002-7097-0235", "clpid": "Schmidt-O-T" }, { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" } ], "abstract": "Coherent features of a turbulent Mach 0.9, Reynolds number 10^6 jet are educed from a high-fidelity large eddy simulation. Besides the well-known Kelvin\u2013Helmholtz instabilities of the shear layer, a new class of trapped acoustic waves is identified in the potential core. A global linear stability analysis based on the turbulent mean flow is conducted. The trapped acoustic waves form branches of discrete eigenvalues in the global spectrum, and the corresponding global modes accurately match the educed structures. Discrete trapped acoustic modes occur in a hierarchy determined by their radial and axial order. A local dispersion relation is constructed from the global modes and found to agree favourably with an empirical dispersion relation educed from the simulation data. The product between direct and adjoint modes is then used to isolate the trapped waves. Under certain conditions, resonance in the form of a beating occurs between trapped acoustic waves of positive and negative group velocities. This resonance explains why the trapped modes are prominently observed in the simulation and as tones in previous experimental studies. In the past, these tones were attributed to external factors. Here, we show that they are an intrinsic feature of high-subsonic jets that can be unambiguously identified by a global linear stability analysis.", "doi": "10.1017/jfm.2017.407", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2017-08-25", "volume": "825", "pages": "1153-1181" }, { "id": "authors:kd90d-fwg72", "collection": "authors", "collection_id": "kd90d-fwg72", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20161114-103855929", "type": "article", "title": "A strongly-coupled immersed-boundary formulation for thin elastic structures", "author": [ { "family_name": "Goza", "given_name": "Andres", "orcid": "0000-0002-9372-7713", "clpid": "Goza-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We present a strongly-coupled immersed-boundary method for flow\u2013structure interaction problems involving thin deforming bodies. The method is stable for arbitrary choices of solid-to-fluid mass ratios and for large body motions. As with many strongly-coupled immersed-boundary methods, our method requires the solution of a nonlinear algebraic system at each time step. The system is solved through iteration, where the iterates are obtained by linearizing the system and performing a block-LU factorization. This restricts all iterations to small-dimensional subsystems that scale with the number of discretization points on the immersed surface, rather than on the entire flow domain. Moreover, the iteration procedure we propose does not involve heuristic regularization parameters, and has converged in a small number of iterations for all problems we have considered. We derive our method for general deforming surfaces, and verify the method with two-dimensional test problems of geometrically nonlinear flags undergoing large amplitude flapping behavior.", "doi": "10.1016/j.jcp.2017.02.027", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2017-05-01", "volume": "336", "pages": "401-411" }, { "id": "authors:9q6es-f5z32", "collection": "authors", "collection_id": "9q6es-f5z32", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20161219-080814106", "type": "article", "title": "A fast immersed boundary method for external incompressible viscous flows using lattice Green's functions", "author": [ { "family_name": "Liska", "given_name": "Sebastian", "orcid": "0000-0003-4139-9364", "clpid": "Liska-S" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A new parallel, computationally efficient immersed boundary method for solving three-dimensional, viscous, incompressible flows on unbounded domains is presented. Immersed surfaces with prescribed motions are generated using the interpolation and regularization operators obtained from the discrete delta function approach of the original (Peskin's) immersed boundary method. Unlike Peskin's method, boundary forces are regarded as Lagrange multipliers that are used to satisfy the no-slip condition. The incompressible Navier\u2013Stokes equations are discretized on an unbounded staggered Cartesian grid and are solved in a finite number of operations using lattice Green's function techniques. These techniques are used to automatically enforce the natural free-space boundary conditions and to implement a novel block-wise adaptive grid that significantly reduces the run-time cost of solutions by limiting operations to grid cells in the immediate vicinity and near-wake region of the immersed surface. These techniques also enable the construction of practical discrete viscous integrating factors that are used in combination with specialized half-explicit Runge\u2013Kutta schemes to accurately and efficiently solve the differential algebraic equations describing the discrete momentum equation, incompressibility constraint, and no-slip constraint. Linear systems of equations resulting from the time integration scheme are efficiently solved using an approximation-free nested projection technique. The algebraic properties of the discrete operators are used to reduce projection steps to simple discrete elliptic problems, e.g. discrete Poisson problems, that are compatible with recent parallel fast multipole methods for difference equations. Numerical experiments on low-aspect-ratio flat plates and spheres at Reynolds numbers up to 3700 are used to verify the accuracy and physical fidelity of the formulation.", "doi": "10.1016/j.jcp.2016.11.034", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2017-02-15", "volume": "331", "pages": "257-279" }, { "id": "authors:hyvtg-k9w41", "collection": "authors", "collection_id": "hyvtg-k9w41", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170407-133420635", "type": "article", "title": "Transition to bluff-body dynamics in the wake of vertical-axis wind turbines", "author": [ { "family_name": "Araya", "given_name": "Daniel B.", "clpid": "Arayba-D-B" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "abstract": "We present experimental data to demonstrate that the far wake of a vertical-axis wind turbine (VAWT) exhibits features that are quantitatively similar to that of a circular cylinder with the same aspect ratio. For a fixed Reynolds number (Re \u2248 0.8 \u00d7 10^5) and variable tip-speed ratio, two-dimensional particle image velocimetry (PIV) is used to measure the velocity field in the wake of four different laboratory-scale models: a 2-bladed, 3-bladed and 5-bladed VAWT, as well as a circular cylinder. With these measurements, we use spectral analysis and proper orthogonal decomposition (POD) to evaluate statistics of the velocity field and investigate the large-scale coherent motions of the wake. In all cases, we observe three distinct regions in the VAWT wake: (i) the near wake, where periodic blade vortex shedding dominates; (ii) a transition region, where growth of a shear-layer instability occurs; (iii) the far wake, where bluff-body wake oscillations dominate. We define a dynamic solidity parameter, \u03c3_D , that relates the characteristic scales of the flow to the streamwise transition location in the wake. In general, we find that increasing \u03c3_D leads to an earlier transition, a greater initial velocity deficit and a faster rate of recovery in the wake. We propose a coordinate transformation using \u03c3_D in which the minimum velocity recovery profiles of the VAWT wake closely match that of the cylinder wake. The results have implications for manipulating VAWT wake recovery within a wind farm.", "doi": "10.1017/jfm.2016.862", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2017-02", "volume": "813", "pages": "346-381" }, { "id": "authors:th2xh-mbw54", "collection": "authors", "collection_id": "th2xh-mbw54", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20160714-085129783", "type": "article", "title": "Accurate computation of surface stresses and forces with immersed boundary methods", "author": [ { "family_name": "Goza", "given_name": "Andres", "orcid": "0000-0002-9372-7713", "clpid": "Goza-A" }, { "family_name": "Liska", "given_name": "Sebastian", "orcid": "0000-0003-4139-9364", "clpid": "Liska-S" }, { "family_name": "Morley", "given_name": "Benjamin", "clpid": "Morley-B" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Many immersed boundary methods solve for surface stresses that impose the velocity boundary conditions on an immersed body. These surface stresses may contain spurious oscillations that make them ill-suited for representing the physical surface stresses on the body. Moreover, these inaccurate stresses often lead to unphysical oscillations in the history of integrated surface forces such as the coefficient of lift. While the errors in the surface stresses and forces do not necessarily affect the convergence of the velocity field, it is desirable, especially in fluid\u2013structure interaction problems, to obtain smooth and convergent stress distributions on the surface. To this end, we show that the equation for the surface stresses is an integral equation of the first kind whose ill-posedness is the source of spurious oscillations in the stresses. We also demonstrate that for sufficiently smooth delta functions, the oscillations may be filtered out to obtain physically accurate surface stresses. The filtering is applied as a post-processing procedure, so that the convergence of the velocity field is unaffected. We demonstrate the efficacy of the method by computing stresses and forces that converge to the physical stresses and forces for several test problems.", "doi": "10.1016/j.jcp.2016.06.014", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2016-09-15", "volume": "321", "pages": "860-873" }, { "id": "authors:jqbj2-cwh81", "collection": "authors", "collection_id": "jqbj2-cwh81", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20160523-085237573", "type": "article", "title": "A fast lattice Green's function method for solving viscous incompressible flows on unbounded domains", "author": [ { "family_name": "Liska", "given_name": "Sebastian", "orcid": "0000-0003-4139-9364", "clpid": "Liska-S" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A computationally efficient method for solving three-dimensional, viscous, incompressible flows on unbounded domains is presented. The method formally discretizes the incompressible Navier\u2013Stokes equations on an unbounded staggered Cartesian grid. Operations are limited to a finite computational domain through a lattice Green's function technique. This technique obtains solutions to inhomogeneous difference equations through the discrete convolution of source terms with the fundamental solutions of the discrete operators. The differential algebraic equations describing the temporal evolution of the discrete momentum equation and incompressibility constraint are numerically solved by combining an integrating factor technique for the viscous term and a half-explicit Runge\u2013Kutta scheme for the convective term. A projection method that exploits the mimetic and commutativity properties of the discrete operators is used to efficiently solve the system of equations that arises in each stage of the time integration scheme. Linear complexity, fast computation rates, and parallel scalability are achieved using recently developed fast multipole methods for difference equations. The accuracy and physical fidelity of solutions are verified through numerical simulations of vortex rings.", "doi": "10.1016/j.jcp.2016.04.023", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2016-07-01", "volume": "316", "pages": "360-384" }, { "id": "authors:7hbsa-t5v43", "collection": "authors", "collection_id": "7hbsa-t5v43", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20160204-153835203", "type": "article", "title": "Parabolized stability analysis of jets from serrated nozzles", "author": [ { "family_name": "Sinha", "given_name": "Aniruddha", "orcid": "0000-0002-7122-3549", "clpid": "Sinha-A" }, { "family_name": "Gudmundsson", "given_name": "Kristj\u00e1n", "clpid": "Gudmundsson-K" }, { "family_name": "Xia", "given_name": "Hao", "clpid": "Xia-Hao" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We study the viscous spatial linear stability characteristics of the time-averaged flow in turbulent subsonic jets issuing from serrated (chevroned) nozzles, and compare them to analogous round jet results. Linear parabolized stability equations (PSE) are used in the calculations to account for the non-parallel base flow. By exploiting the symmetries of the mean flow due to the regular arrangement of serrations, we obtain a series of coupled two-dimensional PSE problems from the original three-dimensional problem. This reduces the solution cost and manifests the symmetries of the stability modes. In the parallel-flow linear stability theory (LST) calculations that are performed near the nozzle to initiate the PSE, we find that the serrated nozzle reduces the growth rates of the most unstable eigenmodes of the jet, but their phase speeds are approximately similar. We obtain encouraging validation of our linear PSE instability wave results vis-\u00e0-vis near-field hydrodynamic pressure data acquired on a phased microphone array in experiments, after filtering the latter with proper orthogonal decomposition (POD) to extract the energetically dominant coherent part. Additionally, a large-eddy simulation database of the same serrated jet is investigated, and its POD-filtered pressure field is found to compare favourably with the corresponding PSE solution within the jet plume. We conclude that the coherent hydrodynamic pressure fluctuations of jets from both round and serrated nozzles are reasonably consistent with the linear instability modes of the turbulent mean flow.", "doi": "10.1017/jfm.2015.719", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2016-02", "volume": "789", "pages": "36-63" }, { "id": "authors:sz6a3-fza02", "collection": "authors", "collection_id": "sz6a3-fza02", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20160303-101150959", "type": "article", "title": "Coriolis Effect on Dynamic Stall in a Vertical Axis Wind Turbine", "author": [ { "family_name": "Tsai", "given_name": "Hsieh-Chen", "clpid": "Tsai-Hsieh-Chen" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The immersed boundary method is used to simulate the flow around a two-dimensional cross section of a rotating NACA 0018 airfoil in order to investigate the dynamic stall occurring on a vertical axis wind turbine. The influence of dynamic stall on the force is characterized as a function of tip-speed ratio and Rossby number. The influence of the Coriolis effect is isolated by comparing the rotating airfoil to one undergoing an equivalent planar motion that is composed of surging and pitching motions that produce an equivalent speed and angle-of-attack variation over the cycle. Planar motions consisting of sinusoidally varying pitch and surge are also examined. At lower tip-speed ratios, the Coriolis force leads to the capture of a vortex pair when the angle of attack of a rotating airfoil begins to decrease in the upwind half cycle. This wake-capturing phenomenon leads to a significant decrease in lift during the downstroke phase. The appearance of this feature depends subtly on the tip-speed ratio. On the one hand, it is strengthened due to the intensifying Coriolis force, but on the other hand, it is attenuated because of the comitant decrease in angle of attack. While the present results are restricted to two-dimensional flow at low Reynolds numbers, they compare favorably with experimental observations at much higher Reynolds numbers. Moreover, the wake-capturing is observed only when the combination of surging, pitching, and Coriolis force is present.", "doi": "10.2514/1.J054199", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2016-01", "series_number": "1", "volume": "54", "issue": "1", "pages": "216-226" }, { "id": "authors:1d0vq-sb702", "collection": "authors", "collection_id": "1d0vq-sb702", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20160211-085648396", "type": "article", "title": "Modeling and experimental analysis of acoustic cavitation bubbles for Burst Wave Lithotripsy", "author": [ { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" }, { "family_name": "Kreider", "given_name": "Wayne", "clpid": "Kreider-Wayne" }, { "family_name": "Maxwell", "given_name": "Adam", "clpid": "Maxwell-Adam-D" }, { "family_name": "Cunitz", "given_name": "Bryan", "clpid": "Cunitz-Bryan" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Bailey", "given_name": "Michael", "clpid": "Bailey-Michael-R" } ], "abstract": "Cavitation bubbles initiated by focused ultrasound waves are investigated through experiments and modeling. Pulses of focused ultrasound with a frequency of 335 kHz and a peak negative pressure of 8 MPa is generated in a water tank by a piezoelectric transducer to initiate cavitation. The pressure field is modeled by solving the Euler equations and used to simulate single bubble oscillation. The characteristics of cavitation bubbles observed by highspeed photography qualitatively agree with the simulation results. Finally, bubble clouds are captured using acoustic B-mode imaging that works synchronized with high-speed photography.", "doi": "10.1088/1742-6596/656/1/012027", "pmcid": "PMC4831575", "issn": "1742-6596", "publisher": "IOP", "publication": "Journal of Physics: Conference Series", "publication_date": "2015-12-03", "volume": "656", "pages": "Art. No. 012027" }, { "id": "authors:2nkh7-j6r42", "collection": "authors", "collection_id": "2nkh7-j6r42", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151015-074155757", "type": "article", "title": "One-way spatial integration of hyperbolic equations", "author": [ { "family_name": "Towne", "given_name": "Aaron", "orcid": "0000-0002-7315-5375", "clpid": "Towne-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "In this paper, we develop and demonstrate a method for constructing well-posed one-way approximations of linear hyperbolic systems. We use a semi-discrete approach that allows the method to be applied to a wider class of problems than existing methods based on analytical factorization of idealized dispersion relations. After establishing the existence of an exact one-way equation for systems whose coefficients do not vary along the axis of integration, efficient approximations of the one-way operator are constructed by generalizing techniques previously used to create nonreflecting boundary conditions. When physically justified, the method can be applied to systems with slowly varying coefficients in the direction of integration. To demonstrate the accuracy and computational efficiency of the approach, the method is applied to model problems in acoustics and fluid dynamics via the linearized Euler equations; in particular we consider the scattering of sound waves from a vortex and the evolution of hydrodynamic wavepackets in a spatially evolving jet. The latter problem shows the potential of the method to offer a systematic, convergent alternative to ad hoc regularizations such as the parabolized stability equations.", "doi": "10.1016/j.jcp.2015.08.015", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2015-11-01", "volume": "300", "pages": "844-861" }, { "id": "authors:efkfh-atp20", "collection": "authors", "collection_id": "efkfh-atp20", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151027-114411683", "type": "article", "title": "Piezoelectric Energy Harvesting in Internal Fluid Flow", "author": [ { "family_name": "Lee", "given_name": "Hyeong Jae", "clpid": "Lee-Hyeong-Jae" }, { "family_name": "Sherrit", "given_name": "Stewart", "orcid": "0000-0003-0656-4889", "clpid": "Sherrit-Stewart" }, { "family_name": "Tosi", "given_name": "Luis Phillipe", "orcid": "0000-0002-0819-4765", "clpid": "Tosi-L-P" }, { "family_name": "Walkemeyer", "given_name": "Phillip", "clpid": "Walkemeyer-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We consider piezoelectric flow energy harvesting in an internal flow environment with the ultimate goal powering systems such as sensors in deep oil well applications. Fluid motion is coupled to structural vibration via a cantilever beam placed in a converging-diverging flow channel. Two designs were considered for the electromechanical coupling: first; the cantilever itself is a piezoelectric bimorph; second; the cantilever is mounted on a pair of flextensional actuators. We experimentally investigated varying the geometry of the flow passage and the flow rate. Experimental results revealed that the power generated from both designs was similar; producing as much as 20 mW at a flow rate of 20 L/min. The bimorph designs were prone to failure at the extremes of flow rates tested. Finite element analysis (FEA) showed fatigue failure was imminent due to stress concentrations near the bimorph's clamped region; and that robustness could be improved with a stepped-joint mounting design. A similar FEA model showed the flextensional-based harvester had a resonant frequency of around 375 Hz and an electromechanical coupling of 0.23 between the cantilever and flextensional actuators in a vacuum. These values; along with the power levels demonstrated; are significant steps toward building a system design that can eventually deliver power in the Watts range to devices down within a well.", "doi": "10.3390/s151026039", "issn": "1424-8220", "publisher": "MDPI", "publication": "Sensors", "publication_date": "2015-10", "series_number": "10", "volume": "15", "issue": "10", "pages": "26039-26062" }, { "id": "authors:tkjgp-7m230", "collection": "authors", "collection_id": "tkjgp-7m230", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150814-110653004", "type": "article", "title": "Spatial Stability Analysis of Subsonic Jets Modified for Low-Frequency Noise Reduction", "author": [ { "family_name": "Uzun", "given_name": "Ali", "clpid": "Uzun-A" }, { "family_name": "Alvi", "given_name": "Farrukh S.", "clpid": "Alvi-F-S" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hussaini", "given_name": "M. Yousuff", "clpid": "Hussaini-M-Y" } ], "abstract": "This study performs a spatial stability analysis of several jets that have previously been investigated in terms of their noise radiation. The cases include a round jet and two chevron nozzle jets with varying penetration angle. The instability wave evolution in the near-nozzle region is examined to seek for clues as to how and why the mean flow azimuthal inhomogeneity introduced by chevrons modifies the low-frequency noise component. A biglobal stability analysis is performed to determine the most unstable modes on an initial plane. The downstream evolution of the most unstable modes is then computed via three-dimensional parabolized stability equations. The azimuthal mean flow inhomogeneity introduced by chevrons is found to modify instability wave growth rates and phase speeds. Findings indicate that the near-field hydrodynamic pressure oscillations of round jet instability modes are suppressed by chevron jets. For the same modal excitation amplitude at the inlet, the two chevron jets generate considerably lower pressure fluctuations than the round jet. It is also shown that the chevron jet with the lowest hydrodynamic pressure fluctuation levels is the jet with the lowest far-field low-frequency noise output among the three jets.", "doi": "10.2514/1.J053719", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2015-08", "series_number": "8", "volume": "53", "issue": "8", "pages": "2335-2358" }, { "id": "authors:vra8x-6vm60", "collection": "authors", "collection_id": "vra8x-6vm60", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150924-105657541", "type": "article", "title": "Optimal control of circular cylinder wakes using long control horizons", "author": [ { "family_name": "Flinois", "given_name": "Thibault L. B.", "clpid": "Flinois-T-L-B" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The classical problem of suppressing vortex shedding in the wake of a circular cylinder by using body rotation is revisited in an adjoint-based optimal control framework. The cylinder's unsteady and fully unconstrained rotation rate is optimized at Reynolds numbers between 75 and 200 and over horizons that are longer than in previous studies, where they are typically of the order of a vortex shedding period or shorter. In the best configuration, the drag is reduced by 19%, the vortex shedding is effectively suppressed, and this low drag state is maintained with minimal cylinder rotation after transients. Unlike open-loop control, the optimal control is shown to maintain a specific phase relationship between the actuation and the shedding in order to stabilize the wake. A comparison is also given between the performance of optimizations for different Reynolds numbers, cost functions, and horizon lengths. It is shown that the long horizons used are necessary in order to stabilize the vortex shedding efficiently.", "doi": "10.1063/1.4928896", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2015-08", "series_number": "8", "volume": "27", "issue": "8", "pages": "Art. No. 087105" }, { "id": "authors:7md42-hnw11", "collection": "authors", "collection_id": "7md42-hnw11", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150702-123336452", "type": "article", "title": "Numerical simulations of the early stages of high-speed droplet breakup", "author": [ { "family_name": "Meng", "given_name": "J. C.", "orcid": "0000-0002-8966-2291", "clpid": "Meng-Jomela-Chen-Chen" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Experiments reported in the literature are reproduced\nusing numerical simulations to investigate the early\nstages of the breakup of water cylinders in the flow behind\nnormal shocks. Qualitative features of breakup observed in\nthe numerical results, such as the initial streamwise flattening of the cylinder and the formation of tips at its periphery, support previous experimental observations of stripping breakup. Additionally, the presence of a transitory recirculation region at the cylinder's equator and a persistent upstream jet in the wake is noted and discussed. Within the uncertainties inherent to the different methods used to extract measurements from experimental and numerical results, comparisons with experimental data of various cylinder deformation metrics show good agreement. To study the effects of the transition between subsonic and supersonic post-shock flow, we extend the range of incident shock Mach numbers beyond those investigated by the experiments. Supersonic post-shock flow velocities are not observed to significantly alter the cylinder's behavior, i.e., we are able to effectively collapse the drift, acceleration, and drag curves for all simulated shock Mach numbers. Using a new method that minimizes noise errors, the cylinder's acceleration is calculated; acceleration curves for all shock Mach numbers are subsequently collapsed by scaling with the pressure ratio across the incident shock. Furthermore, we find that accounting for the cylinder's deformed diameter in the calculation of its unsteady drag coefficient allows the drag coefficient to be approximated as a constant over the initial breakup period.", "doi": "10.1007/s00193-014-0546-z", "issn": "0938-1287", "publisher": "Springer", "publication": "Shock Waves", "publication_date": "2015-07", "series_number": "4", "volume": "25", "issue": "4", "pages": "399-414" }, { "id": "authors:wb377-d7a14", "collection": "authors", "collection_id": "wb377-d7a14", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150420-134149863", "type": "article", "title": "Surging and plunging oscillations of an airfoil at low Reynolds number", "author": [ { "family_name": "Choi", "given_name": "Jeesoon", "clpid": "Choi-Jeesoon" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Williams", "given_name": "David R.", "clpid": "Williams-D-R" } ], "abstract": "We investigate the forces and unsteady flow structures associated with harmonic oscillations of an airfoil in the streamwise (surging) and transverse (plunging) directions in two-dimensional simulations at low Reynolds number. For the surging case, we show that there are specific frequencies where the wake instability synchronizes with the unsteady motion of the airfoil, leading to significant changes in the mean forces. Resonant behaviour of the time-averaged forces is observed near the vortex shedding frequency and its subharmonic; the behaviour is reminiscent of the dynamics of the generic nonlinear oscillator known as the Arnol'd tongue or the resonance horn. Below the wake instability frequency, there are two regimes where the fluctuating forces are amplified and attenuated, respectively. A detailed study of the flow structures associated with leading-edge vortex (LEV) growth and detachment are used to relate this behaviour with the LEV acting either in phase with the quasi-steady component of the forces for the amplification case, or out of phase for the attenuation case. Comparisons with wind tunnel measurements show that phenomenologically similar dynamics occur at higher Reynolds number. Finally, we show that qualitatively similar phenomena occur during both surging and plunging.", "doi": "10.1017/jfm.2014.674", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2015-01", "volume": "763", "pages": "237-253" }, { "id": "authors:hbpzk-j9561", "collection": "authors", "collection_id": "hbpzk-j9561", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150217-092612513", "type": "article", "title": "A study of linear wavepacket models for subsonic turbulent jets using local eigenmode decomposition of PIV data", "author": [ { "family_name": "Rodr\u00edguez", "given_name": "Daniel", "clpid": "Rodr\u00edguez-D" }, { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" } ], "abstract": "Locally-parallel linear stability theory (LST) of jet velocity profiles is revisited to study the evolution of the wavepackets and the manner in which the parabolized stability equations (PSE) approach models them. An adjoint-based eigenmode decomposition technique is used to project cross-sectional velocity profiles measured using time-resolved particle image velocimetry (PIV) on the different families of eigenmodes present in the LST eigenspectrum. Attention is focused on the evolution of the Kelvin\u2013Helmholtz (K\u2013H) eigenmode and the projection of experimental fluctuations on it, since in subsonic jets the inflectional K\u2013H instability is the only possible mechanism for linear amplification of the large-scale fluctuations, and governs the wavepacket evolution. Comparisons of the fluctuations extracted by projection onto K\u2013H eigenmode with PSE solutions and PIV measurements are made. We show that the jet can be divided into three main regions, classified with respect to the LST eigenspectrum. Near the jet exit, there is significant amplification of the K\u2013H mode; the PSE solution is shown to comprise almost exclusively the K\u2013H mode, and the agreement with experiments shows that the evolution of this mode dominates the near-nozzle fluctuations. For downstream positions, the Kelvin\u2013Helmholtz mode becomes stable and eventually merges with other branches of the eigenspectrum. The comparison between PSE, experiment and the projection onto the K\u2013H mode for downstream positions suggests that the mechanism of saturation and decay of wavepackets is related to a combination of several marginally stable modes, which is reasonably well modeled by linear PSE, but cannot be obtained in the usual application of locally-parallel stability dealing exclusively with the K\u2013H mode. In addition, the projection of empirical data on the K\u2013H eigenmode at a near-nozzle cross-section is shown to be a well-founded method for the determination of the amplitudes of the linear wavepacket models.", "doi": "10.1016/j.euromechflu.2014.03.004", "issn": "0997-7546", "publisher": "Elsevier", "publication": "European Journal of Mechanics B/Fluids", "publication_date": "2015-01", "volume": "49", "pages": "308-321" }, { "id": "authors:j86f3-86381", "collection": "authors", "collection_id": "j86f3-86381", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20141030-085927201", "type": "article", "title": "A parallel fast multipole method for elliptic difference equations", "author": [ { "family_name": "Liska", "given_name": "Sebastian", "orcid": "0000-0003-4139-9364", "clpid": "Liska-S" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A new fast multipole formulation for solving elliptic difference equations on unbounded domains and its parallel implementation are presented. These difference equations can arise directly in the description of physical systems, e.g. crystal structures, or indirectly through the discretization of PDEs. In the analog to solving continuous inhomogeneous differential equations using Green's functions, the proposed method uses the fundamental solution of the discrete operator on an infinite grid, or lattice Green's function. Fast solutions O(N)O(N) are achieved by using a kernel-independent interpolation-based fast multipole method. Unlike other fast multipole algorithms, our approach exploits the regularity of the underlying Cartesian grid and the efficiency of FFTs to reduce the computation time. Our parallel implementation allows communications and computations to be overlapped and requires minimal global synchronization. The accuracy, efficiency, and parallel performance of the method are demonstrated through numerical experiments on the discrete 3D Poisson equation.", "doi": "10.1016/j.jcp.2014.07.048", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2014-12-01", "volume": "278", "pages": "76-91" }, { "id": "authors:hmn69-p2t05", "collection": "authors", "collection_id": "hmn69-p2t05", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150108-133817483", "type": "article", "title": "Effect of direct bubble-bubble interactions on linear-wave propagation in bubbly liquids", "author": [ { "family_name": "Fuster", "given_name": "D.", "clpid": "Fuster-D" }, { "family_name": "Conoir", "given_name": "J. M.", "clpid": "Conoir-J-M" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We study the influence of bubble-bubble interactions on the propagation of linear acoustic waves in bubbly liquids. Using the full model proposed by Fuster and Colonius J. Fluid Mech. 688, 253 ( 2011)], numerical simulations reveal that direct bubble-bubble interactions have an appreciable effect for frequencies above the natural resonance frequency of the average size bubble. Based on the new results, a modification of the classical wave propagation theory is proposed. The results obtained are in good agreement with previously reported experimental data where the classical linear theory systematically overpredicts the effective attenuation and phase velocity.", "doi": "10.1103/PhysRevE.90.063010", "issn": "1539-3755", "publisher": "American Physical Society", "publication": "Physical Review E", "publication_date": "2014-12", "series_number": "6", "volume": "90", "issue": "6", "pages": "Art. No." }, { "id": "authors:z72vz-waz16", "collection": "authors", "collection_id": "z72vz-waz16", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140911-085308346", "type": "article", "title": "Finite-volume WENO scheme for viscous compressible multicomponent flows", "author": [ { "family_name": "Coralic", "given_name": "Vedran", "clpid": "Coralic-V" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We develop a shock- and interface-capturing numerical method that is suitable for the simulation of multicomponent flows governed by the compressible Navier\u2013Stokes equations. The numerical method is high-order accurate in smooth regions of the flow, discretely conserves the mass of each component, as well as the total momentum and energy, and is oscillation-free, i.e. it does not introduce spurious oscillations at the locations of shockwaves and/or material interfaces. The method is of Godunov-type and utilizes a fifth-order, finite-volume, weighted essentially non-oscillatory (WENO) scheme for the spatial reconstruction and a Harten\u2013Lax\u2013van Leer contact (HLLC) approximate Riemann solver to upwind the fluxes. A third-order total variation diminishing (TVD) Runge\u2013Kutta (RK) algorithm is employed to march the solution in time. The derivation is generalized to three dimensions and nonuniform Cartesian grids. A two-point, fourth-order, Gaussian quadrature rule is utilized to build the spatial averages of the reconstructed variables inside the cells, as well as at cell boundaries. The algorithm is therefore fourth-order accurate in space and third-order accurate in time in smooth regions of the flow. We corroborate the properties of our numerical method by considering several challenging one-, two- and three-dimensional test cases, the most complex of which is the asymmetric collapse of an air bubble submerged in a cylindrical water cavity that is embedded in 10% gelatin.", "doi": "10.1016/j.jcp.2014.06.003", "pmcid": "PMC4122134", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2014-10-01", "volume": "274", "pages": "95-121" }, { "id": "authors:4cmzg-23s42", "collection": "authors", "collection_id": "4cmzg-23s42", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140515-110845791", "type": "article", "title": "Unsteady effects in dense, high speed, particle laden flows", "author": [ { "family_name": "Regele", "given_name": "J. D.", "clpid": "Regele-J-D" }, { "family_name": "Rabinovitch", "given_name": "J.", "orcid": "0000-0002-1914-7964", "clpid": "Rabinovitch-J" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Blanquart", "given_name": "G.", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "abstract": "Dense high speed non-compacted multiphase flows exist in variable phase turbines, explosions, and ejector nozzles, where the particle volume fraction is in the range 0.001<\u03b1_d<0.5. A canonical problem that can be used to study modeling issues related to these types of flows is a shock wave impacting a planar particle cloud. Thus far, prior work has modeled the flow using a 1-D volume-averaged point particle approach and developed momentum and energy coupling terms that reproduce accurately the trajectory of particles in the experiments. Although these early results are promising, it is appropriate to question whether all aspects of the experimental flow can be captured using a one-dimensional model that is traditionally only used for dilute flows. Thus the objective of this work is to set-up a two-dimensional configuration that captures qualitatively the multidimensional behavior of a real three-dimensional particle cloud, but can be used as an exact solution to compare with an equivalent volume-averaged model. The 2-D data is phase-averaged to reduce it to one dimension, and x\u2013t diagrams are used to characterize the flow behavior. These results show the importance of the Reynolds stress term inside the particle cloud and in its turbulent wake. A one-dimensional (1-D) model is developed for direct comparison with the 2-D simulation. While the 1-D model characterizes the overall steady-state flow behavior well, it fails to capture aspects of the unsteady behavior inside and behind the particle cloud because it neglects important unclosed terms.", "doi": "10.1016/j.ijmultiphaseflow.2013.12.007", "issn": "0301-9322", "publisher": "Elsevier", "publication": "International Journal of Multiphase Flow", "publication_date": "2014-05", "volume": "61", "pages": "1-13" }, { "id": "authors:9trks-81d64", "collection": "authors", "collection_id": "9trks-81d64", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140421-160135900", "type": "article", "title": "Wavepacket models for supersonic jet noise", "author": [ { "family_name": "Sinha", "given_name": "Aniruddha", "orcid": "0000-0002-7122-3549", "clpid": "Sinha-A" }, { "family_name": "Rodr\u00edguez", "given_name": "Daniel", "clpid": "Rodr\u00edguez-D" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Gudmundsson and Colonius (J. Fluid Mech., vol. 689, 2011, pp. 97\u2013128) have recently shown that the average evolution of low-frequency, low-azimuthal modal large-scale structures in the near field of subsonic jets are remarkably well predicted as linear instability waves of the turbulent mean flow using parabolized stability equations. In this work, we extend this modelling technique to an isothermal and a moderately heated Mach 1.5 jet for which the mean flow fields are obtained from a high-fidelity large-eddy simulation database. The latter affords a rigourous and extensive validation of the model, which had only been pursued earlier with more limited experimental data. A filter based on proper orthogonal decomposition is applied to the data to extract the most energetic coherent components. These components display a distinct wavepacket character, and agree fairly well with the parabolized stability equations model predictions in terms of near-field pressure and flow velocity. We next apply a Kirchhoff surface acoustic propagation technique to the near-field pressure model and obtain an encouraging match for far-field noise levels in the peak aft direction. The results suggest that linear wavepackets in the turbulence are responsible for the loudest portion of the supersonic jet acoustic field.", "doi": "10.1017/jfm.2013.660", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2014-03", "volume": "742", "pages": "71-95" }, { "id": "authors:2fayj-7tq31", "collection": "authors", "collection_id": "2fayj-7tq31", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20131202-084606542", "type": "article", "title": "Inlet conditions for wave packet models in turbulent jets based on eigenmode decomposition of large eddy simulation data", "author": [ { "family_name": "Rodr\u00edguez", "given_name": "Daniel", "clpid": "Rodr\u00edguez-D" }, { "family_name": "Sinha", "given_name": "Aniruddha", "orcid": "0000-0002-7122-3549", "clpid": "Sinha-A" }, { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "This paper makes contributions towards reduced-order models of wave packets in supersonic, turbulent jets. Wave packets are large-scale turbulent structures that are correlated and advected over distances that are large compared to the integral scales of turbulence, i.e., many jet diameters at the lowest frequencies. They are thought to be responsible for the peak noise radiated at shallow angles to the jet axis. Linear wave packet models based on the Parabolized Stability Equations (PSE) have been shown in the past to be in excellent agreement with statistical structures educed from experimental pressure and velocity data in subsonic jets. Here, we extend these models to supersonic jets and validate them using a Large Eddy Simulation (LES) database for an isothermal and a moderately heated Mach 1.5 turbulent jets. For supersonic jets, inlet conditions for PSE models are ambiguous, as a parallel flow stability analysis shows several unstable modes at the inlet cross section. We develop a bi-orthogonal decomposition and project the LES data onto the relevant families of instability waves. These serve as inlet conditions, including the amplitude and shape functions, for PSE solutions which are then favorably compared to the near-field pressure fields educed from LES.", "doi": "10.1063/1.4824479", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2013-10", "series_number": "10", "volume": "25", "issue": "10", "pages": "Art. No. 105107" }, { "id": "authors:amkhf-j8620", "collection": "authors", "collection_id": "amkhf-j8620", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130718-085055399", "type": "article", "title": "Generalized characteristic relaxation boundary conditions for unsteady compressible flow simulations", "author": [ { "family_name": "Pirozzoli", "given_name": "Sergio", "clpid": "Pirozzoli-S" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We develop numerical boundary conditions for the compressible Navier\u2013Stokes equations based on a generalized relaxation approach (GRCBC), which hinges on locally one-dimensional characteristic projection at the computational boundaries, supplemented with available information from the flow exterior. The basic idea is to estimate the amplitude of incoming characteristic waves through first-order one-sided finite-difference approximations which involve the value of the reference flow state at the first exterior (ghost) point. Unlike other characteristic-based relaxation methods, the present one requires minimal user-supplied input, including the reference flow state, which may be totally or partially known, and in general may vary both in space and time. Furthermore, it can be applied to any type of computational boundary, either inflow or outflow, either subsonic or supersonic. The method is theoretically predicted to convey reduced reflection of waves at computational boundaries compared to other ones, and to have better properties of frequency response to injected disturbances. Numerical tests confirm the improvement of the nonreflecting performance, and demonstrate high degree of flexibility, also for problems with non-trivial far-field boundary conditions (e.g. flows in rotating reference frames) and for the artificial stimulation of subsonic turbulent boundary layers.", "doi": "10.1016/j.jcp.2013.04.021", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2013-09-01", "volume": "248", "pages": "109-126" }, { "id": "authors:1t44b-keq14", "collection": "authors", "collection_id": "1t44b-keq14", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130917-093943668", "type": "article", "title": "On a transitional and turbulent natural convection in spherical shells", "author": [ { "family_name": "Feldman", "given_name": "Yuri", "clpid": "Feldman-Y" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Laminar and turbulent natural convection inside concentric spherical shells with isothermal cold and hot boundaries is numerically investigated up to Rayleigh number values Ra \u2a7d 10^(12) and Pr = 0.71. The study utilizes direct numerical simulation (DNS), large eddy simulation (LES) and Reynolds averaged Navier\u2013Stokes (RANS) approaches for investigation of the laminar, transitional and fully developed turbulent flow regimes, respectively. Three-dimensional flow patterns for slightly supercritical oscillatory flow regime inside the shell, with internal/external diameter ratio equal to D_i/D_o = 0.714 are presented and may be potentially useful for verification of the future linear stability analysis results. Particular attention has been given to the complex, fully three-dimensional unsteady flows occurring in narrow shell geometries characterized by 0.85 \u2a7d D_i/D_o \u2a7d 0.95. For this geometry considerable deviations in predicted heat flux rate through the shell boundaries are observed when compared with existing heat transfer correlations for the entire range of Ra numbers. The deviations tend to increase for transitional and fully turbulent flows. A new correlation for the heat transfer rate is suggested for laminar and transitional flow regimes.", "doi": "10.1016/j.ijheatmasstransfer.2013.04.042", "issn": "0017-9310", "publisher": "Elsevier", "publication": "International Journal of Heat and Mass Transfer", "publication_date": "2013-09", "volume": "64", "pages": "514-525" }, { "id": "authors:66nhj-xgz84", "collection": "authors", "collection_id": "66nhj-xgz84", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20131010-095128306", "type": "article", "title": "Wavepackets in the velocity field of turbulent jets", "author": [ { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Rodr\u00edguez", "given_name": "Daniel", "clpid": "Rodr\u00edguez-D" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Gervais", "given_name": "Yves", "clpid": "Gervais-Y" } ], "abstract": "We study the velocity fields of unforced, high Reynolds number, subsonic jets, issuing from round nozzles with turbulent boundary layers. The objective of the study is to educe wavepackets in such flows and to explore their relationship with the radiated sound. The velocity field is measured using a hot-wire anemometer and a stereoscopic, time-resolved PIV system. The field can be decomposed into frequency and azimuthal Fourier modes. The low-angle sound radiation is measured synchronously with a microphone ring array. Consistent with previous observations, the azimuthal wavenumber spectra of the velocity and acoustic pressure fields are distinct. The velocity spectrum of the initial mixing layer exhibits a peak at azimuthal wavenumbers ranging from 4 to 11, and the peak is found to scale with the local momentum thickness of the mixing layer. The acoustic pressure field is, on the other hand, predominantly axisymmetric, suggesting an increased relative acoustic efficiency of the axisymmetric mode of the velocity field, a characteristic that can be shown theoretically to be caused by the radial compactness of the sound source. This is confirmed by significant correlations, as high as 10 %, between the axisymmetric modes of the velocity and acoustic pressure fields, these values being significantly higher than those reported for two-point flow\u2013acoustic correlations in subsonic jets. The axisymmetric and first helical modes of the velocity field are then compared with solutions of linear parabolized stability equations (PSE) to ascertain if these modes correspond to linear wavepackets. For all but the lowest frequencies close agreement is obtained for the spatial amplification, up to the end of the potential core. The radial shapes of the linear PSE solutions also agree with the experimental results over the same region. The results suggests that, despite the broadband character of the turbulence, the evolution of Strouhal numbers 0.3 \u2264 St \u2264 0.9 and azimuthal modes 0 and 1 can be modelled as linear wavepackets, and these are associated with the sound radiated to low polar angles.", "doi": "10.1017/jfm.2013.346", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2013-09", "volume": "730", "pages": "559-592" }, { "id": "authors:ttese-95c96", "collection": "authors", "collection_id": "ttese-95c96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130605-141321734", "type": "article", "title": "Shock-induced collapse of a bubble inside a deformable vessel", "author": [ { "family_name": "Coralic", "given_name": "Vedran", "clpid": "Coralic-V" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Shockwave lithotripsy repeatedly focuses shockwaves on kidney stones to induce their fracture, partially\nthrough cavitation erosion. A typical side effect of the procedure is hemorrhage, which is potentially\nthe result of the growth and collapse of bubbles inside blood vessels. To identify the mechanisms by\nwhich shock-induced collapse could lead to the onset of injury, we study an idealized problem involving\na preexisting bubble in a deformable vessel. We utilize a high-order accurate, shock- and interfacecapturing,\nfinite-volume scheme and simulate the three-dimensional shock-induced collapse of an air\nbubble immersed in a cylindrical water column which is embedded in a gelatin/water mixture. The\nmixture is a soft tissue simulant, 10% gelatin by weight, and is modeled by the stiffened gas equation\nof state. The bubble dynamics of this model configuration are characterized by the collapse of the bubble\nand its subsequent jetting in the direction of the propagation of the shockwave. The vessel wall, which is\ndefined by the material interface between the water and gelatin/water mixture, is invaginated by the\ncollapse and distended by the impact of the jet. The present results show that the highest measured\npressures and deformations occur when the volumetric confinement of the bubble is strongest, the bubble\nis nearest the vessel wall and/or the angle of incidence of the shockwave reduces the distance between\nthe jet tip and the nearest vessel surface. For a particular case considered, the 40 MPa shockwave utilized\nin this study to collapse the bubble generated a vessel wall pressure of almost 450 MPa and produced both\nan invagination and distention of nearly 50% of the initial vessel radius on a O(10) ns timescale. These\nresults are indicative of the significant potential of shock-induced collapse to contribute to the injury of\nblood vessels in shockwave lithotripsy.", "doi": "10.1016/j.euromechflu.2013.01.003", "pmcid": "PMC3763519", "issn": "0997-7546", "publisher": "Elsevier", "publication": "European Journal of Mechanics B/Fluids", "publication_date": "2013-07", "volume": "40", "pages": "64-74" }, { "id": "authors:51jj6-hc925", "collection": "authors", "collection_id": "51jj6-hc925", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130716-141834071", "type": "article", "title": "Second-mode attenuation and cancellation by porous coatings in a high-speed boundary layer", "author": [ { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" }, { "family_name": "Inkman", "given_name": "Matthew", "clpid": "Inkman-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Fedorov", "given_name": "Alexander V.", "clpid": "Fedorov-A-V" } ], "abstract": "Numerical simulations of the linear and nonlinear two-dimensional Navier\u2013Stokes equations, and linear stability theory are used to parametrically investigate hypersonic boundary layers over ultrasonic absorptive coatings. The porous coatings consist of a uniform array of rectangular pores (slots) with a range of porosities and pore aspect ratios. For the numerical simulations, temporally (rather than spatially) evolving boundary layers are considered and we provide evidence that this approximation is appropriate for slowly growing second-mode instabilities. We consider coatings operating in the typical regime where the pores are relatively deep and acoustic waves and second-mode instabilities are attenuated by viscous effects inside the pores, as well as regimes with phase cancellation or reinforcement associated with reflection of acoustic waves from the bottom of the pores. These conditions are defined as attenuative and cancellation/reinforcement regimes, respectively. The focus of the present study is on the cases which have not been systematically studied in the past, namely the reinforcement regime (which represents a worst-case scenario, i.e. minimal second-mode damping) and the cancellation regime (which corresponds to the configuration with the most potential improvement). For all but one of the cases considered, the linear simulations show good agreement with the results of linear instability theory that employs an approximate porous-wall boundary condition, and confirm that the porous coating stabilizing performance is directly related to their acoustic scattering performance. A particular case with relatively shallow pores and very high porosity showed the existence of a shorter-wavelength instability that was not initially predicted by theory. Our analysis shows that this new mode is associated with acoustic resonances in the pores and can be more unstable than the second mode. Modifications to the theoretical model are suggested to account for the new mode and to provide estimates of the porous coating parameters that avoid this detrimental instability. Finally, nonlinear simulations confirm the conclusions of the linear analysis; in particular, we did not observe any tripping of the boundary layer by small-scale disturbances associated with individual pores.", "doi": "10.1017/jfm.2013.206", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2013-07", "volume": "726", "pages": "312-337" }, { "id": "authors:grnfr-k1q39", "collection": "authors", "collection_id": "grnfr-k1q39", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130506-104903569", "type": "article", "title": "Wave Packets and Turbulent Jet Noise", "author": [ { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Turbulent jet noise is a controversial fluid mechanical puzzle that has amused and bewildered researchers for more than half a century. Whereas numerical simulations are now capable of simultaneously predicting turbulence and its\nradiated sound, the theoretical framework that would guide noise-control efforts is incomplete. Wave packets are intermittent, advecting disturbances that are correlated over distances far exceeding the integral scales of turbulence. Their signatures are readily distinguished in the vortical, turbulent region; the irrotational, evanescent near field; and the propagating far field. We review evidence of the existence, energetics, dynamics, and acoustic efficiency of wave packets. We highlight how extensive data available from simulations and modern measurement techniques can be used to distill acoustically relevant turbulent motions. The evidence supports theories that seek to represent wave packets as instability waves, or more general modal solutions of the governing equations, and confirms the acoustic importance of these structures in the aft-angle radiation of high subsonic and supersonic\njets. The resulting unified view of wave packets provides insights that can help guide control strategies.", "doi": "10.1146/annurev-fluid-011212-140756", "issn": "0066-4189", "publisher": "Annual Reviews", "publication": "Annual Review of Fluid Mechanics", "publication_date": "2013-01", "volume": "45", "pages": "173-195" }, { "id": "authors:4wvjz-stn24", "collection": "authors", "collection_id": "4wvjz-stn24", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130201-102028177", "type": "article", "title": "The impulse response of a high-speed jet forced with localized arc filament plasma actuators", "author": [ { "family_name": "Sinha", "given_name": "Aniruddha", "orcid": "0000-0002-7122-3549", "clpid": "Sinha-A" }, { "family_name": "Alkandry", "given_name": "Hind", "clpid": "Alkandry-H" }, { "family_name": "Kearney-Fischer", "given_name": "Martin", "clpid": "Kearney-Fischer-M" }, { "family_name": "Samimy", "given_name": "Mo", "clpid": "Samimy-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We present experimental and theoretical analyses of the response of high-speed, high-Reynolds-number, round jets to impulsive forcing with arc-filament-plasma actuators. The impulse response is obtained with forcing Strouhal numbers, based on the nozzle exit diameter and exit center line velocity, less than 0.1. The resulting phase-averaged near-field pressure signature displays a compact wave with a positive peak preceding a negative one, indicative of a large scale structure in the shear layer of the jet. Scaling laws derived by operating the jet at four subsonic Mach numbers are used to distinguish this hydrodynamic component of the phase-averaged jet response from the direct actuator noise. As the forcing frequency increases, the compact waves in the near-field pressure signal overlap each other, indicating interaction of the growing seeded structures. For this regime, the phase-averaged response is approximately replicated by linear superposition of the impulse response, thereby demonstrating the quasi-linearity of structure interaction. A novel application of linear parabolized stability theory yields a successful model of the impulse response.", "doi": "10.1063/1.4772191", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2012-12", "series_number": "12", "volume": "24", "issue": "12", "pages": "Art. No. 125104" }, { "id": "authors:2czvw-bpz81", "collection": "authors", "collection_id": "2czvw-bpz81", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121211-102852653", "type": "article", "title": "Simulation and Cryogenic Experiments of Natural\n Convection for the Titan Montgolfiere", "author": [ { "family_name": "Feldman", "given_name": "Yuri", "clpid": "Feldman-Y" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Pauken", "given_name": "Michael T.", "orcid": "0000-0002-3993-675X", "clpid": "Pauken-M-T" }, { "family_name": "Hall", "given_name": "Jeffrey L.", "clpid": "Hall-J-L" }, { "family_name": "Jones", "given_name": "Jack A.", "clpid": "Jones-J-A" } ], "abstract": "Natural convection in a spherical geometry is considered for prediction of the buoyancy of single- and double-walled\nballoons in a cryogenic environment such as Titan's atmosphere. The steady-state flow characteristics\nobtained by solving the Reynolds-averaged Navier\u2013Stokes equations with a standard turbulence model are used to\ndetermine the net buoyancy as a function of heat input. Thermal radiation effects are shown to have a minor impact\non the buoyancy, as would be expected at cryogenic conditions. The predicted buoyancy and temperature fields\ncompare favorably with experiments preformed on a 1-m-diameter Montgolfiere prototype in a cryogenic facility. In\naddition, both numerical and experimental results were compared with correlations for the heat transfer coefficients\nfor free convection internal and external to the balloon as well as in the concentric gap of the double-walled balloons.\nFinally, scaling issues related to inferring the performance of the full-scale Montgolfiere from the model-scale results\nare examined.", "doi": "10.2514/1.J051672", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2012-11", "series_number": "11", "volume": "50", "issue": "11", "pages": "2483-2491" }, { "id": "authors:kyz36-qkt52", "collection": "authors", "collection_id": "kyz36-qkt52", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20120817-102356325", "type": "article", "title": "Axisymmetric superdirectivity in subsonic jets", "author": [ { "family_name": "Cavalieri", "given_name": "Andr\u00e9 V. G.", "orcid": "0000-0003-4283-0232", "clpid": "Cavalieri-A-V-G" }, { "family_name": "Jordan", "given_name": "Peter", "orcid": "0000-0001-8576-5587", "clpid": "Jordan-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Gervais", "given_name": "Yves", "clpid": "Gervais-Y" } ], "abstract": "We present experimental results for the acoustic field of jets with Mach numbers\nbetween 0.35 and 0.6. An azimuthal ring array of six microphones, whose polar\nangle, \u03b8, was progressively varied, allows the decomposition of the acoustic pressure\ninto azimuthal Fourier modes. In agreement with past observations, the sound field for\nlow polar angles (measured with respect to the jet axis) is found to be dominated by\nthe axisymmetric mode, particularly at the peak Strouhal number. The axisymmetric\nmode of the acoustic field can be clearly associated with an axially non-compact\nsource, in the form of a wavepacket: the sound pressure level for peak frequencies is\nfound be superdirective for all Mach numbers considered, with exponential decay as a\nfunction of (1 \u2013 M_c cos \u03b8)^2, where M_c is the Mach number based on the phase velocity\nU_c of the convected wave. While the mode m = 1 spectrum scales with Strouhal\nnumber, suggesting that its energy content is associated with turbulence scales,\nthe axisymmetric mode scales with Helmholtz number \u2013 the ratio between source\nlength scale and acoustic wavelength. The axisymmetric radiation has a stronger\nvelocity dependence than the higher-order azimuthal modes, again in agreement with\npredictions of wavepacket models. We estimate the axial extent of the source of\nthe axisymmetric component of the sound field to be of the order of six to eight\njet diameters. This estimate is obtained in two different ways, using, respectively,\nthe directivity shape and the velocity exponent of the sound radiation. The analysis\nfurthermore shows that compressibility plays a significant role in the wavepacket\ndynamics, even at this low Mach number. Velocity fluctuations on the jet centreline\nare reduced as the Mach number is increased, an effect that must be accounted for\nin order to obtain a correct estimation of the velocity dependence of sound radiation.\nFinally, the higher-order azimuthal modes of the sound field are considered, and a\nmodel for the low-angle sound radiation by helical wavepackets is developed. The\nmeasured sound for azimuthal modes 1 and 2 at low Strouhal numbers is seen to\ncorrespond closely to the predicted directivity shapes.", "doi": "10.1017/jfm.2012.247", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2012-08-10", "volume": "704", "pages": "388-420" }, { "id": "authors:jhgkz-esh83", "collection": "authors", "collection_id": "jhgkz-esh83", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20120807-103744882", "type": "article", "title": "Effects of Actuation Frequency on Flow Control Applied to a Wall-Mounted Hump", "author": [ { "family_name": "Franck", "given_name": "Jennifer A.", "clpid": "Franck-J-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Active flow control, and particularly unsteady mass injection (synthetic jets), promotes reattachment of naturally separated flows and can increase lift and decrease drag on airfoils at high angles of attack. Synthetic-jet actuators are typically characterized by the\nunsteady momentum coefficient =(\u03c1_sh_s)/(0.5\u03c1_(\u221e)U^2_(\u221e)c) and the actuation frequency F^(+)= fX_sep/U_\u221e, where \u03c1_s and u_s are the density and velocity of the injected fluid, respectively; h_s is the slot velocity; f is the actuation frequency; and X_sep is the separation distance or chord length, depending on the author or application. Actuation frequencies are often chosen to be on the order of the natural large-scale shedding frequency, or F^(+) ~ O(1). At this frequency, large-scale vortex shedding is induced, which increases the entrainment rate and promotes deflection of the separated shear layer toward the surface. Recent investigations have looked at a second regime of actuation frequencies, which are typically an order of magnitude higher than the most dominant natural frequency in the separated flow and are designed to excite Kelvin\u2013Helmholtz instabilities in the boundary layer. In the airfoil experiments of Amitay and Glezer and Glezer et al., high-frequency excitation was found to be more effective in improving the aerodynamic performance than actuation at lower frequencies by increasing the suction force immediately after the leading edge actuation location. Simulations performed by Visbal and Visbal et al. have shown that plasma-based actuation pulsed at frequencies in the range of F^(+) = 4\u20138 are effective in promoting laminar-turbulent transition and suppressing separation. However, when applied to a fully turbulent boundary layer, the plasma actuation required significantly more power to achieve a reduction in separation bubble size. Other airfoil simulations at low or transitional Reynolds number have found that actuation close to the fundamental shear-layer frequency is ineffective, and forcing closest to the natural shedding frequency is more optimal; however, such two-dimensional simulations may not be accurately capturing the three-dimensional flow physics. Similar results are seen in the numerical experiments by Dandois et al. on a backward-facing step, in which the separation bubble length increased with high-frequency forcing. \nHere, we briefly report results for a compressible, large-eddy simulation (LES) of flow past a wall-mounted hump at high Reynolds number where oscillatory control is applied at F^(+) ~ O(10). The LES with F^(+) ~ O(1) was previously validated and found to compare favorably with extensive experimental results of this geometry. Many other computational results in the low-frequency regime were reported in the NASA Langley Computational Fluid Dynamics Validation workshop. We simulate the flow at Re_c = 500,000 and Mach numbers of 0.25 and 0.6, the latter of which is significantly higher than previous simulations of high-frequency actuation.", "doi": "10.2514/1.J051183", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2012-07", "series_number": "7", "volume": "50", "issue": "7", "pages": "1631-1634" }, { "id": "authors:2pqy6-0g834", "collection": "authors", "collection_id": "2pqy6-0g834", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20120224-125133227", "type": "article", "title": "A contact model for normal immersed collisions between a particle and a wall", "author": [ { "family_name": "Li", "given_name": "Xiaobai", "clpid": "Li-Xiaobai" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The incompressible Navier\u2013Stokes equations are solved numerically to predict the coupled motion of a falling particle and the surrounding fluid as the particle impacts and rebounds from a planar wall. The method is validated by comparing the numerical simulations of a settling sphere with experimental measurements of the sphere trajectory and the accompanying flow field. The normal collision process is then studied for a range of impact Stokes numbers. A contact model of the liquid\u2013solid interaction and elastic effect is developed that incorporates the elasticity of the solids to permit the rebound trajectory to be simulated accurately. The contact model is applied when the particle is sufficiently close to the wall that it becomes difficult to resolve the thin lubrication layer. The model is calibrated with new measurements of the particle trajectories and reproduces the observed coefficient of restitution over a range of impact Stokes numbers from 1 to 1000.", "doi": "10.1017/jfm.2011.461", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2012-01-25", "volume": "691", "pages": "123-145" }, { "id": "authors:0sr9j-09p72", "collection": "authors", "collection_id": "0sr9j-09p72", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20120203-100537724", "type": "article", "title": "Instability wave models for the near-field fluctuations of turbulent jets", "author": [ { "family_name": "Gudmundsson", "given_name": "K.", "clpid": "Gudmundsson-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Previous work has shown that aspects of the evolution of large-scale structures, particularly in forced and transitional mixing layers and jets, can be described by linear and nonlinear stability theories. However, questions persist as to the choice of the basic (steady) flow field to perturb, and the extent to which disturbances in natural (unforced), initially turbulent jets may be modelled with the theory. For unforced jets, identification is made difficult by the lack of a phase reference that would permit a portion of the signal associated with the instability wave to be isolated from other, uncorrelated fluctuations. In this paper, we investigate the extent to which pressure and velocity fluctuations in subsonic, turbulent round jets can be described as linear perturbations to the mean flow field. The disturbances are expanded about the experimentally measured jet mean flow field, and evolved using linear parabolized stability equations (PSE) that account, in an approximate way, for the weakly non-parallel jet mean flow field. We utilize data from an extensive microphone array that measures pressure fluctuations just outside the jet shear layer to show that, up to an unknown initial disturbance spectrum, the phase, wavelength, and amplitude envelope of convecting wavepackets agree well with PSE solutions at frequencies and azimuthal wavenumbers that can be accurately measured with the array. We next apply the proper orthogonal decomposition to near-field velocity fluctuations measured with particle image velocimetry, and show that the structure of the most energetic modes is also similar to eigenfunctions from the linear theory. Importantly, the amplitudes of the modes inferred from the velocity fluctuations are in reasonable agreement with those identified from the microphone array. The results therefore suggest that, to predict, with reasonable accuracy, the evolution of the largest-scale structures that comprise the most energetic portion of the turbulent spectrum of natural jets, nonlinear effects need only be indirectly accounted for by considering perturbations to the mean turbulent flow field, while neglecting any non-zero frequency disturbance interactions.", "doi": "10.1017/jfm.2011.401", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2011-12-25", "volume": "689", "pages": "97-128" }, { "id": "authors:4mgnf-w2x74", "collection": "authors", "collection_id": "4mgnf-w2x74", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20120123-141053654", "type": "article", "title": "Modelling bubble clusters in compressible liquids", "author": [ { "family_name": "Fuster", "given_name": "D.", "clpid": "Fuster-D" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We present a new model for bubbly cavitating flows. Based on volume-averaged equations, a subgrid model is added to account for a bubble, or multiple bubbles, within each computational cell. The model converges to the solution of ensemble-averaged bubbly flow equations for weak oscillations and monodisperse systems. In the other extreme, it also converges to the theoretical solution for a single oscillating bubble, and captures the bubble radius evolution and the pressure disturbance induced in the liquid. A substantial saving of computational time is achieved compared to ensemble-averaged models for polydisperse mixtures.", "doi": "10.1017/jfm.2011.380", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2011-12", "volume": "688", "pages": "352-389" }, { "id": "authors:qx2a2-rm460", "collection": "authors", "collection_id": "qx2a2-rm460", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110812-110736366", "type": "article", "title": "Flow around a NACA0018 airfoil with a cavity and its dynamical response to acoustic forcing", "author": [ { "family_name": "Olsman", "given_name": "W. F. J.", "clpid": "Olsman-W-F-J" }, { "family_name": "Willems", "given_name": "J. F. H.", "clpid": "Willems-J-F-H" }, { "family_name": "Hirschberg", "given_name": "A.", "clpid": "Hirschberg-A" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Trieling", "given_name": "R. R.", "clpid": "Trieling-R-R" } ], "abstract": "Trapping of vortices in a cavity has been explored in recent years as a drag reduction measure for thick airfoils. If, however, trapping fails, then oscillation of\nthe cavity flow may couple with elastic vibration modes of\nthe airfoil. To examine this scenario, the effect of small\namplitude vertical motion on the oscillation of the shear\nlayer above the cavity is studied by acoustic forcing simulating a vertical translation of a modified NACA0018\nprofile. At low Reynolds numbers based on the chord\n(O(10^4)), natural instability modes of this shear layer are\nobserved for Strouhal numbers based on the cavity width of\norder unity. Acoustic forcing sufficiently close to the natural instability frequency induces a strong non-linear\nresponse due to lock-in of the shear layer. At higher Reynolds numbers (above 10^5) for Strouhal number 0.6 or\nlower, no natural instabilities of the shear layer and only a linear response to forcing were observed. The dynamical\npressure difference across the airfoil is then dominated by\nadded mass effects, as was confirmed by numerical simulations.", "doi": "10.1007/s00348-011-1065-7", "issn": "0723-4864", "publisher": "Springer", "publication": "Experiments in Fluids", "publication_date": "2011-08", "series_number": "2", "volume": "51", "issue": "2", "pages": "493-509" }, { "id": "authors:brzft-41e74", "collection": "authors", "collection_id": "brzft-41e74", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110908-120901325", "type": "article", "title": "Closed-Loop Control of Lift for Longitudinal Gust Suppression\n at Low Reynolds Numbers", "author": [ { "family_name": "Kerstens", "given_name": "Wesley", "clpid": "Kerstens-W" }, { "family_name": "Pfeiffer", "given_name": "Jens", "clpid": "Pfeiffer-J" }, { "family_name": "Williams", "given_name": "David", "clpid": "Williams-D" }, { "family_name": "King", "given_name": "Rudibert", "clpid": "King-R" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "contributor": [ { "family_name": "Naguib", "given_name": "A.", "clpid": "Naguib-A" } ], "abstract": "Experiments are conducted to investigate the ability of variable-pressure pulsed-blowing actuation to maintain a\nconstant lift force on a low-aspect-ratio semicircular wing in a longitudinally gusting flow. Dynamic models of the lift\nresponse to actuation and the lift response to longitudinal gusting are obtained through modern system identification\nmethods. Robust closed-loop controllers are synthesized using a mixed-sensitivity loop-shaping approach. An\nadditional feedforward disturbance compensator is designed based on a model of the unsteady aerodynamics. The\ncontrollers show suppression of lift fluctuations at low gust frequencies, f < 0.8 Hz(reduced frequency, k < 0.09). At higher frequencies, the control performance degrades due to limitations related to the time for a disturbance, created by the actuators, to convect over the wing and establish the flowfield that leads to enhanced lift on the wing.", "doi": "10.2514/1.J050954", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2011-08", "series_number": "8", "volume": "49", "issue": "8", "pages": "1721-1728" }, { "id": "authors:y5yf5-3a588", "collection": "authors", "collection_id": "y5yf5-3a588", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110713-152954571", "type": "article", "title": "Numerical simulation of shock propagation in a polydisperse bubbly liquid", "author": [ { "family_name": "Ando", "given_name": "Keita", "clpid": "Ando-Keita" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "abstract": "The effect of distributed bubble nuclei sizes on shock propagation in a bubbly liquid is numerically investigated. An ensemble-averaged technique is employed to derive the statistically averaged conservation laws for polydisperse bubbly flows. A finite-volume method is developed to solve the continuum bubbly flow equations coupled to a single-bubble-dynamic equation that incorporates the effects of heat transfer, liquid viscosity and compressibility. The one-dimensional shock computations reveal that the distribution of equilibrium bubble sizes leads to an apparent damping of the averaged shock dynamics due to phase cancellations in oscillations of the different-sized bubbles. If the distribution is sufficiently broad, the phase cancellation effect can dominate over the single-bubble-dynamic dissipation and the averaged shock profile is smoothed out.", "doi": "10.1016/j.ijmultiphaseflow.2011.03.007", "issn": "0301-9322", "publisher": "Elsevier", "publication": "International Journal of Multiphase Flow", "publication_date": "2011-07", "series_number": "6", "volume": "37", "issue": "6", "pages": "596-608" }, { "id": "authors:x6w2g-rb975", "collection": "authors", "collection_id": "x6w2g-rb975", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110613-112849752", "type": "article", "title": "Special issue on global flow instability and control", "author": [ { "family_name": "Theofilis", "given_name": "Vassilis", "clpid": "Theofilis-V" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "This special issue is intended to provide a snapshot of current research in the area of \"Global Flow Instability and Control\". The original papers, and to a certain extent the topic itself, are intimately linked with the series of symposia by the same name that were held in Crete, Greece, between 2001 and 2009. As members of the organizing committees of the Crete symposia, we invited all past participants to contribute, and all papers were reviewed following the strict standards of the journal. This preface gives a brief historical account of events that have shaped ideas in the field over the past decade, followed by a synopsis of the papers published herein.", "doi": "10.1007/s00162-010-0217-3", "issn": "0935-4964", "publisher": "Springer", "publication": "Theoretical and Computational Fluid Dynamics", "publication_date": "2011-06", "series_number": "1-4", "volume": "25", "issue": "1-4", "pages": "1-6" }, { "id": "authors:e99c1-sbs06", "collection": "authors", "collection_id": "e99c1-sbs06", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110613-140739970", "type": "article", "title": "Feedback control of vortex shedding from an inclined flat plate", "author": [ { "family_name": "Joe", "given_name": "Won Tae", "clpid": "Joe-Won-Tae" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "MacMynowski", "given_name": "Douglas G.", "orcid": "0000-0003-1987-9417", "clpid": "MacMartin-D-G" } ], "abstract": "Open- and closed-loop control of vortex shedding in two-dimensional flow over a flat plate at high\nangle of attack is numerically investigated at a Reynolds number of 300. Unsteady actuation is modeled as\na body force near the leading or trailing edge and is directed either upstream or downstream. For moderate\nangles of attack, sinusoidal forcing at the natural shedding frequency results in phase locking, with a periodic\nvariation of lift at the same frequency, leading to higher unsteady lift than the natural shedding. However, at\nsufficiently high angles of attack, a subharmonic of the forcing frequency is also excited and the average lift\nover the forcing period varies from cycle-to-cycle in a complex manner. It is observed that the periods with the\nhighest averaged lift are associated with particular phase differences between the forcing and the lift, but that\nthis highest-lift shedding cycle is not always stably maintained with open-loop forcing.We design a feedback\nalgorithm to lock the forcing with the phase shift associated with the highest period-averaged lift. It is shown\nthat the compensator results in a stable phase-locked limit cycle for a broader range of forcing frequencies\nthan the open-loop control, and that it is able to stabilize otherwise unstable high-lift limit cycles that cannot\nbe obtained with open-loop control. For example, at an angle of attack of 40\u25e6, the feedback controller can\nincrease the averaged magnitude of force on the plate by 76% and increase the averaged lift coefficient from\n1.33 to 2.43.", "doi": "10.1007/s00162-010-0204-8", "issn": "0935-4964", "publisher": "Springer", "publication": "Theoretical and Computational Fluid Dynamics", "publication_date": "2011-06", "series_number": "1-4", "volume": "25", "issue": "1-4", "pages": "221-232" }, { "id": "authors:0g6nk-bns78", "collection": "authors", "collection_id": "0g6nk-bns78", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110328-100709222", "type": "article", "title": "Control of vortex shedding on two- and three-dimensional aerofoils", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Williams", "given_name": "David R.", "clpid": "Williams-D-R" } ], "abstract": "We review and expand on the control of separated flows over flat plates and aerofoils at low Reynolds numbers associated with micro air vehicles. Experimental observations of the steady-state and transient lift response to actuation, and its dependence on the actuator, aerofoil geometry and flow conditions, are discussed and an attempt is made to unify them in terms of their excitation of periodic and transient vortex shedding. We also examine strategies for closed-loop flow and flight control using actuation of leading-edge vortices.", "doi": "10.1098/rsta.2010.0355", "issn": "1364-503X", "publisher": "Royal Society of London", "publication": "Philosophical Transactions A: Mathematical, Physical and Engineering Sciences", "publication_date": "2011-04-13", "series_number": "1940", "volume": "369", "issue": "1940", "pages": "1525-1539" }, { "id": "authors:s7m63-wm447", "collection": "authors", "collection_id": "s7m63-wm447", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190712-112323080", "type": "article", "title": "Reply by the Authors to G. E. Dorrington", "author": [ { "family_name": "Samanta", "given_name": "Arnab", "clpid": "Samanta-A" }, { "family_name": "Appel\u00f6", "given_name": "Daniel", "clpid": "Appel\u00f6-D" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Nott", "given_name": "Julian", "clpid": "Nott-J" }, { "family_name": "Hall", "given_name": "Jeffery", "clpid": "Hall-J" } ], "abstract": "[no abstract]", "doi": "10.2514/1.J050961", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2011-04", "series_number": "4", "volume": "49", "issue": "4", "pages": "877-878" }, { "id": "authors:5btrz-77q55", "collection": "authors", "collection_id": "5btrz-77q55", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110328-100709784", "type": "article", "title": "Shock propagation through a bubbly liquid in a deformable tube", "author": [ { "family_name": "Ando", "given_name": "Keita", "clpid": "Ando-Keita" }, { "family_name": "Sanada", "given_name": "T.", "clpid": "Sanada-Toshiyuki" }, { "family_name": "Inaba", "given_name": "K.", "clpid": "Inaba-Kazuaki" }, { "family_name": "Damazo", "given_name": "J. S.", "orcid": "0000-0002-4155-7177", "clpid": "Damazo-J-S" }, { "family_name": "Shepherd", "given_name": "J. E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "C. E.", "clpid": "Brennen-C-E" } ], "abstract": "Shock propagation through a bubbly liquid contained in a deformable tube is considered. Quasi-one-dimensional mixture-averaged flow equations that include fluid\u2013structure interaction are formulated. The steady shock relations are derived and the nonlinear effect due to the gas-phase compressibility is examined. Experiments\nare conducted in which a free-falling steel projectile impacts the top of an air/water mixture in a polycarbonate tube, and stress waves in the tube material and pressure\non the tube wall are measured. The experimental data indicate that the linear theory is incapable of properly predicting the propagation speeds of finite-amplitude waves\nin a mixture-filled tube; the shock theory is found to more accurately estimate the measured wave speeds.", "doi": "10.1017/S0022112010005707", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2011-03", "volume": "671", "pages": "339-363" }, { "id": "authors:89wpc-7rd03", "collection": "authors", "collection_id": "89wpc-7rd03", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110222-101256399", "type": "article", "title": "Numerical Simulation of Flow over an Airfoil with a Cavity", "author": [ { "family_name": "Olsman", "given_name": "W. F. J.", "clpid": "Olsman-W-F-J" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Two-dimensional direct numerical simulation of the flow over a NACA0018 airfoil with a cavity is presented. The\nlow Reynolds number simulations are validated by means of flow visualizations carried out in a water channel. From\nthe simulations, it follows that there are two main regimes of flow inside the cavity. Depending on the angle of attack,\nthe first or the second shear-layer mode (Rossiter tone) is present. The global effect of the cavity on the flow around\nthe airfoil is the generation of vortices that reduce flow separation downstream of the cavity. At high positive angles of\nattack, the flow separates in front of the cavity, and the separated flow interacts with the cavity, causing the\ngeneration of smaller-scale structures and a narrower wake compared with the case when no cavity is present. At\ncertain angles of attack, the numerical results suggest the possibility of a higher lift-to-drag ratio for the airfoil with\ncavity compared with the airfoil without cavity.", "doi": "10.2514/1.J050542", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2011-01", "series_number": "1", "volume": "49", "issue": "1", "pages": "143-149" }, { "id": "authors:1wweh-1yn35", "collection": "authors", "collection_id": "1wweh-1yn35", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190712-112322993", "type": "article", "title": "A Computational Study of High-Speed Droplet Impact", "author": [ { "family_name": "Sanada", "given_name": "T.", "clpid": "Sanada-T" }, { "family_name": "Ando", "given_name": "K.", "clpid": "Ando-Keita" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "When a droplet impacts a solid surface at high speed, the contact periphery expands very quickly and liquid compressibility plays an important role in the initial dynamics and the formation of lateral jets. The high speed impact results in high pressures that can account for the surface erosion. In this study, we numerically investigated a high speed droplet impacts on a solid wall. The multicomponent Euler equations with the stiffened equation of state are computed using a FV-WENO scheme with an HLLC Riemann solver that accurately captures shocks and interfaces. In order to compare the available theories and experiments, 1D, 2D and axisymmetric solutions are obtained. The generated pressures, shock speeds, and differences in the dimensionality are investigated. In addition, the effect of target compliance is evaluated.", "doi": "10.3970/fdmp.2011.007.329", "issn": "1555-2578", "publisher": "Tech Science Press", "publication": "Fluid Dynamics & Material Processing", "publication_date": "2011", "series_number": "4", "volume": "7", "issue": "4", "pages": "329-340" }, { "id": "authors:2r3yh-6xy61", "collection": "authors", "collection_id": "2r3yh-6xy61", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20101214-085632676", "type": "article", "title": "Simulation of the effects of cavitation and anatomy in the shock path of model lithotripters", "author": [ { "family_name": "Krimmel", "given_name": "Jeff", "clpid": "Krimmel-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Tanguay", "given_name": "Michel", "clpid": "Tanguay-M" } ], "abstract": "We report on recent efforts to develop predictive models for the pressure and other flow variables in the focal region of shock wave lithotripters. Baseline simulations of three representative lithotripters (electrohydraulic, electromagnetic, and piezoelectric) compare favorably with in vitro experiments (in a water bath). We proceed to model and investigate how shock focusing is altered by the presence of material interfaces associated with different types of tissue encountered along the shock path, and by the presence of cavitation bubbles that are excited by tensile pressures associated with the focused shock wave. We use human anatomical data, but simplify the description by assuming that the tissue behaves as a fluid, and by assuming cylindrical symmetry along the shock path. Scattering by material interfaces is significant, and regions of high pressure amplitudes (both compressive and tensile) are generated almost 4 cm postfocus. Bubble dynamics generate secondary shocks whose strength depends on the density of bubbles and the pulse repetition frequency (PRF). At sufficiently large densities, the bubbles also attenuate the shock. Together with experimental evidence, the simulations suggest that high PRF may be counterproductive for stone comminution. Finally, we discuss how the lithotripter simulations can be used as input to more detailed physical models that attempt to characterize the mechanisms by which collapsing cavitation models erode stones, and by which shock waves and bubbles may damage tissue.", "doi": "10.1007/s00240-010-0332-z", "pmcid": "PMC3032941", "issn": "0300-5623", "publisher": "Springer", "publication": "Urological Research", "publication_date": "2010-12", "series_number": "6", "volume": "38", "issue": "6", "pages": "505-518" }, { "id": "authors:kjydr-yaz92", "collection": "authors", "collection_id": "kjydr-yaz92", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100831-130506686", "type": "article", "title": "Lift Enhancement for Low-Aspect-Ratio Wings with Periodic Excitation", "author": [ { "family_name": "Taira", "given_name": "Kunihiko", "orcid": "0000-0002-3762-8075", "clpid": "Taira-Kunihiko" }, { "family_name": "Rowley", "given_name": "Clarence W.", "orcid": "0000-0002-9099-5739", "clpid": "Rowley-C-W" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Williams", "given_name": "David R.", "clpid": "Williams-D-R" } ], "abstract": "In an effort to enhance lift on low-aspect-ratio rectangular flat-plate wings in low-Reynolds-number\npost-stall flows, periodic injection of momentum is considered along the trailing edge in this numerical\nstudy. The purpose of actuation is not to reattach the flow but to change the dynamics of the wake\nvortices such that the resulting lift force is increased. Periodic forcing is observed to be effective\nin increasing lift for various aspect ratios and angles of attack, achieving a similar lift enhancement\nattained by steady forcing with less momentum input. Through the investigation on the influence of\nthe actuation frequency, it is also found that there exists a frequency at which the flow locks on to a\ntime-periodic high-lift state.", "doi": "10.2514/1.J050248", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2010-08", "series_number": "8", "volume": "48", "issue": "8", "pages": "1785-1790" }, { "id": "authors:k7ftv-qy132", "collection": "authors", "collection_id": "k7ftv-qy132", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100810-142241271", "type": "article", "title": "Wave-packet models for large-scale mixing noise", "author": [ { "family_name": "Reba", "given_name": "Ramons", "clpid": "Reba-R" }, { "family_name": "Narayanan", "given_name": "Satish", "clpid": "Narayanan-S" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A wave-packet Ansatz is used to model jet noise generation by large-scale turbulence. In this approach, an equivalent source is defined based on the two-point space-time correlation of hydrodynamic pressure on a conical surface surrounding the jet plume. The surface is sufficiently near the turbulent flow region to be dominated by non-propagating hydrodynamic signatures of large-scale turbulent structures, yet sufficiently far that linear behavior can be assumed in extending the near-field pressure to the acoustic field. In the present study, a 78-microphone array was used to measure hydrodynamic pressure on the conical surface in order to identify parameters for the model and to validate the approach. Six microphones were distributed in the azimuthal direction at each of 13 axial locations spanning the first 8 jet diameters, allowing decomposition of azimuthal modes m = 0 and m = 1. We show that a source model based on a Gaussian correlation function provides a consistently good representation of the noise source attributed to large-scale structures. The results provide evidence that large-scale wave-like structures, known to dominate aft radiation at supersonic phase speeds, are also relevant at subsonic speeds.", "doi": "10.1260/1475-472X.9.4-5.533", "issn": "1475-472X", "publisher": "Multi-Science Publishing", "publication": "International Journal of Aeroacoustics", "publication_date": "2010-06", "series_number": "4-5", "volume": "9", "issue": "4-5", "pages": "533-557" }, { "id": "authors:q1njr-bft11", "collection": "authors", "collection_id": "q1njr-bft11", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100623-152555163", "type": "article", "title": "Compressible Large-Eddy Simulation of Separation Control on a Wall-Mounted Hump", "author": [ { "family_name": "Franck", "given_name": "Jennifer A.", "clpid": "Franck-J-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Compressible large-eddy simulations of turbulent flow over a wall-mounted hump with active flow control are\nperformed and compared with previous experiments. The flow is characterized by the unsteady separation before the\nsteep trailing edge, which naturally reattaches downstream of the hump to form an unsteady turbulent separation\nbubble. The low Mach number large-eddy simulation demonstrated a good prediction of surface pressure coefficient,\nseparation-bubble length, and velocity profiles compared with experiments. The effect of compressibility on the\nbaseline flow is documented and analyzed and is found to increase the separation-bubble size, due to a reduced\ngrowth rate. Control is applied just before the natural separation point via steady suction and zero-net-mass-flux\noscillatory forcing, and steady suction is shown to be more effective in decreasing the size of the separation bubble and\npressure drag for the control parameters investigated. Controlled flow at a compressible subsonic Mach number is\napplied, and found to be slightly less effective than the same control parameters at low Mach numbers.", "doi": "10.2514/1.44756", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2010-06", "series_number": "6", "volume": "48", "issue": "6", "pages": "1098-1107" }, { "id": "authors:ynx4y-x5694", "collection": "authors", "collection_id": "ynx4y-x5694", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170408-171700963", "type": "article", "title": "Computational Modeling and Experiments of Natural Convection for a Titan Montgolfiere", "author": [ { "family_name": "Samanta", "given_name": "Arnab", "clpid": "Samanta-A" }, { "family_name": "Appel\u00f6", "given_name": "Daniel", "clpid": "Appel\u00f6-D" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Nott", "given_name": "Julian", "clpid": "Nott-J" }, { "family_name": "Hall", "given_name": "Jeffrey", "clpid": "Hall-J" } ], "abstract": "Computational models are developed to predict the natural convection heat transfer and buoyancy for a Montgolfiere under conditions relevant to the Titan atmosphere. Idealized single- and double-walled balloon geometries are simulated using algorithms suitable for both laminar and (averaged) turbulent convection. Steady-state performance results are compared with existing heat transfer coefficient correlations. The laminar results, in particular, are used to test the validity of the correlations in the absence of uncertainties associated with turbulence modeling. Some discrepancies are observed, which appear to be primarily associated with temperature nonuniformity on the balloon surface. The predicted buoyancy for both the single- and double-walled balloons in the turbulent convection regime, predicted with standard two-equation turbulence models, showed trends similar to those with the empirical correlations. There was also good agreement with recently conducted experiments in a cryogenic facility designed to simulate the Titan atmosphere.", "doi": "10.2514/1.45854", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2010-05", "series_number": "5", "volume": "48", "issue": "5", "pages": "1007-1016" }, { "id": "authors:fmcs4-rhk52", "collection": "authors", "collection_id": "fmcs4-rhk52", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100520-150834651", "type": "article", "title": "The leading-edge vortex and quasisteady vortex shedding on an accelerating plate", "author": [ { "family_name": "Chen", "given_name": "Kevin K.", "clpid": "Chen-Kevin-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Taira", "given_name": "Kunihiko", "orcid": "0000-0002-3762-8075", "clpid": "Taira-Kunihiko" } ], "abstract": "A computational inquiry focuses on leading-edge vortex (LEV) growth and shedding during acceleration of a two-dimensional flat plate at a fixed 10\u00b0\u201360\u00b0 angle of attack and low Reynolds number. The plate accelerates from rest with a velocity given by a power of time ranging from 0 to 5. During the initial LEV growth, subtraction of the added mass lift from the computed lift reveals an LEV-induced lift augmentation evident across all powers and angles of attack. For the range of Reynolds numbers considered, a universal time scale exists for the peak when \u03b1 \u2265 30\u00b0, with augmentation lasting about four to five chord lengths of translation. This time scale matches well with the half-stroke of a flying insect. An oscillating pattern of leading- and trailing-edge vortex shedding follows the shedding of the initial LEV. The nondimensional frequency of shedding and lift coefficient minima and maxima closely match their values in the absence of acceleration. These observations support a quasisteady theory of vortex shedding, where dynamics are determined primarily by velocity and not acceleration. Finally, the nondimensional vortex formation time is found to be a function of the Reynolds number, but only weakly when the Reynolds number is high.", "doi": "10.1063/1.3327282", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2010-03", "series_number": "3", "volume": "22", "issue": "3", "pages": "Art. No. 033601" }, { "id": "authors:knhq1-chv66", "collection": "authors", "collection_id": "knhq1-chv66", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100308-144328976", "type": "article", "title": "Acoustic Properties of Porous Coatings for Hypersonic Boundary-Layer Control", "author": [ { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Fedorov", "given_name": "Alexander V.", "clpid": "Fedorov-A-V" } ], "abstract": "Numerical simulations are performed to investigate the interaction of acoustic waves with an array of equally\nspaced two-dimensional microcavities on an otherwise flat plate without external boundary-layer flow. This acoustic\nscattering problem is important in the design of ultrasonic absorptive coatings for hypersonic laminar flow control.\nThe reflection coefficient, characterizing the ratio of the reflected wave amplitude to the incident wave amplitude, is\ncomputed as a function of the acoustic wave frequency and angle of incidence, for coatings of different porosities, at\nvarious acoustic Reynolds numbers relevant to hypersonic flight. Overall, the numerical results validate predictions\nfrom existing theoretical modeling. In general, the amplitude of the reflection coefficient has local minima at some\nspecific frequencies. A simple model to predict these frequencies is presented. The simulations also highlight the\npresence of resonant acoustic modes caused by coupling of small-scale scattered waves near the coating surface.\nFinally, the cavity depth and the porosity are identified as the most important parameters for coating design.\nGuidelines for the choice of these parameters are suggested.", "doi": "10.2514/1.40811", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2010-02", "series_number": "2", "volume": "48", "issue": "2", "pages": "267-274" }, { "id": "authors:qe3p9-faa39", "collection": "authors", "collection_id": "qe3p9-faa39", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110321-092415409", "type": "article", "title": "Development of Arbitrary-Order Hermite Methods for Simulation and Analysis of Turbulent Jet Noise", "author": [ { "family_name": "Appel\u00f6", "given_name": "Daniel", "clpid": "Appel\u00f6-D" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hagstrom", "given_name": "Thomas", "clpid": "Hagstrom-T" }, { "family_name": "Inkman", "given_name": "Matthew", "clpid": "Inkman-M" } ], "abstract": "In this short note a brief description of Hermite methods is given. Previous and ongoing development of arbitrary\norder Hermite methods for the simulation of turbulent jets is also presented. In addition we outline how Hermite\nmethods can be hybridized with discontinuous Galerkin methods to handle boundary conditions in a straightforward\nway.", "doi": "10.1016/j.proeng.2010.09.003", "issn": "1877-7058", "publisher": "Elsevier", "publication": "Procedia Engineering", "publication_date": "2010", "volume": "6", "pages": "19-27" }, { "id": "authors:7x47c-dnj36", "collection": "authors", "collection_id": "7x47c-dnj36", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110318-153515340", "type": "article", "title": "Parabolized stability equation models of large-scale jet mixing noise", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Samanta", "given_name": "Arnab", "clpid": "Samanta-Arnab" }, { "family_name": "Gudmundsson", "given_name": "Kristjan", "clpid": "Gudmundsson-K" } ], "contributor": [ { "family_name": "Astley", "given_name": "R. J.", "clpid": "Astley-R-J" }, { "family_name": "Gabard", "given_name": "G.", "clpid": "Gabard-G" } ], "abstract": "We report on the development of parabolized stability equation models to predict the evolution of low frequencies, large-scale wavepacket\n structures in turbulent jets and their radiated sound. We consider\n computations and data corresponding to high subsonic and supersonic\n jets from circular nozzles. Previous methods are extended to consider\n nonlinear interactions amongst the waves and use a Kirchhoff-surface\n type approach to project the near-field wavepacket amplitudes to the\n far-field. Linear PSE, whose initial conditions are chosen to provide\n an overall amplitude reference, show excellent agreement for the\n wavepacket amplitudes and phases with microphone array data just\n outside the jet shear layers, especially when the microphone data are\n processed to filter out contributions from uncorrelated fluctuations.\n Far-field sound predictions based on the linear PSE are also in\n reasonable agreement with far-field data. In order to investigate\n nonlinearity, we use an LES database to evaluate initial conditions for\n the PSE modes, and then compare their later evolution along the jet.\n Preliminary cases show some sensitivity to the initial amplitudes and\n their phases, and that nonlinear effects may be important in predicting\n the far-field sound based on the initial (near-nozzle) spectrum of\n disturbances.", "doi": "10.1016/j.proeng.2010.09.008", "issn": "1877-7058", "publisher": "Elsevier", "publication": "Procedia Engineering", "publication_date": "2010", "volume": "6", "pages": "64-73" }, { "id": "authors:rtdx6-f4z42", "collection": "authors", "collection_id": "rtdx6-f4z42", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20091223-094524380", "type": "article", "title": "Lift Response of a Stalled Wing to Pulsatile Disturbances", "author": [ { "family_name": "Williams", "given_name": "David R.", "clpid": "Williams-D-R" }, { "family_name": "Tadmor", "given_name": "Gilead", "clpid": "Tadmor-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Kerstens", "given_name": "Wesley", "clpid": "Kerstens-W" }, { "family_name": "Quach", "given_name": "Vien", "clpid": "Quach-V" }, { "family_name": "Buntain", "given_name": "Seth", "clpid": "Buntain-S" } ], "abstract": "The transient lift response of a low-Reynolds-number wing subjected to small amplitude pulsatile disturbances is investigated. The wing has a small aspect ratio and a semicircular planform, and it is fully stalled at a 20 deg angle of attack. Microvalve actuators distributed along the leading edge of the wing produce the transient disturbance. It is shown that the lift response to a single pulse increases with increasing actuator supply pressure and that the lift response curves are similar to each other when scaled by the total impulse. Furthermore, for fixed actuator supply pressure, the amplitude and total impulse of the transient lift response curve increases with increasing external flow speed. In this case, the lift response curves are similar when scaled by the dynamic pressure. The lift response to a single pulse can be treated as a filter kernel, and it can be used to predict the lift time history for the arbitrary actuator input signals. The kernel is similar in shape to transient measurements obtained by other investigators on two-dimensional wings and flaps. Comparisons between the model predictions and the experiments using multiple pulse inputs and square-wave modulated input signals at low frequencies are presented.", "doi": "10.2514/1.45407", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2009-12", "series_number": "12", "volume": "47", "issue": "12", "pages": "3031-3037" }, { "id": "authors:jcp2f-xng50", "collection": "authors", "collection_id": "jcp2f-xng50", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20090916-093310681", "type": "article", "title": "Improvement of acoustic theory of ultrasonic waves in dilute bubbly liquids", "author": [ { "family_name": "Ando", "given_name": "Keita", "clpid": "Ando-Keita" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "abstract": "The theory of the acoustics of dilute bubbly liquids is reviewed, and the dispersion relation is modified by including the effect of liquid compressibility on the natural frequency of the bubbles. The modified theory is shown to more accurately predict the trend in measured attenuation of ultrasonic waves. The model limitations associated with such high-frequency waves are discussed.", "doi": "10.1121/1.3182858", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2009-07-27", "series_number": "3", "volume": "126", "issue": "3", "pages": "EL69" }, { "id": "authors:vzca5-fha04", "collection": "authors", "collection_id": "vzca5-fha04", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20090904-093528277", "type": "article", "title": "Numerical simulations of non-spherical bubble collapse", "author": [ { "family_name": "Johnsen", "given_name": "Eric", "clpid": "Johnsen-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A high-order accurate shock- and interface-capturing scheme is used to simulate the collapse of a gas bubble in water. In order to better understand the damage caused by collapsing bubbles, the dynamics of the shock-induced and Rayleigh collapse of a bubble near a planar rigid surface and in a free field are analysed. Collapse times, bubble displacements, interfacial velocities and surface pressures are quantified as a function of the pressure ratio driving the collapse and of the initial bubble stand-off distance from the wall; these quantities are compared to the available theory and experiments and show good agreement with the data for both the bubble dynamics and the propagation of the shock emitted upon the collapse. Non-spherical collapse involves the formation of a re-entrant jet directed towards the wall or in the direction of propagation of the incoming shock. In shock-induced collapse, very high jet velocities can be achieved, and the finite time for shock propagation through the bubble may be non-negligible compared to the collapse time for the pressure ratios of interest. Several types of shock waves are generated during the collapse, including precursor and water-hammer shocks that arise from the re-entrant jet formation and its impact upon the distal side of the bubble, respectively. The water-hammer shock can generate very high pressures on the wall, far exceeding those from the incident shock. The potential damage to the neighbouring surface is quantified by measuring the wall pressure. The range of stand-off distances and the surface area for which amplification of the incident shock due to bubble collapse occurs is determined.", "doi": "10.1017/S0022112009006351", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2009-06-25", "volume": "629", "pages": "231-262" }, { "id": "authors:27fx2-jcb90", "collection": "authors", "collection_id": "27fx2-jcb90", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20090721-120325880", "type": "article", "title": "A high-order super-grid-scale absorbing layer and its application to linear hyperbolic systems", "author": [ { "family_name": "Appel\u00f6", "given_name": "Daniel", "clpid": "Appel\u00f6-D" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We continue the development of the super-grid-scale model initiated in [T. Colonius, H. Ran, A super-grid-scale model for simulating compressible flow on unbounded domains, J. Comput. Phys. 182 (1) (2002) 191\u2013212] and consider its application to linear hyperbolic systems. The super-grid-scale model consists of two parts: reduction of an unbounded to a bounded domain by a smooth coordinate transformation and a damping of those scales. For linear problems the super-grid scales are analogous to spurious numerical waves. We damp these waves by high-order undivided differences. We compute reflection coefficients for different orders of the damping and find that significant improvements are obtained when high-order damping is used.\nIn numerical experiments with Maxwell's equations, we show that when the damping is of high order, the error from the boundary condition converges at the order of the interior scheme. We also demonstrate that the new method achieves perfectly matched layer-like accuracy.\nWhen applied to linear hyperbolic systems the stability of the super-grid-scale method follows from its construction. This makes our method particularly suitable for problems for which perfectly matched layers are unstable. We present results for two such problems: elastic waves in anisotropic media and isotropic elastic waves in wave guides with traction-free surfaces.", "doi": "10.1016/j.jcp.2009.02.030", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2009-06-20", "series_number": "11", "volume": "228", "issue": "11", "pages": "4200-4217" }, { "id": "authors:mjba7-nzm77", "collection": "authors", "collection_id": "mjba7-nzm77", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100819-150529622", "type": "article", "title": "Numerical simulation of the sound radiated by a turbulent vortex ring", "author": [ { "family_name": "Ran", "given_name": "Hongyu", "clpid": "Ran-Hongyu" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The acoustic field radiated by a turbulent vortex ring is studied. Direct Numerical Simulations\n(ONS) of the fully compressible, three-dimensional Navier-Stokes equations are used to generate\nan axisymmetric vortex ring to which 3D stochastic disturbances are added. The disturbances\ncause instability and turbulent transition of the vortex ring. Detailed information about temporal\nevolution of sound pressure level, spectrum and directivity associated with modes of oscillation\nand their turbulent breakdown are investigated. The peak frequency agrees well with experiments.\nand the modal directivities agree well with predictions of vortex sound theory. Based on the self-similar\ndecay of the turbulent near field, the self-similar decay of the sound field is investigated.\nWe also explore the connections with jet noise by modeling the jet as a de-correlated train of\nvortex rings.", "issn": "1475-472X", "publisher": "Multi-Science Publishing", "publication": "International Journal of Aeroacoustics", "publication_date": "2009-06", "series_number": "4", "volume": "8", "issue": "4", "pages": "317-336" }, { "id": "authors:phrw0-8sg81", "collection": "authors", "collection_id": "phrw0-8sg81", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100819-150126006", "type": "article", "title": "Turbulence and sound-field POD analysis of a turbulent jet", "author": [ { "family_name": "Freund", "given_name": "J. B.", "clpid": "Freund-J-B" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A Proper Orthogonal Decomposition (POD) is constructed for a Mach 0.9 turbulent jet using a\nwell-validated direct numerical simulation database. Norms are defined based on near-field\nvolume integrals of pressure, turbulence kinetic energy, streamwisc velocity, and total enthalpy,\ntwo-dimensional integrals of streamswise velocity (to match experimental measurements), and\nfar-field integrals of pressure over a sphere. We find substantially different POD modes for the\ndifferent norms, and their efficiency at representing the full data is strongly dependent upon the\nnorm and specifically which data we attempt to represent. To reproduce near-field turbulence\nstatistics requires relatively few modes computed by a kinetic energy or pressure norm. However.\na large number of the POD modes computed using a near-field norm are required to represent the\nsound field. The dominant near-field POD modes computed with either the near-field pressure\nnorm or the sound field norm have the structure of wave packets.", "issn": "1475-472X", "publisher": "Multi-Science Publishing", "publication": "International Journal of Aeroacoustics", "publication_date": "2009-06", "series_number": "4", "volume": "8", "issue": "4", "pages": "337-354" }, { "id": "authors:frnmk-28536", "collection": "authors", "collection_id": "frnmk-28536", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20090629-092825317", "type": "article", "title": "Three-dimensional flows around low-aspect-ratio flat-plate wings at low Reynolds numbers", "author": [ { "family_name": "Kunihiko", "given_name": "Taira", "clpid": "Kunihiko-Taira" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Three-dimensional flows over impulsively translated low-aspect-ratio flat plates are investigated for Reynolds numbers of 300 and 500, with a focus on the unsteady vortex dynamics at post-stall angles of attack. Numerical simulations, validated by an oil tow-tank experiment, are performed to study the influence of aspect ratio, angle of attack and planform geometry on the wake vortices and the resulting forces on the plate. Immediately following the impulsive start, the separated flows create wake vortices that share the same topology for all aspect ratios. At large time, the tip vortices significantly influence the vortex dynamics and the corresponding forces on the wings. Depending on the aspect ratio, angle of attack and Reynolds number, the flow at large time reaches a stable steady state, a periodic cycle or aperiodic shedding. For cases of high angles of attack, an asymmetric wake develops in the spanwise direction at large time. The present results are compared to higher Reynolds number flows. Some non-rectangular planforms are also considered to examine the difference in the wakes and forces. After the impulsive start, the time at which maximum lift occurs is fairly constant for a wide range of flow conditions during the initial transient. Due to the influence of the tip vortices, the three-dimensional dynamics of the wake vortices are found to be quite different from the two-dimensional von K\u00e1rm\u00e1n vortex street in terms of stability and shedding frequency.", "doi": "10.1017/S0022112008005314", "issn": "0022-1120", "publisher": "Cambridge", "publication": "Journal of Fluid Mechanics", "publication_date": "2009-03-25", "volume": "623", "pages": "187-207" }, { "id": "authors:nx7jr-csp53", "collection": "authors", "collection_id": "nx7jr-csp53", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20091009-082632397", "type": "article", "title": "Effect of Tip Vortices in Low-Reynolds-Number Poststall Flow Control", "author": [ { "family_name": "Taira", "given_name": "Kunihiko", "orcid": "0000-0002-3762-8075", "clpid": "Taira-Kunihiko" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We numerically investigate the application of steady blowing to three-dimensional stalled flows around low-aspect-ratio rectangular flat-plate wings at a Reynolds number of 300. The objective of this study is to explore\ntechniques to enhance lift by directly modifying the dynamics of the wake vortices. Out of various combinations of\nforcing location and direction considered, we identify two configurations that provide significant lift enhancement. In\nthese cases, actuation appears to strengthen the tip vortices for increased downward induced velocity upon the\nleading-edge vortices. This in turn moves the low-pressure core directly above the top surface of the wing to greatly\nenhance lift.", "doi": "10.2514/1.40615", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2009-03", "series_number": "3", "volume": "47", "issue": "3", "pages": "749-756" }, { "id": "authors:ckqq1-wqs11", "collection": "authors", "collection_id": "ckqq1-wqs11", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:JOHjasa08", "type": "article", "title": "Shock-induced collapse of a gas bubble in shockwave lithotripsy", "author": [ { "family_name": "Johnsen", "given_name": "Eric", "clpid": "Johnsen-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "The shock-induced collapse of a pre-existing nucleus near a solid surface in the focal region of a lithotripter is investigated. The entire flow field of the collapse of a single gas bubble subjected to a lithotripter pulse is simulated using a high-order accurate shock- and interface-capturing scheme, and the wall pressure is considered as an indication of potential damage. Results from the computations show the same qualitative behavior as that observed in experiments: a re-entrant jet forms in the direction of propagation of the pulse and penetrates the bubble during collapse, ultimately hitting the distal side and generating a water-hammer shock. As a result of the propagation of this wave, wall pressures on the order of 1 GPa may be achieved for bubbles collapsing close to the wall. The wall pressure decreases with initial stand-off distance and pulse width and increases with pulse amplitude. For the stand-off distances considered in the present work, the wall pressure due to bubble collapse is larger than that due to the incoming shockwave; the region over which this holds may extend to ten initial radii. The present results indicate that shock-induced collapse is a mechanism with high potential for damage in shockwave lithotripsy.", "doi": "10.1121/1.2973229", "issn": "0001-4966", "publisher": "Acoustical Society of America", "publication": "Journal of the Acoustical Society of America", "publication_date": "2008-10", "series_number": "4", "volume": "124", "issue": "4", "pages": "2011-2020" }, { "id": "authors:knvkc-frk46", "collection": "authors", "collection_id": "knvkc-frk46", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20100505-140942697", "type": "article", "title": "A fast immersed boundary method using a nullspace approach and multi-domain far-field boundary conditions", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Taira", "given_name": "Kunihiko", "orcid": "0000-0002-3762-8075", "clpid": "Taira-Kunihiko" } ], "abstract": "We report on the continued development of a projection approach for implementing the immersed boundary method for incompressible flows in two and three dimensions. Boundary forces and pressure are regarded as Lagrange multipliers that enable the no-slip and divergence-free constraints to be implicitly determined to arbitrary precision with no associated time-step restrictions. In order to accelerate the method, we further implement a nullspace (discrete streamfunction) method that allows the divergence-free constraint to be automatically satisfied to machine roundoff. By employing a fast sine transform technique, the linear system to determine the forces can be solved efficiently with direct or iterative techniques. A multi-domain technique is developed in order to improve far-field boundary conditions that are compatible with the fast sine transform and account for the extensive potential flow induced by the body as well as vorticity that advects/diffuses to large distance from the body. The multi-domain and fast techniques are validated by comparing to the exact solutions for the potential flow induced by stationary and propagating Oseen vortices and by an impulsively-started circular cylinder. Speed-ups of more than an order-of-magnitude are achieved with the new method.", "doi": "10.1016/j.cma.2007.08.014", "issn": "0045-7825", "publisher": "Elsevier", "publication": "Computer Methods in Applied Mechanics and Engineering", "publication_date": "2008-04-15", "series_number": "25-28", "volume": "197", "issue": "25-28", "pages": "2131-2146" }, { "id": "authors:rj4h4-z6k80", "collection": "authors", "collection_id": "rj4h4-z6k80", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:COLpof08", "type": "article", "title": "Statistical equilibrium of bubble oscillations in dilute bubbly flows", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hagmeijer", "given_name": "Rob", "clpid": "Hagmeijer-R" }, { "family_name": "Ando", "given_name": "Keita", "clpid": "Ando-Keita" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "abstract": "The problem of predicting the moments of the distribution of bubble radius in bubbly flows is considered. The particular case where bubble oscillations occur due to a rapid (impulsive or step change) change in pressure is analyzed, and it is mathematically shown that in this case, inviscid bubble oscillations reach a stationary statistical equilibrium, whereby phase cancellations among bubbles with different sizes lead to time-invariant values of the statistics. It is also shown that at statistical equilibrium, moments of the bubble radius may be computed using the period-averaged bubble radius in place of the instantaneous one. For sufficiently broad distributions of bubble equilibrium (or initial) radius, it is demonstrated that bubble statistics reach equilibrium on a time scale that is fast compared to physical damping of bubble oscillations due to viscosity, heat transfer, and liquid compressibility. The period-averaged bubble radius may then be used to predict the slow changes in the moments caused by the damping. A benefit is that period averaging gives a much smoother integrand, and accurate statistics can be obtained by tracking as few as five bubbles from the broad distribution. The period-averaged formula may therefore prove useful in reducing computational effort in models of dilute bubbly flow wherein bubbles are forced by shock waves or other rapid pressure changes, for which, at present, the strong effects caused by a distribution in bubble size can only be accurately predicted by tracking thousands of bubbles. Some challenges associated with extending the results to more general (nonimpulsive) forcing and strong two-way coupled bubbly flows are briefly discussed.", "doi": "10.1063/1.2912517", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2008-04", "series_number": "4", "volume": "20", "issue": "4", "pages": "Art. No. 040902" }, { "id": "authors:tzsbh-1f356", "collection": "authors", "collection_id": "tzsbh-1f356", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:BREjfm08", "type": "article", "title": "Three-dimensional instabilities in compressible flow over open cavities", "author": [ { "family_name": "Br\u00e8s", "given_name": "Guillaume A.", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-G-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Direct numerical simulations are performed to investigate the three-dimensional stability of compressible flow over open cavities. A linear stability analysis is conducted to search for three-dimensional global instabilities of the two-dimensional mean flow for cavities that are homogeneous in the spanwise direction. The presence of such instabilities is reported for a range of flow conditions and cavity aspect ratios. For cavities of aspect ratio (length to depth) of 2 and 4, the three-dimensional mode has a spanwise wavelength of approximately one cavity depth and oscillates with a frequency about one order of magnitude lower than two-dimensional Rossiter (flow/acoustics) instabilities. A steady mode of smaller spanwise wavelength is also identified for square cavities. The linear results indicate that the instability is hydrodynamic (rather than acoustic) in nature and arises from a generic centrifugal instability mechanism associated with the mean recirculating vortical flow in the downstream part of the cavity. These three-dimensional instabilities are related to centrifugal instabilities previously reported in flows over backward-facing steps, lid-driven cavity flows and Couette flows. Results from three-dimensional simulations of the nonlinear compressible Navier\u2013Stokes equations are also reported. The formation of oscillating (and, in some cases, steady) spanwise structures is observed inside the cavity. The spanwise wavelength and oscillation frequency of these structures agree with the linear analysis predictions. When present, the shear-layer (Rossiter) oscillations experience a low-frequency modulation that arises from nonlinear interactions with the three-dimensional mode. The results are consistent with observations of low-frequency modulations and spanwise structures in previous experimental and numerical studies on open cavity flows.", "doi": "10.1017/S0022112007009925", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2008-03-25", "volume": "599", "pages": "309-339" }, { "id": "authors:g69gt-k6264", "collection": "authors", "collection_id": "g69gt-k6264", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:PREpof07", "type": "article", "title": "A reduced-order model of diffusive effects on the dynamics of bubbles", "author": [ { "family_name": "Preston", "given_name": "A. T.", "clpid": "Preston-A-T" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "C. E.", "clpid": "Brennen-C-E" } ], "abstract": "We propose a new reduced-order model for spherical bubble dynamics that accurately captures the effects of heat and mass diffusion. The objective is to reduce the full system of partial differential equations to a set of coupled ordinary differential equations that are efficient enough to implement into complex bubbly flow computations. Comparisons to computations of the full partial differential equations and of other reduced-order models are used to validate the model and establish its range of validity.", "doi": "10.1063/1.2825018", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2007-12", "series_number": "12", "volume": "19", "issue": "12", "pages": "Art. No. 123302" }, { "id": "authors:qtb98-cpe50", "collection": "authors", "collection_id": "qtb98-cpe50", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190718-165125075", "type": "article", "title": "A Cumulative Shear Mechanism for Tissue Damage Initiation in Shock-Wave Lithotripsy", "author": [ { "family_name": "Freund", "given_name": "Jonathan B.", "clpid": "Freund-J-B" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Evan", "given_name": "Andrew P.", "clpid": "Evan-A-P" } ], "abstract": "Evidence suggests that inertial cavitation plays an important role in the renal injury incurred during shock-wave lithotripsy. However, it is unclear how tissue damage is initiated, and significant injury typically occurs only after a sufficient dose of shock waves. Although it has been suggested that shock-induced shearing might initiate injury, estimates indicate that individual shocks do not produce sufficient shear to do so. In this paper, we hypothesize that the cumulative shear of the many shocks is damaging. This mechanism depends on whether there is sufficient time between shocks for tissue to relax to its unstrained state. We investigate the mechanism with a physics-based simulation model, wherein the basement membranes that define the tubules and vessels in the inner medulla are represented as elastic shells surrounded by viscous fluid. Material properties are estimated from in-vitro tests of renal basement membranes and documented mechanical properties of cells and extracellular gels. Estimates for the net shear deformation from a typical lithotripter shock (similar to 0.1 %) are found from a separate dynamic shock simulation. The results suggest that the larger interstitial volume (similar to 40%) near the papilla tip gives the tissue there a relaxation time comparable to clinical shock delivery rates (similar to 1 Hz), thus allowing shear to accumulate. Away from the papilla tip, where the interstitial volume is smaller (similar to 20%), the model tissue relaxes completely before the next shock would be delivered. Implications of the model are that slower delivery rates and broader focal zones should both decrease injury, consistent with some recent observations.", "doi": "10.1016/j.ultrasmedbio.2007.03.001", "issn": "0301-5629", "publisher": "Elsevier", "publication": "Ultrasound in Medicine and Biology", "publication_date": "2007-09", "series_number": "9", "volume": "33", "issue": "9", "pages": "1495-1503" }, { "id": "authors:5cggc-2dg45", "collection": "authors", "collection_id": "5cggc-2dg45", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190718-165128384", "type": "article", "title": "The Immersed Boundary Method: A Projection Approach", "author": [ { "family_name": "Taira", "given_name": "Kunihiko", "orcid": "0000-0002-3762-8075", "clpid": "Taira-Kunihiko" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A new formulation of the immersed boundary method with a structure algebraically identical to the traditional fractional step method is presented for incompressible flow over bodies with prescribed surface motion. Like previous methods, a boundary force is applied at the immersed surface to satisfy the no-slip constraint. This extra constraint can be added to the incompressible Navier\u2013Stokes equations by introducing regularization and interpolation operators. The current method gives prominence to the role of the boundary force acting as a Lagrange multiplier to satisfy the no-slip condition. This role is analogous to the effect of pressure on the momentum equation to satisfy the divergence-free constraint. The current immersed boundary method removes slip and non-divergence-free components of the velocity field through a projection. The boundary force is determined implicitly without any constitutive relations allowing the present formulation to use larger CFL numbers compared to some past methods. Symmetry and positive-definiteness of the system are preserved such that the conjugate gradient method can be used to solve for the flow field. Examples show that the current formulation achieves second-order temporal accuracy and better than first-order spatial accuracy in L2-norms for one- and two-dimensional test problems. Results from two-dimensional simulations of flows over stationary and moving cylinders are in good agreement with those from previous experimental and numerical studies.", "doi": "10.1016/j.jcp.2007.03.005", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2007-08-10", "series_number": "2", "volume": "225", "issue": "2", "pages": "2118-2137" }, { "id": "authors:smjdh-yhx43", "collection": "authors", "collection_id": "smjdh-yhx43", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190718-165128280", "type": "article", "title": "Implementation of WENO Schemes in Compressible Multicomponent Flow Problems", "author": [ { "family_name": "Johnsen", "given_name": "Eric", "clpid": "Johnsen-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "High-order accurate shock-capturing schemes are capable of properly resolving discontinuities with correct wave speeds in single-fluid Riemann problems. However, when different fluids are present, oscillations develop at interfaces. A class of existing interface-capturing methods that suppress these oscillations is based on first- and second-order accurate reconstructions with Roe solvers. In this paper, we extend these methods to high-order accurate WENO schemes and the HLLC approximate Riemann solver. In particular, we show that a finite volume formulation where the appropriately averaged primitive variables are reconstructed leads to the oscillation-free advection of an isolated interface. Furthermore, numerical experiments show no spurious oscillations for problems where shockwaves and interfaces interact. We solve the Euler equations supplemented by a stiffened equation of state to model flows of gas and liquid components. Our method is high-order accurate, quasi-conservative, shock-capturing and interface-capturing; these properties are additionally verified by considering one-dimensional multicomponent Riemann problems and a two-dimensional shock\u2013bubble interaction.", "doi": "10.1016/j.jcp.2006.04.018", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2006-12-10", "series_number": "2", "volume": "219", "issue": "2", "pages": "715-732" }, { "id": "authors:z459a-4n506", "collection": "authors", "collection_id": "z459a-4n506", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:SUZjfm06b", "type": "article", "title": "Instability waves in a subsonic round jet detected using a near-field phased microphone array", "author": [ { "family_name": "Suzuki", "given_name": "Takao", "clpid": "Suzuki-Takao" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We propose a diagnostic technique to detect instability waves in a subsonic round jet using a phased microphone array. The detection algorithm is analogous to the beam-forming technique, which is typically used with a far-field microphone array to localize noise sources. By replacing the reference solutions used in the conventional beam-forming with eigenfunctions from linear stability analysis, the amplitudes of instability waves in the axisymmetric and first two azimuthal modes are inferred. Experimental measurements with particle image velocimetry and a database from direct numerical simulation are incorporated to design a conical array that is placed just outside the mixing layer near the nozzle exit. The proposed diagnostic technique is tested in experiments by checking for consistency of the radial decay, streamwise evolution and phase correlation of hydrodynamic pressure. The results demonstrate that in a statistical sense, the pressure field is consistent with instability waves evolving in the turbulent mean flow from the nozzle exit to the end of the potential core, particularly near the most amplified frequency of each azimuthal mode. We apply this technique to study the effects of jet Mach number and temperature ratio on the azimuthal mode balance and evolution of instability waves. We also compare the results from the beam-forming algorithm with the proper orthogonal decomposition and discuss some implications for jet noise.", "doi": "10.1017/S0022112006001613", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2006-10", "volume": "565", "pages": "197-226" }, { "id": "authors:kq0p5-yz865", "collection": "authors", "collection_id": "kq0p5-yz865", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190718-165125363", "type": "article", "title": "Progress in Lithotripsy Research", "author": [ { "family_name": "Bailey", "given_name": "Michael R.", "clpid": "Bailey-M-R" }, { "family_name": "McAteer", "given_name": "James A.", "clpid": "McAteer-J-A" }, { "family_name": "Pishchalnikov", "given_name": "Yuri A.", "clpid": "Pishchalnikov-Y-A" }, { "family_name": "Hamilton", "given_name": "Mark F.", "clpid": "Hamilton-Mark-F" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Shock wave lithotripsy (SWL) for the non-invasive treatment of kidney stones was introduced in the United States in 1984. SWL virtually eliminated the need for open surgery to remove kidney stones, and it did not take long for physicians and patients to endorse this revolutionary technology. Early reports told of the efficient removal of even the most troublesome stones without apparent complications, and SWL quickly became the \"treatment modality of choice.\" It was not long, however, before concerned physicians began to report the occurrence of adverse effects in SWL, particularly involving vascular trauma and including cases of severe hemorrhage in the kidney and acute renal failure \u2014 significant side effects of serious consequence. Researchers quickly recognized the challenge and opportunity to determine the mechanisms of shock wave action in lithotripsy, and in 1988, the Acoustical Society of America held the first in a series of popular sessions devoted to the topic of shock waves in medicine. The goal of the inaugural session was to improve the fundamental understanding of lithotripsy \u2014 to bring better devices and treatments to patients. The goal of this paper is to report on progress in this effort.", "issn": "1557-0215", "publisher": "Acoustical Society of America", "publication": "Acoustics Today", "publication_date": "2006-04", "series_number": "2", "volume": "2", "issue": "2", "pages": "18-29" }, { "id": "authors:cs1cy-78146", "collection": "authors", "collection_id": "cs1cy-78146", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:ROWjfm06", "type": "article", "title": "Linear models for control of cavity flow oscillations", "author": [ { "family_name": "Rowley", "given_name": "Clarence W.", "orcid": "0000-0002-9099-5739", "clpid": "Rowley-C-W" }, { "family_name": "Williams", "given_name": "David R.", "clpid": "Williams-D-R" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "MacMynowski", "given_name": "Douglas G.", "orcid": "0000-0003-1987-9417", "clpid": "MacMartin-D-G" } ], "abstract": "Models for understanding and controlling oscillations in the flow past a rectangular cavity are developed. These models may be used to guide control designs, to understand performance limits of feedback, and to interpret experimental results. Traditionally, cavity oscillations are assumed to be self-sustained: no external disturbances are necessary to maintain the oscillations, and amplitudes are limited by nonlinearities. We present experimental data which suggests that in some regimes, the oscillations may not be self-sustained, but lightly damped: oscillations are sustained by external forcing, such as boundary-layer turbulence. In these regimes, linear models suffice to describe the behaviour, and the final amplitude of oscillations depends on the characteristics of the external disturbances. These linear models are particularly appropriate for describing cavities in which feedback has been used for noise suppression, as the oscillations are small and nonlinearities are less likely to be important. It is shown that increasing the gain too much in such feedback control experiments can lead to a peak-splitting phenomenon, which is explained by the linear models. Fundamental performance limits indicate that peak splitting is likely to occur for narrow-bandwidth actuators and controllers.", "doi": "10.1017/S0022112005007299", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2006-01-25", "volume": "547", "pages": "317-330" }, { "id": "authors:yfga7-6tv34", "collection": "authors", "collection_id": "yfga7-6tv34", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:SUZjfm04", "type": "article", "title": "Vortex shedding in a two-dimensional diffuser: theory and simulation of separation control by periodic mass injection", "author": [ { "family_name": "Suzuki", "given_name": "Takao", "clpid": "Suzuki-Takao" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Pirozzoli", "given_name": "Sergio", "clpid": "Pirozzoli-S" } ], "abstract": "We develop a reduced-order model for large-scale unsteadiness (vortex shedding) in a two-dimensional diffuser and use the model to show how periodic mass injection near the separation point reduces stagnation pressure loss. The model estimates the characteristic frequency of vortex shedding and stagnation pressure loss by accounting for the accumulated circulation due to the vorticity flux into the separated region. The stagnation pressure loss consists of two parts: a steady part associated with the time-averaged static pressure distribution on the wall, and an unsteady part caused by vortex shedding. To validate the model, we perform numerical simulations of compressible unsteady laminar diffuser flows in two dimensions. The model and simulation show good agreement as we vary the Mach number and the area ratio of the diffuser. To investigate the effects of periodic mass injection near the separation point, we also perform simulations over a range of the injection frequencies. Periodic mass injection causes vortices to be pinched off with a smaller size as observed in experiments. Consequently, their convective velocity is increased, absorption of circulation from the wall is enhanced, and the reattached point is shifted upstream. Thus, in accordance with the model, the stagnation pressure loss, particularly the unsteady part, is substantially reduced even though the separation point is nearly unchanged. This study helps explain experimental results of separation control using unsteady mass injection in diffusers and on airfoils.", "doi": "10.1017/S0022112004001405", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2004-12-10", "volume": "520", "pages": "187-213" }, { "id": "authors:fbxcw-3en51", "collection": "authors", "collection_id": "fbxcw-3en51", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190718-165126641", "type": "article", "title": "A Model for Kidney Tissue Damage under High Speed Loading", "author": [ { "family_name": "Weinberg", "given_name": "Kerstin", "clpid": "Weinberg-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "abstract": "In a medical procedure to comminute kidney stones the patient is subjected to hypersonic waves focused at the stone. Unfortunately such shock waves also damage the surrounding kidney tissue. We present here a model for the mechanical response of the soft tissue to such a high speed loading regime. \n\nThe material model combines shear induced plasticity with irreversible volumetric expansion as induced, e.g., by cavitating bubbles. The theory is based on a multiplicative decomposition of the deformation gradient and on an internal variable formulation of continuum thermodynamics. By the use of logarithmic and exponential mappings the stress update algorithms are extended from small\u2010strain to the finite deformation range. In that way the time\u2010discretized version of the porous\u2010viscoplastic constitutive updates is described in a fully variational manner. \n\nBy numerical experiments we study the shock\u2010wave propagation into the tissue and analyze the resulting stress states. A first finite element simulation shows localized damage in the human kidney.", "doi": "10.1002/pamm.200410098", "issn": "1617-7061", "publisher": "Wiley", "publication": "Proceedings in Applied Mathematics and Mechanics", "publication_date": "2004-12", "series_number": "1", "volume": "4", "issue": "1", "pages": "234-235" }, { "id": "authors:twjng-6yq95", "collection": "authors", "collection_id": "twjng-6yq95", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190214-075725469", "type": "article", "title": "Computational aeroacoustics: progress on nonlinear problems of sound generation", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Lele", "given_name": "Sanjiva K.", "clpid": "Lele-S-K" } ], "abstract": "Computational approaches are being developed to study a range of problems in aeroacoustics. These aeroacoustic problems may be classified based on the physical processes responsible for the sound radiation, and range from linear problems of radiation, refraction, and scattering in known base flows or by solid bodies, to sound generation by turbulence. In this article, we focus mainly on the challenges and successes associated with numerically simulating sound generation by turbulent flows. \n\nWe discuss a hierarchy of computational approaches that range from semi-empirical schemes that estimate the noise sources using mean-flow and turbulence statistics, to high-fidelity unsteady flow simulations that resolve the sound generation process by direct application of the fundamental conservation principles. We stress that high-fidelity methods such as Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) have their merits in helping to unravel the flow physics and the mechanisms of sound generation. They also provide rich databases for modeling activities that will ultimately be needed to improve existing predictive capabilities. \n\nSpatial and temporal discretization schemes that are well-suited for aeroacoustic calculations are analyzed, including the effects of artificial dispersion and dissipation on uniform and nonuniform grids. We stress the importance of the resolving power of the discretization as well as computational efficiency of the overall scheme. Boundary conditions to treat the flow of disturbances in and out of the computational domain, as well as methods to mimic anechoic domain extension are discussed. Test cases on some benchmark problems are included to provide a realistic assessment of several boundary condition treatments.\n\nFinally, highlights of recent progress are given using selected model problems. These include subsonic cavity noise and jet noise. In the end, the current challenges in aeroacoustic modeling and in simulation algorithms are revisited with a look toward the future developments.", "doi": "10.1016/j.paerosci.2004.09.001", "issn": "0376-0421", "publisher": "Elsevier", "publication": "Progress in Aerospace Sciences", "publication_date": "2004-08", "series_number": "6", "volume": "40", "issue": "6", "pages": "345-416" }, { "id": "authors:2fex1-0xr95", "collection": "authors", "collection_id": "2fex1-0xr95", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190214-075224908", "type": "article", "title": "Model reduction for compressible flows using POD and Galerkin projection", "author": [ { "family_name": "Rowley", "given_name": "Clarence W.", "orcid": "0000-0002-9099-5739", "clpid": "Rowley-C-W" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" } ], "abstract": "We present a framework for applying the method of proper orthogonal decomposition (POD) and Galerkin projection to compressible fluids. For incompressible flows, only the kinematic variables are important, and the techniques are well known. In a compressible flow, both the kinematic and thermodynamic variables are dynamically important, and must be included in the configuration space. We introduce an energy-based inner product which may be used to obtain POD modes for this configuration space. We then obtain an approximate version of the Navier\u2013Stokes equations, valid for cold flows at moderate Mach number, and project these equations onto a POD basis. The resulting equations of motion are quadratic, and are much simpler than projections of the full compressible Navier\u2013Stokes equations.", "doi": "10.1016/j.physd.2003.03.001", "issn": "0167-2789", "publisher": "Elsevier", "publication": "Physica D", "publication_date": "2004-02-15", "series_number": "1-2", "volume": "189", "issue": "1-2", "pages": "115-129" }, { "id": "authors:6s5ph-btg61", "collection": "authors", "collection_id": "6s5ph-btg61", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:COLarfm04", "type": "article", "title": "Modeling Artificial Boundary Conditions for Compressible Flow", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "We review artificial boundary conditions (BCs) for simulation of inflow, outflow, and far-field (radiation) problems, with an emphasis on techniques suitable for compressible turbulent shear flows. BCs based on linearization near the boundary are usually appropriate for inflow and radiation problems. A variety of accurate techniques have been developed for this case, but some robustness and implementation issues remain. At an outflow boundary, the linearized BCs are usually not accurate enough. Various ad hoc models have been proposed for the nonlinear case, including absorbing layers and fringe methods. We discuss these techniques and suggest directions for future modeling efforts.", "doi": "10.1146/annurev.fluid.36.050802.121930", "issn": "0066-4189", "publisher": "Annual Reviews, Inc.", "publication": "Annual Review of Fluid Mechanics", "publication_date": "2004-01", "volume": "36", "pages": "315-345" }, { "id": "authors:5y0p5-1gd86", "collection": "authors", "collection_id": "5y0p5-1gd86", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190718-165125193", "type": "article", "title": "Inverse-Imaging Method for Detection of a Vortex in a Channel", "author": [ { "family_name": "Suzuki", "given_name": "Takao", "clpid": "Suzuki-Takao" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A simple vortex-imaging algorithm using a least-square method is considered. The position and the circulation of a vortex convected in a channel are identified from the time history of pressure at a limited number of points on the wall. The capabilities of the algorithm are demonstrated using two-dimensional direct numerical simulations. A few observer points on one side of the wall are sufficient to detect the position and the circulation of a compact vortex to a reasonable degree of accuracy. An advanced algorithm can even detect two nearby vortices to the same degree of accuracy. Because the algorithm assumes a point vortex solution, the accuracy of the detection deteriorates as the vorticity distribution spreads.", "doi": "10.2514/2.7292", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2003-09", "series_number": "9", "volume": "41", "issue": "9", "pages": "1743-1751" }, { "id": "authors:ngawq-0xp65", "collection": "authors", "collection_id": "ngawq-0xp65", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170409-075315005", "type": "article", "title": "Cavitation Bubble Cluster Activity in the Breakage of Kidney Stones by Lithotripter Shock Waves", "author": [ { "family_name": "Pishchalnikov", "given_name": "Yuriy A.", "clpid": "Pishchalnikov-Y-A" }, { "family_name": "Sapozhnikov", "given_name": "Oleg A.", "clpid": "Sapozhnikov-O-A" }, { "family_name": "Bailey", "given_name": "Michael R.", "clpid": "Bailey-M-R" }, { "family_name": "Williams", "given_name": "James C., Jr.", "clpid": "Williams-J-C-Jr" }, { "family_name": "Cleveland", "given_name": "Robin O.", "clpid": "Cleveland-R-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Crum", "given_name": "Lawrence A.", "clpid": "Crum-L-A" }, { "family_name": "Evan", "given_name": "Andrew P.", "clpid": "Evan-A-P" }, { "family_name": "McAteer", "given_name": "James A.", "clpid": "McAteer-J-A" } ], "abstract": "Background and Purpose: There is strong evidence that cavitation bubble activity contributes to stone breakage and that shockwave-bubble interactions are involved in the tissue trauma associated with shockwave lithotripsy. Cavitation control may thus be a way to improve lithotripsy. \n\nMaterials and Methods: High-speed photography was used to analyze cavitation bubble activity at the surface of artificial and natural kidney stones during exposure to lithotripter shockwaves in vitro. \n\nResults: Numerous individual bubbles formed on the surfaces of stones, but these bubbles did not remain independent but rather combined to form clusters. Bubble clusters formed at the proximal and distal ends and at the sides of stones. Each cluster collapsed to a narrow point of impact. Collapse of the proximal cluster eroded the leading face of the stone, and the collapse of clusters at the sides of stones appeared to contribute to the growth of cracks. Collapse of the distal cluster caused minimal damage. \n\nConclusion: Cavitation-mediated damage to stones is attributable, not to the action of solitary bubbles, but to the growth and collapse of bubble clusters.", "doi": "10.1089/089277903769013568", "pmcid": "PMC2442573", "issn": "1557-900X", "publisher": "Mary Ann Leibert", "publication": "Journal of Endourology", "publication_date": "2003-09", "series_number": "7", "volume": "17", "issue": "7", "pages": "435-446" }, { "id": "authors:sbjcv-sdt76", "collection": "authors", "collection_id": "sbjcv-sdt76", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190214-081643851", "type": "article", "title": "A Super-Grid-Scale Model for Simulating Compressible Flow on Unbounded Domains", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Ran", "given_name": "Hongyu", "clpid": "Ran-Hongyu" } ], "abstract": "A new buffer region (absorbing layer, sponge layer, fringe region) technique for computing compressible flows on unbounded domains is proposed. We exploit the connection between coordinate mapping from bounded to unbounded domains and filtering of the equations of motion in Fourier space in order to develop a model to damp flow disturbances (advective and acoustic) that propagate outside an arbitrarily defined near field. This effectively simulates a free-space boundary condition. Damping the solution in the far field is accomplished in a simple and effective way by applying a filter (similar to that used in large-eddy simulation) on a mesh in Fourier space, followed by a secondary filtering of the equations on the physical grid and implementation of a model for the unresolved scales. The final form of the buffer region is given in real space, independent of any discretization of the equations. Here we use a dealiased, Fourier spectral collocation method to demonstrate the efficacy of the buffer region for several model problems: acoustic wave propagation, convection of a finite-amplitude vortex, and a viscous starting jet in two dimensions. The results compare favorably to previous nonreflecting and absorbing boundary conditions.", "doi": "10.1006/jcph.2002.7161", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2002-10-10", "series_number": "1", "volume": "182", "issue": "1", "pages": "191-212" }, { "id": "authors:tkjgt-qg954", "collection": "authors", "collection_id": "tkjgt-qg954", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:ELDjot02", "type": "article", "title": "A dilating vortex particle method for compressible flow", "author": [ { "family_name": "Eldredge", "given_name": "Jeff D.", "clpid": "Eldredge-J-D" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "abstract": "Vortex methods have become useful tools for the computation of incompressible fluid flow. In this work, a vortex particle method for the simulation of unsteady two-dimensional compressible flow is developed. By decomposing the velocity into irrotational and solenoidal parts, and using particles that are able to change volume and that carry vorticity, dilatation, enthalpy, entropy and density, the equations of motion are satisfied. Spatial derivatives are treated using the method of particle strength exchange with high-order-accurate, non-dissipative kernels. The new vortex method is applied to co-rotating and leapfrogging vortices in compressible flow, with the far acoustic field computed using a two-dimensional Kirchhoff surface.", "issn": "1468-5248", "publisher": "IOP", "publication": "Journal of Turbulence", "publication_date": "2002-09-20", "series_number": "3", "volume": "2002", "issue": "3", "pages": "Art. No. 036" }, { "id": "authors:3cj7q-41a41", "collection": "authors", "collection_id": "3cj7q-41a41", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190709-092058511", "type": "article", "title": "An evaluation of linear instability waves as sources of sound in a supersonic turbulent jet", "author": [ { "family_name": "Mohseni", "given_name": "Kamran", "orcid": "0000-0002-1382-221X", "clpid": "Mohseni-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Freund", "given_name": "Jonathan B.", "clpid": "Freund-J-B" } ], "abstract": "Mach wave radiation from supersonic jets is revisited to better quantify the extent to which linearized equations represent the details of the actual mechanism. To this end, we solve the linearized Navier\u2013Stokes equations (LNS) with precisely the same mean flow and inflow disturbances as a previous direct numerical simulation (DNS) of a perfectly expanded turbulent M = 1.92 jet [Freund et al., AIAA J. 38, 2023 (2000)]. We restrict our attention to the first two azimuthal modes, n = 0 and n = 1, which constitute most of the acoustic field. The direction of peak radiation and the peak Strouhal number matches the DNS reasonably well, which is in accord with previous experimental justification of the linear theory. However, it is found that the sound pressure level predicted by LNS is significantly lower than that from DNS. In order to investigate the discrepancy, individual frequency components of the solution are examined. These confirm that near the peak Strouhal number, particularly for the first helical mode n = 1, the amplification of disturbances in the LNS closely matches the DNS. However, away from the peak frequency (and generally for the azimuthal mode n = 0), modes in the LNS are damped while those in the DNS grow at rates comparable to those at the peak Strouhal number.", "doi": "10.1063/1.1501545", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2002-09-05", "series_number": "10", "volume": "14", "issue": "10", "pages": "3593-3600" }, { "id": "authors:tbabg-06308", "collection": "authors", "collection_id": "tbabg-06308", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190708-164305261", "type": "article", "title": "A General Deterministic Treatment of Derivatives in Particle Methods", "author": [ { "family_name": "Eldredge", "given_name": "Jeff D.", "clpid": "Eldredge-J-D" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "A unified approach to approximating spatial derivatives in particle methods using integral operators is presented. The approach is an extension of particle strength exchange, originally developed for treating the Laplacian in advection-diffusion problems. Kernels of high order of accuracy are constructed that can be used to approximate derivatives of any degree. A new treatment for computing derivatives near the edge of particle coverage is introduced, using \"one-sided\" integrals that only look for information where it is available. The use of these integral approximations in wave propagation applications is considered and their error is analyzed in this context using Fourier methods. Finally, simple tests are performed to demonstrate the characteristics of the treatment, including an assessment of the effects of particle dispersion, and their results are discussed.", "doi": "10.1006/jcph.2002.7112", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2002-08-08", "series_number": "2", "volume": "180", "issue": "2", "pages": "686-709" }, { "id": "authors:pyp8a-pv568", "collection": "authors", "collection_id": "pyp8a-pv568", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190708-164305134", "type": "article", "title": "A Vortex Particle Method for Two-Dimensional Compressible Flow", "author": [ { "family_name": "Eldredge", "given_name": "Jeff D.", "clpid": "Eldredge-J-D" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "abstract": "A vortex particle method is developed for simulating two-dimensional, unsteady compressible (low. The method uses the Helmholtz decomposition of the velocity field to separately treat the irrotational and solenoidal portions of the Now, and the particles are allowed to change volume to conserve mass. In addition to having vorticity and dilatation properties, the particles also carry density, enthalpy, and entropy. The resulting evolution equations contain terms that are computed with techniques used in some incompressible methods. Truncation of unbounded domains via a nonreflecting boundary condition is also considered. The fast multipole method is adapted to compressible particles in order to make the method computationally efficient. The new method is applied to several problems, including sound generation by corotating vortices and generation of vorticity by baroclinic torque.", "doi": "10.1006/jcph.2002.7060", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2002-07-01", "series_number": "2", "volume": "179", "issue": "2", "pages": "371-399" }, { "id": "authors:1gt3e-gqy44", "collection": "authors", "collection_id": "1gt3e-gqy44", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190709-092102278", "type": "article", "title": "Cavitation Damage to Kidney Stones in SWL Involves the Action of Bubble Clusters: New Observations Using Ultra-High Speed Imaging in Vitro", "author": [ { "family_name": "Pishchalnikov", "given_name": "Yuriy A.", "clpid": "Pishchalnikov-Y-A" }, { "family_name": "Sapozhnikov", "given_name": "Oleg A.", "clpid": "Sapozhnikov-O-A" }, { "family_name": "Bailey", "given_name": "Michael R.", "clpid": "Bailey-M-R" }, { "family_name": "Williams", "given_name": "James C.", "clpid": "Williams-J-C-Jr" }, { "family_name": "Cleveland", "given_name": "Robin O.", "clpid": "Cleveland-R-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Crum", "given_name": "Lawrence A.", "clpid": "Crum-L-A" }, { "family_name": "Evan", "given_name": "Andrew P.", "clpid": "Evan-A-P" }, { "family_name": "McAteer", "given_name": "James A.", "clpid": "McAteer-J-A" } ], "abstract": "There is strong evidence io support the idea that cavitation plays a role in stone breakage in SWL. In order to better understand how cavitation bubbles interact with stones we recorded ultra-high speed movies of the bubble activity induced by single shock waves fired by an electrohydraulic lithotripter.", "issn": "0022-5347", "publisher": "Elsevier", "publication": "Journal of Urology Supplement", "publication_date": "2002-04", "series_number": "4", "volume": "167", "issue": "4", "pages": "S261-S262" }, { "id": "authors:jzmer-yr644", "collection": "authors", "collection_id": "jzmer-yr644", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:ROWjfm02", "type": "article", "title": "On self-sustained oscillations in two-dimensional compressible flow over rectangular cavities", "author": [ { "family_name": "Rowley", "given_name": "Clarence W.", "orcid": "0000-0002-9099-5739", "clpid": "Rowley-C-W" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Basu", "given_name": "Amit J.", "clpid": "Basu-A-J" } ], "abstract": "Numerical simulations are used to investigate the resonant instabilities in two-dimensional flow past an open cavity. The compressible Navier\u2013Stokes equations are solved directly (no turbulence model) for cavities with laminar boundary layers upstream. The computational domain is large enough to directly resolve a portion of the radiated acoustic field, which is shown to be in good visual agreement with schlieren photographs from experiments at several different Mach numbers. The results show a transition from a shear-layer mode, primarily for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear-layer mode is characterized well by the acoustic feedback process described by Rossiter (1964), and disturbances in the shear layer compare well with predictions based on linear stability analysis of the Kelvin\u2013Helmholtz mode. The wake mode is characterized instead by a large-scale vortex shedding with Strouhal number independent of Mach number. The wake mode oscillation is similar in many ways to that reported by Gharib & Roshko (1987) for incompressible flow with a laminar upstream boundary layer. Transition to wake mode occurs as the length and/or depth of the cavity becomes large compared to the upstream boundary-layer thickness, or as the Mach and/or Reynolds numbers are raised. Under these conditions, it is shown that the Kelvin\u2013Helmholtz instability grows to sufficient strength that a strong recirculating flow is induced in the cavity. The resulting mean flow is similar to wake profiles that are absolutely unstable, and absolute instability may provide an explanation of the hydrodynamic feedback mechanism that leads to wake mode. Predictive criteria for the onset of shear-layer oscillations (from steady flow) and for the transition to wake mode are developed based on linear theory for amplification rates in the shear layer, and a simple model for the acoustic efficiency of edge scattering.", "doi": "10.1017/S0022112001007534", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2002-03-25", "volume": "455", "pages": "315-346" }, { "id": "authors:26e04-3q183", "collection": "authors", "collection_id": "26e04-3q183", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:PREpf02", "type": "article", "title": "A Numerical Investigation of Unsteady Bubbly Cavitating Nozzle Flows", "author": [ { "family_name": "Preston", "given_name": "A. T.", "clpid": "Preston-A-T" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "C. E.", "clpid": "Brennen-C-E" } ], "abstract": "The effects of unsteady bubbly dynamics on cavitating flow through a converging-diverging nozzle are investigated numerically. A continuum model that couples the Rayleigh-Plesset equation with the continuity and momentum equations is used to formulate unsteady, quasi-one-dimensional partial differential equations. Flow regimes studied include those where steady-state solutions exist, and those where steady-state solutions diverge at the so-called flashing instability. these latter flows consist of unsteady bubbly shock waves traveling downstream in the diverging section of the nozzle. An approximate analytical expression is developed to predict the critical backpressure for choked flow. The results agree with previous barotropic models for those flows where bubbly dynamics are not important, but show that in many instances the neglect of bubbly dynamics cannot be justified. Finally the computations show reasonable agreement with an experiment that measures the spatial variation of pressure, velocity and void fraction for steady shockfree flows, and good agreement with an experiment that measures the throat pressure and shock position for flows with bubbly shocks. In the model, damping of the bubbly raidal motion is restricted to a simple \"effective\" viscosity, but many features of the flow are shown to be independent of the specific damping mechanism.", "doi": "10.1063/1.1416497", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2002-01", "series_number": "1", "volume": "14", "issue": "1", "pages": "300-311" }, { "id": "authors:gfe62-pb328", "collection": "authors", "collection_id": "gfe62-pb328", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:MOHjfm01", "type": "article", "title": "Numerical experiments on vortex ring formation", "author": [ { "family_name": "Mohseni", "given_name": "Kamran", "orcid": "0000-0002-1382-221X", "clpid": "Mohseni-K" }, { "family_name": "Ran", "given_name": "Hongyu", "clpid": "Ran-Hongyu" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Numerical simulations are used to study the formation of vortex rings that are generated by applying a non-conservative force of long duration, simulating experimental vortex ring generation with large stroke ratio. For sufficiently long-duration forces, we investigate the extent to which properties of the leading vortex ring are invariant to the force distribution. The results confirm the existence of a universal 'formation number' defined by Gharib, Rambod & Shariff (1998), beyond which the leading vortex ring is separated from a trailing jet. We find that the formation process is governed by two non-dimensional parameters that are formed with three integrals of the motion (energy, circulation, and impulse) and the translation velocity of the leading vortex ring. Limiting values of the normalized energy and circulation of the leading vortex ring are found to be around 0.3 and 2.0, respectively, in agreement with the predictions of Mohseni & Gharib (1998). It is shown that under this normalization smaller variations in the circulation of the leading vortex ring are obtained than by scaling the circulation with parameters associated with the forcing. We show that by varying the spatial extent of the forcing or the forcing amplitude during the formation process, thicker rings with larger normalized circulation can be generated. Finally, the normalized energy and circulation of the leading vortex rings compare well with the same properties for vortices in the Norbury family with the same mean core radius.", "doi": "10.1017/S0022112000003025", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "2001", "volume": "430", "pages": "267-282" }, { "id": "authors:dv11f-bt291", "collection": "authors", "collection_id": "dv11f-bt291", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:COLpf00", "type": "article", "title": "Acoustic Saturation in Bubbly Cavitating Flow Adjacent to an Oscillating Wall", "author": [ { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "d'Auria", "given_name": "F.", "clpid": "d'Auria-F" }, { "family_name": "Brennen", "given_name": "C.E.", "clpid": "Brennen-C-E" } ], "abstract": "Bubbly cavitating flow generated by the normal oscillation of a wall bounding a semi-infinite domain of fluid is computed using a continuum two-phase flow model. Bubble dynamics are computed, on the microscale, using the Rayleigh-Plesset equation. A Lagrangian finite volume\nscheme and implicit adaptive time marching are employed to accurately resolve bubbly shock waves and other steep gradients in the flow. The one-dimensional, unsteady computations show that when the wall oscillation frequency is much smaller than the bubble natural frequency, the power radiated away from the wall is limited by an acoustic saturation effect (the radiated power becomes\nindependent of the amplitude of vibration), which is similar to that found in a pure gas. That is, for large enough vibration amplitude, nonlinear steepening of the generated waves leads to shocking of the wave train, and the dissipation associated with the jump conditions across each shock limits the radiated power. In the model, damping of the bubble volume oscillations is restricted to a simple \"effective\" viscosity. For wall oscillation frequency less than the bubble natural frequency, the saturation amplitude of the radiated field is nearly independent of any specific damping mechanism. Finally, implications for noise radiation from cavitating flows are discussed.", "doi": "10.1063/1.1313561", "issn": "1070-6631", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "2000-11", "series_number": "11", "volume": "12", "issue": "11", "pages": "2752-2761" }, { "id": "authors:fekcd-0mg70", "collection": "authors", "collection_id": "fekcd-0mg70", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104729599", "type": "article", "title": "Quasi\u2010linear gradients for capillary liquid chromatography with mass and tandem mass spectrometry", "author": [ { "family_name": "Zhou", "given_name": "Jie", "clpid": "Zhou-J" }, { "family_name": "Rusnak", "given_name": "Felicia", "clpid": "Rusnak-F" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hathaway", "given_name": "Gary M.", "clpid": "Hathaway-G" } ], "abstract": "Gradient elution, capillary liquid chromatography mass spectrometry was performed with linear, static gradients constructed by laminar flowing ten, 1.5 mu L volume steps of decreasing organic concentration into tubing of small internal diameter. Sample loading, gradient formation, and sample elution were accomplished entirely by means of a commercially available micro-autosampler and single-syringe drive pump. The procedure was simple, fast, stable, and reproducible. Essentially linear gradients were produced without the use of additional valves, mixers, pumps or software. It took less than 10 minutes to form a gradient and less than 30 minutes to construct the set of individual buffer vials. The gradients were shown to be stable to storage. One hour after forming, peak retention times were reproduced to +/-0.5%. Long-term retention time reproducibility was found to vary by +/-2%, Chromatographic resolution was comparable or superior to that obtained by gradient elution with conventional dynamic mixing and split how, The procedure was adapted with a 'peak parking' method which extended the time for generating peptide fragmentation data up to 10 minutes per peptide with the triple quadruple mass spectrometer, Using this technique, collision data were collected at the 25 femtomole level on nine of ten tryptic peptides in a single run.", "doi": "10.1002/(SICI)1097-0231(20000331)14:6%3C432::AID-RCM886%3E3.0.CO;2-T", "issn": "0951-4198", "publisher": "Wiley", "publication": "Rapid Communications in Mass Spectrometry", "publication_date": "2000-03-15", "series_number": "6", "volume": "14", "issue": "6", "pages": "432-438" }, { "id": "authors:qddde-84t61", "collection": "authors", "collection_id": "qddde-84t61", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170408-150651603", "type": "article", "title": "Application of Lighthill's Equation to a Mach 1.92 Turbulent Jet", "author": [ { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Freund", "given_name": "J. B.", "clpid": "Freund-J-B" } ], "abstract": "It has often been suggested that simulations of turbulent jets could provide the necessary sound source information for jet noise predictions via Lighthill's acoustic analogy. Such an application of Lighthill's equation is useful for two reasons. First, it provides a framework for identifying and modeling acoustic sources in a turbulent flow. Second, it may provide a less expensive means of computing the sound generated by turbulent flows because the flow equations would need to be computed only in the source region.", "doi": "10.2514/2.966", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "2000-02", "series_number": "2", "volume": "38", "issue": "2", "pages": "368-370" }, { "id": "authors:ag9g2-5de13", "collection": "authors", "collection_id": "ag9g2-5de13", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104729508", "type": "article", "title": "Discretely Nonreflecting Boundary Conditions for Linear Hyperbolic Systems", "author": [ { "family_name": "Rowley", "given_name": "Clarence W.", "orcid": "0000-0002-9099-5739", "clpid": "Rowley-C-W" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Many compressible flow and aeroacoustic computations rely on accurate nonreflecting or radiation boundary conditions. When the equations and boundary conditions are discretized using a finite-difference scheme, the dispersive nature of the discretized equations can lead to spurious numerical reflections not seen in the continuous boundary value problem. Here we construct discretely nonreflecting boundary conditions, which account for the particular finite-difference scheme used, and are designed to minimize these spurious numerical reflections. Stable boundary conditions that are local and nonreflecting to arbitrarily high order of accuracy are obtained, and test cases are presented for the linearized Euler equations. For the cases presented. reflections for a pressure pulse leaving the boundary are reduced by up to two orders of magnitude over typical ad hoc closures, and for a vorticity pulse, reflections are reduced by up to four orders of magnitude.", "doi": "10.1006/jcph.1999.6383", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2000-01-20", "series_number": "2", "volume": "157", "issue": "2", "pages": "500-538" }, { "id": "authors:8bt2k-c4t50", "collection": "authors", "collection_id": "8bt2k-c4t50", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104731562", "type": "article", "title": "Numerical Treatment of Polar Coordinate Singularities", "author": [ { "family_name": "Mohseni", "given_name": "Kamran", "orcid": "0000-0002-1382-221X", "clpid": "Mohseni-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "[no abstract]", "doi": "10.1006/jcph.1999.6382", "issn": "0021-9991", "publisher": "Elsevier", "publication": "Journal of Computational Physics", "publication_date": "2000-01-20", "series_number": "2", "volume": "157", "issue": "2", "pages": "787-795" }, { "id": "authors:tmf91-jbj73", "collection": "authors", "collection_id": "tmf91-jbj73", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190214-124414387", "type": "article", "title": "Transition of Chaotic Flow in a Radially Heated Taylor-Couette System", "author": [ { "family_name": "Kedia", "given_name": "R.", "clpid": "Kedia-R" }, { "family_name": "Hunt", "given_name": "M. L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Numerical simulations have been performed to study the stability of heated, incompressible Taylor-Couette flow for a radius ratio of 0.7 and a Prandtl number of 0.7. As Gr is increased, the Taylor cell that has the same direction of circulation as the natural convection current increases in size and the counterrotating cell becomes smaller. The flow remains axisymmetric and the average heat transfer decreases with the increase in Gr. When the cylinder is impulsively heated, the counterrotating cell vanishes and n = 1 spiral is formed for Gr = 1000. This transition marks an increase in the heat transfer due to an increase in the radial velocity component of the fluid. By slowly varying the Grashof number, the simulations demonstrate the existence of a hysteresis loop. Two different stable states with same heat transfer are found to exist at the same Grashof number. A time-delay analysis of the radial velocity and the local heat transfer coefficient time is performed to determine the dimension at two Grashof numbers. For a fixed Reynolds number of 100, the two-dimensional projection of the reconstructed attractor shows a limit cycle for Gr = \u22121700. The limit cycle behavior disappears at Gr = \u22122100, and the reconstructed attractor becomes irregular. The attractor dimension increases to about 3.2 from a value of 1 for the limit cycle case; similar values were determined for both the local heat transfer and the local radial velocity, indicating that the dynamics of the temperature variations can be inferred from that of the velocity variations.", "doi": "10.1115/1.2826018", "issn": "0022-1481", "publisher": "American Society of Mechanical Engineers", "publication": "Journal of Heat Transfer", "publication_date": "1999-08", "series_number": "3", "volume": "121", "issue": "3", "pages": "574-582" }, { "id": "authors:wh4re-8ma30", "collection": "authors", "collection_id": "wh4re-8ma30", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104729403", "type": "article", "title": "Numerical Simulations of Heat Transfer in Taylor-Couette Flow", "author": [ { "family_name": "Kedia", "given_name": "R.", "clpid": "Kedia-R" }, { "family_name": "Hunt", "given_name": "M. L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "T.", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Numerical simulations have been performed to study the effects of the gravitational and the centrifugal potentials on the stability of heated, incompressible Taylor-Couette flow. The flow is confined between two differentially heated, concentric cylinders, and the inner cylinder is allowed to rotate. The Navier-Stokes equations and the coupled energy equation are solved using a spectral method. To validate the code, comparisons are made with existing linear stability analysis and with experiments. The code is used to calculate the local and average heat transfer coefficients for a fixed Reynolds number (Re = 100) and a range of Grashof numbers. The investigation is primarily restricted to radius ratios 0.5 and 0.7 for fluids with Prandtl number of about 0.7. The variation of the local coefficients of heat transfer on the cylinder surface is investigated, and maps showing different stable states of the flow are presented. Results are also presented in terms of the equivalent conductivity, and show that heat transfer decreases with Grashof number in axisymmetric Taylor vortex flow regime, and increases with Grashof number after the flow becomes nonaxisymmetric.", "doi": "10.1115/1.2830066", "issn": "0022-1481", "publisher": "American Society of Mechanical Engineers", "publication": "Journal of Heat Transfer", "publication_date": "1998-02-01", "series_number": "1", "volume": "120", "issue": "1", "pages": "65-71" }, { "id": "authors:xdd1q-1ph06", "collection": "authors", "collection_id": "xdd1q-1ph06", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104729015", "type": "article", "title": "Numerically Nonreflecting Boundary and Interface Conditions for Compressible Flow and Aeroacoustic Computations", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "Accurate nonreflecting or radiation boundary conditions are important for effective computation of aeroacoustic and compressible flow problems. The performance of such boundary conditions is often degraded upon discretization of the equations with finite difference and time marching methods. In particular, poorly resolved, spurious sawtooth waves are generated at boundaries due to the dispersive nature of the finite difference approximation. These disturbances can lead to spurious self-sustained oscillations in the flow (self-forcing), poor convergence to steady state, and long time instability of the numerics. Exact discretely nonreflecting boundary closures (boundary conditions for a downwind artificial boundary and an upwind physical boundary) are derived by considering a one-dimensional hyperbolic equation discretized with finite difference schemes and Runge-Kutta time advancements. The current methodology leads to stable local finite difference-like boundary closures, which are nonreflecting to an essentially arbitrarily high order of accuracy. These conditions can also be applied at interfaces where there is a discontinuity in the wave speed (a shock) or where there is an abrupt change in the grid spacing. Compared to other boundary treatments, the present boundary and interface conditions can reduce spurious reflected energy in the computational domain by many orders of magnitude.", "doi": "10.2514/2.235", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "1997-07", "series_number": "7", "volume": "35", "issue": "7", "pages": "1126-1133" }, { "id": "authors:n2nnq-xsr39", "collection": "authors", "collection_id": "n2nnq-xsr39", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:COLjfm97", "type": "article", "title": "Sound generation in a mixing layer", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Lele", "given_name": "Sanjiva K.", "clpid": "Lele-S-K" }, { "family_name": "Moin", "given_name": "Parviz", "orcid": "0000-0002-0491-7065", "clpid": "Moin-Parviz" } ], "abstract": "The sound generated by vortex pairing in a two-dimensional compressible mixing layer is investigated. Direct numerical simulations (DNS) of the Navier-Stokes equations are used to compute both the near-field region and a portion of the acoustic field. The acoustic analogy due to Lilley (1974) is also solved with acoustic sources determined from the near-field data of the DNS. It is shown that several commonly made simplifications to the acoustic sources can lead to erroneous predictions for the acoustic field. Predictions based on the quadrupole form of the source terms derived by Goldstein (1976a, 1984) are in excellent agreement with the acoustic field from the DNS. However, despite the low Mach number of the flow, the acoustic far field generated by the vortex pairings cannot be described by considering compact quadrupole sources. The acoustic sources have the form of modulated wave packets and the acoustic far field is described by a superdirective model (Crighton & Huerre 1990). The presence of flow-acoustic interactions in the computed source terms causes the acoustic field predicted by the acoustic analogy to be very sensitive to small changes in the description of the source.", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "1997-01-10", "volume": "330", "pages": "375-409" }, { "id": "authors:ewbg6-j9t46", "collection": "authors", "collection_id": "ewbg6-j9t46", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104731067", "type": "article", "title": "Aeroacoustics", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "abstract": "[no abstract]", "issn": "0740-722X", "publisher": "American Institute of Aeronautics and Astronautics", "publication": "Aerospace America", "publication_date": "1995-12", "series_number": "12", "volume": "33", "issue": "12", "pages": "8" }, { "id": "authors:t35gg-bna56", "collection": "authors", "collection_id": "t35gg-bna56", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104729111", "type": "article", "title": "The scattering of sound waves by a vortex: numerical simulations and analytical solutions", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Lele", "given_name": "Sanjiva K.", "clpid": "Lele-S-K" }, { "family_name": "Moin", "given_name": "Parviz", "orcid": "0000-0002-0491-7065", "clpid": "Moin-Parviz" } ], "abstract": "The scattering of plane sound waves by a vortex is investigated by solving the compressible Navier-Stokes equations numerically, and analytically with asymptotic expansions. Numerical errors associated with discretization and boundary conditions are made small by using high-order-accurate spatial differentiation and time marching schemes along with accurate non-reflecting boundary conditions. The accuracy of computations of flow fields with acoustic waves of amplitude five orders of magnitude smaller than the hydrodynamic fluctuations is directly verified. The properties of the scattered field are examined in detail. The results reveal inadequacies in previous vortex scattering theories when the circulation of the vortex is non-zero and refraction by the slowly decaying vortex flow field is important. Approximate analytical solutions that account for the refraction effect are developed and found to be in good agreement with the computations and experiments.", "doi": "10.1017/S0022112094003514", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "1994-02-10", "volume": "260", "pages": "271-298" }, { "id": "authors:tehjx-7wf54", "collection": "authors", "collection_id": "tehjx-7wf54", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104728916", "type": "article", "title": "Boundary Conditions for Direct Computation of Aerodynamic Sound Generation", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Lele", "given_name": "Sanjiva K.", "clpid": "Lele-S-K" }, { "family_name": "Moin", "given_name": "Parviz", "orcid": "0000-0002-0491-7065", "clpid": "Moin-Parviz" } ], "abstract": "Accurate computation of the far-field sound along with the near-field source terms associated with a free shear flow requires that the Navier-Stokes equations be solved using accurate numerical differentiation and time-marching schemes, with nonreflecting boundary conditions. Nonreflecting boundary conditions have been developed for two-dimensional linearized Euler equations by Giles. These conditions are modified for use with nonlinear Navier-Stokes computations of open flow problems. At an outflow, vortical structures are found to produce large reflections due to nonlinear effects; these reflection errors cannot be improved by increasing the accuracy of the linear boundary conditions. An exit zone just upstream of an outflow where disturbances are significantly attenuated through grid stretching and filtering is developed for use with the nonreflecting boundary conditions; reflections from vortical structures are decreased by 3 orders of magnitude. The accuracy and stability of the boundary conditions are investigated in several model flows that include sound radiation by an energy source in a uniformly sheared viscous flow, the propagation of vortices in a uniform flow, and the spatial evolution of a compressible mixing layer.", "doi": "10.2514/3.11817", "issn": "0001-1452", "publisher": "AIAA", "publication": "AIAA Journal", "publication_date": "1993-09", "series_number": "9", "volume": "31", "issue": "9", "pages": "1574-1582" }, { "id": "authors:88peb-6qp96", "collection": "authors", "collection_id": "88peb-6qp96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190726-104729209", "type": "article", "title": "The free compressible viscous vortex", "author": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Lele", "given_name": "Sanjiva K.", "clpid": "Lele-S-K" }, { "family_name": "Moin", "given_name": "Parviz", "orcid": "0000-0002-0491-7065", "clpid": "Moin-Parviz" } ], "abstract": "The effects of compressibility on free (unsteady) viscous heat-conducting vortices are investigated. Analytical solutions are found in the limit of large, but finite, Reynolds number, and small, but finite, Mach number. The analysis shows that the spreading of the vortex causes a radial flow. This flow is given by the solution of an ordinary differential equation (valid for any Mach number), which gives the dependence of the radial velocity on the tangential velocity, density, and temperature profiles of the vortex; estimates of the radial velocity found by solving this equation are found to be in good agreement with numerical solutions of the full equations. The experiments of Mandella (1987) also report a radial flow in the vortex, but their estimates are much larger than the analytical predictions, and it is found that the flow inferred from the experiments violates the Second Law of Thermodynamics for two-dimensional axisymmetric flow. It is speculated that three-dimensionality is the cause of this discrepancy. To obtain detailed analytical solutions, the equations for the viscous evolution are expanded in powers of Mach number, M. Solutions valid to O(M^2), are discussed for vortices with finite circulation. Two specific initial conditions - vortices with initially uniform entropy and with initially uniform density - are analysed in detail. It is shown that swirling axisymmetric compressible flows generate negative radial velocities far from the vortex core owing to viscous effects, regardless of the initial distributions of vorticity, density and entropy.", "doi": "10.1017/S0022112091000708", "issn": "0022-1120", "publisher": "Cambridge University Press", "publication": "Journal of Fluid Mechanics", "publication_date": "1991-09", "volume": "230", "pages": "45-73" } ]