Submesoscale dynamics, with horizontal scales of O(1\u201310)~km, are ubiquitous in the surface ocean. Recent observations and simulations reveal enhanced sea surface temperature and velocity gradients at these scales, with sharpened buoyancy fronts attracting particular attention for their potential of energy transfer to link large-scale energy reservoirs to the small scales where dissipation occurs. This thesis addresses the following questions: How do the ocean and atmosphere interact at submesoscale? To what extent can balanced dynamics be applied to interpret submesoscale variability, and, given that, how well do balanced frameworks represent energy transfer at submesoscale fronts?
\r\n\r\nAir--sea interaction has been extensively studied at mesoscale, O(100)~km, but remains less well quantified at submesoscale. Mesoscale sea surface temperature and vorticity imprint their own spatial structure on wind stress gradients through thermal and current feedbacks (TFB and CFB). These feedbacks have typically been assessed separately, with limited attention to their collocation and potential joint impacts. Atmospheric wind stress modifications feed back on the ocean by altering surface vertical transports through Ekman dynamics. In submesoscale-resolving air--sea coupled simulations, examining TFB and CFB jointly reveals their combined influence on surface wind stress. The coupled effect produces anomalous wind stress curls an order of magnitude stronger than those at mesoscale. The resulting nonlinear Ekman velocities from wind stress curl changes reach magnitudes roughly ten times larger than at mesoscale, underscoring the potential importance of submesoscale air--sea coupling for vertical exchange in the surface ocean.
\r\n\r\nThe submesoscale range covers a transition from balanced to unbalanced motion as spatial scales decrease. This transition scale is critical for understanding energy transfers and tracer transport in the upper ocean, yet remains poorly constrained. Because balanced motions evolve on time scales much longer than Earth\u2019s rotation period, the temporal scale of submesoscale motions can indicate their dynamical regime. Spatio-temporal analyses in submesoscale-resolving simulations and surface mooring observations show that flow-following Lagrangian time scale of submesoscale flow is much longer than that registered by a fixed Eulerian observer, placing the transition to unbalanced dynamics at ~1 km, much shorter than suggested by Eulerian diagnostics. This implies that balanced theory remain applicable to submesoscale motions down to O(1)~km.
\r\n\r\nAlthough balanced quasi-geostrophic (QG) and semi-geostrophic (SG) theories reproduce submesoscale frontal development, the associated energy transfers under balanced conditions are less well understood. We derive and assess the full kinetic energy budget in spectral space and in physical space using coarse-graining for an isolated front in QG and SG frameworks. A geostrophic strain field by itself is sufficient to produce forward transfer of kinetic energy in QG frontogenesis. In SG, ageostrophic advection generates a dipole in energy transfer linked to the convergent--divergent surface flow structure, but this balanced ageostrophic circulation is too weak to overcome the dominant forward transfer from the strain field. These results demonstrate that balanced dynamics can generate surface convergence and drive downscale transfers, and that ageostrophic circulation is essential to produce the asymmetry of energy fluxes at submesoscale fronts.
", "doi": "10.7907/x7ve-tx51", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17622", "collection": "thesis", "collection_id": "17622", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08132025-212958945", "primary_object_url": { "basename": "fnxfzgsvkxxdpgwywysdbcrbfcphkckp.pdf", "content": "final", "filesize": 9109118, "license": "other", "mime_type": "application/pdf", "url": "/17622/1/fnxfzgsvkxxdpgwywysdbcrbfcphkckp.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Nonlinear Disturbance Evolution in Boundary Layers Using the One-Way Navier-Stokes Equations", "author": [ { "family_name": "Sleeman", "given_name": "Michael Kenneth Elliott", "orcid": "0000-0001-5949-9289", "clpid": "Sleeman-Michael-Kenneth-Elliott" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Laminar-turbulent transition prediction for boundary-layer flows is a pacing item in engineering design. This work extends the one-way Navier Stokes (OWNS) equations to support nonlinear interactions between waves of different frequencies which enables nonlinear disturbance evolution in spatially-developing shear flows, with the goal of predicting transition for a reduced computational cost relative to direct numerical simulation (DNS). The OWNS approach linearizes the Navier-Stokes equations about a user-specified equilibrium solution, and then evolves disturbances to the equilibrium solution by solving a spatial initial-value problem in the frequency domain. OWNS yields a reduced computational cost compared to global linear stability analysis, while also conferring numerous advantages over the parabolized stability equations (PSE) that we seek to extend to nonlinear OWNS (NOWNS). We validate NOWNS for two- and three-dimensional disturbances to a low-speed Blasius boundary layer by comparing to DNS results from the literature. We further demonstrate that NOWNS can be used to for transition prediction since it accurately predicts the onset of laminar-turbulent transition in low-speed boundary-layer flows, relative to DNS. Subsequently, we extend the approach to high-speed boundary-layer flows, where we apply it to study oblique-wave breakdown of Mack's first and second modes. Finally, we formulate a greedy algorithm for choosing optimal OWNS recursion parameters, which achieves rapid error convergence and a net decrease in computational cost compared to previous approaches to recursion parameter selection.", "doi": "10.7907/wwv8-v242", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17049", "collection": "thesis", "collection_id": "17049", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03102025-172015248", "type": "thesis", "title": "Resolvent Analysis of Non-Stationary Turbulent Flows and Transient Flow Phenomena", "author": [ { "family_name": "Ballouz", "given_name": "Eric", "orcid": "0009-0003-7034-1898", "clpid": "Ballouz-Eric" } ], "thesis_advisor": [ { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" } ], "thesis_committee": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Lozano-Duran", "given_name": "Adrian", "orcid": "0000-0001-9306-0261", "clpid": "Lozano-Duran-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this work, we develop a wavelet-based formulation of resolvent analysis in order to extend the method to transient phenomena and non-stationary flows. We apply this method in two ways: first, to analyze systems that were not previously amenable to traditional resolvent analysis, and second, to probe the limits of the resolvent forcing modes' \"optimality\" in a nonlinear simulation as well as investigate the mechanisms that suppress their effectiveness. In wavelet-based resolvent analysis, the Navier-Stokes equations are linearized about a mean profile, Fourier-transformed in the homogeneous directions, and wavelet-transformed in time. The nonlinear terms are represented as forcing terms acting on the system, and a maximally perturbing forcing mode and the response it produces are then computed for this linear system. The wavelet formulation enables the forcing and response modes to represent transient trajectories. By windowing the wavelet-based resolvent operator, we can also compute optimal forcing modes restricted to a time-localized pulse along with their transient response.
\r\n\r\nFor the first application of the method, we use the windowing approach to study bursting in channel flow. The optimal response mode grows and decays in time scales that match turbulent data, and we show that this optimal burst exploits the Orr mechanism.\r\nWe also study channel flow subjected to a spanwise pressure gradient. The corresponding resolvent modes mirror the mean flow and gradually realign themselves according to the new flow conditions. More interestingly, they exhibit a collapse of the lift-up mechanism during this realignment, which offers an explanation to the depletion of tangential Reynolds stresses in the turbulent system.
\r\n\r\nFor the second application of the method, we inject time-localized resolvent forcing modes for the minimal flow unit into a simulation of the system, at different intensities. The principal resolvent forcing mode is much more effective than a randomly generated forcing structure at amplifying the near-wall streak. For initial times and close to the wall, the turbulent minimal flow unit matches the principal response mode well, but due to nonlinear effects, the response decays prematurely. By computing the nonlinear energy transfer to secondary scales, we find that the breakdown of the actuated mode proceeds similarly across all forcing intensities: in the near-wall region, the induced streak forks into two branches, while in the outer region, the streak breaks up in the streamwise direction. In both regions, spanwise gradients account for the dominant share of nonlinear energy transfer.
", "doi": "10.7907/t9sw-b215", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16619", "collection": "thesis", "collection_id": "16619", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08072024-203148023", "primary_object_url": { "basename": "HeidtLiam_CaltechThesis.pdf", "content": "final", "filesize": 32563876, "license": "other", "mime_type": "application/pdf", "url": "/16619/1/HeidtLiam_CaltechThesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Modal Analysis of Harmonically Forced Turbulent Flows with Application to Jets", "author": [ { "family_name": "Heidt", "given_name": "Liam Frank Raven", "orcid": "0000-0003-1967-6847", "clpid": "Heidt-Liam-Frank-Raven" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909" } ], "thesis_committee": [ { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Many turbulent flows exhibit time-periodic statistics. These include flows in turbomachinery, the wakes of bluff bodies, and flows exposed to harmonic actuation. However, many existing techniques for identifying and modeling coherent structures, most notably spectral proper orthogonal decomposition (SPOD) and resolvent analysis, assume statistical stationarity. In this thesis, we develop extensions to study turbulent flows with periodic statistics. We focus on the application of turbulent jets and jet noise reduction through harmonic actuation, which is of interest for both commercial and military aviation due to its success in reducing noise by up to 5dB.
\r\n\r\nTo analyze the coherent structures in harmonically forced flows, we develop the cyclostationary spectral proper orthogonal decomposition (CS-SPOD). We examine the resulting properties of CS-SPOD and develop a theoretical connection between CS-SPOD and harmonic resolvent analysis (HRA), thereby providing the theoretical basis for HRA to be used as a model for coherent structures of cyclostationary flows. We develop and validate a computationally efficient algorithm and then illustrate its efficacy using the linearized (complex) Ginzburg-Landau equation.
\r\n\r\nWe next employ cyclostationary analysis to investigate the impact of an axisymmetric acoustic harmonic forcing on the mean, turbulence, and coherent structures of a round turbulent jet with a Mach number of 0.4 and a Reynolds number of 450000. We perform large-eddy simulations for four cases at two forcing frequencies and amplitudes. Both low-frequency (Strouhal number of 0.3) and high-frequency (Strouhal number of 1.5) forcing is found to generate an energetic, nonlinear, tonal response consisting of the rollup of vortices via the Kelvin-Helmholtz mechanism. However, the impact of forcing on the broadband turbulence and coherent structures is limited, particularly at the low forcing amplitude associated with jet-noise-reduction devices. Additionally, the dominant coherent structures for the forced jets are similar in their energy, structure, and mechanism. At high forcing amplitudes, phase-dependent features arise in the dominant coherent structures and are associated with coupling to the high-velocity/shear regions of the mean. Overall, our results support the existing hypotheses that jet noise reduction can be associated with the deformation of the mean flow field rather than through direct interaction between the forcing and the turbulence. Lastly, we find that HRA predicts the dominant coherent structures well. This shows that HRA can be used to develop models of forced jets in a similar manner to how resolvent is employed for natural jets, which may be useful to guide future sound-source models of jets subjected to active control.
", "doi": "10.7907/e6fe-kz94", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17083", "collection": "thesis", "collection_id": "17083", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03202025-173020131", "primary_object_url": { "basename": "Thesis_Hou.pdf", "content": "final", "filesize": 21073399, "license": "other", "mime_type": "application/pdf", "url": "/17083/1/Thesis_Hou.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Fast Algorithms for Spanwise Periodic Incompressible External Flows: From Simulation to Analysis", "author": [ { "family_name": "Hou", "given_name": "Wei", "orcid": "0000-0001-8023-6395", "clpid": "Hou-Wei" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "External flows over spanwise-homogeneous geometries are ubiquitous in science and engineering applications. In this thesis, we propose algorithms to simulate and analyze these flows using the lattice Green's function (LGF) approach. The LGF is the analytical inverse of a discrete elliptic operator that automatically incorporates exact far-field boundary conditions and minimizes computational expense by allowing snug computational regions encompassing only vortical flow regions. By combining LGFs with adaptive mesh refinement (AMR) and immersed boundary (IB) methods, we present two numerical algorithms specially designed for spanwise periodic incompressible external flows: one to directly solve the nonlinear equations of motion and one to compute stability and resolvent analyses.
\r\n\r\nFor these algorithms, the LGFs of the screened Poisson equation must be computed at runtime. To enable efficient flow simulation and analysis algorithms, we propose a fast numerical algorithm to tabulate these LGFs. We derive convergence results for the algorithms and show that they are orders of magnitude faster than existing algorithms. Armed with the LGF for the screened Poisson equation, we further develop algorithms to solve the Navier-Stokes equations and associated linearized eigenvalue problems.
\r\n\r\nWe present two applications of these algorithms. We perform simulations to validate the starting vortex theory proposed by Pullin and Sader (2021), and we perform stability analyses of flow past a rotating cylinder with a control cylinder in its wake.
", "doi": "10.7907/eygj-k325", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17360", "collection": "thesis", "collection_id": "17360", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022025-131020183", "primary_object_url": { "basename": "Tawney_Jacqueline_Thesis_submission.pdf", "content": "final", "filesize": 25223233, "license": "other", "mime_type": "application/pdf", "url": "/17360/1/Tawney_Jacqueline_Thesis_submission.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Aqueous Metallo-Megasupramolecules: From Stability to Extensional Flow Properties", "author": [ { "family_name": "Tawney", "given_name": "Jacqueline Rose", "orcid": "0000-0002-4276-0652", "clpid": "Tawney-Jacqueline-Rose" } ], "thesis_advisor": [ { "family_name": "Kornfield", "given_name": "Julia A.", "clpid": "Kornfield-J-A" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Nelson", "given_name": "Chris W.", "clpid": "Nelson-Chris-W" }, { "family_name": "Kornfield", "given_name": "Julia A.", "orcid": "0000-0001-6746-8634", "clpid": "Kornfield-J-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "The addition of long, flexible polymers (> 1 Mg/mol) to a fluid is known to reduce turbulent drag and control droplet behavior, which has the potential to significantly enhance the efficiency of engineering flows across various industries, from agriculture to aviation. However, hydrodynamic forces can break the polymers and diminish their effectiveness, which is presently a major roadblock to their practical utilization in both applications and research. To address this challenge, the Kornfield group developed end-associative, self-healing polymers for use in fuel and, more recently, for use in water\u2014aqueous terpyridine-ended polyacrylamide (TPAM) supramolecules. This thesis examines the relationships between the molecular structure of TPAM, the amount of metal provided to link pairs of chain ends, and kinetic processes of the resulting supramolecules and the rheological properties and performance they provide. The most useful polymers for reducing turbulent drag, controlling mist, and tailoring droplet impact behavior combine high efficacy at low concentration (< 0.1 wt%), minimal impact on shear viscosity (< 2x), and long extensional relaxation time (> 1 ms), enabling them to stretch and resist elongational flow in turbulent eddies or fluid filaments. This thesis explores the fundamental nature of TPAM supramolecules and their potential utility as a rheological modifier, using measurements of molecular weight distributions and extensional relaxation times to illuminate the relationship between supramolecular structure and flow behavior.
\r\n\r\nFirst, we examine chemical degradation (desirable in the environment, but not during use), revealing that its rate can be controlled by limiting air exposure, avoiding an excess of metal ions relative to ligands, and storing samples in refrigerated conditions (4℃). Next, we assess how changes in metal-to-ligand ratios (M:L) and unimer lengths influence TPAM\u2019s megasupramolecular size, equilibration, and decay dynamics, showing that the presence of supramolecules comprising over 10 unimers gives rise to a relaxation time around 2 ms at 0.04 wt%\u2014long and dilute enough to cause drag reduction. In pursuit of even longer supramolecules (and thus longer relaxation times) with the same amount of TPAM, we modified the solution preparation protocol by introducing metal ions to a more concentrated TPAM solution prior to dilution. This exposed new and intriguing topologies with molecular weights extending beyond our measurable limit (10 Mg/mol), expanding the envelope of the longest accessible relaxation times (from ~2 to ~6 ms with M:L = 1:2 for Ni(II):terpyridine). We evaluated their potential as chain scission-resistant, turbulent drag-reducing agents. Initially, they reduce drag while maintaining backbone integrity; however, their supramolecular structure and extended relaxation time are not retained after multiple passes through contraction, turbulent, and expansion flows. The preservation of backbone integrity, along with the broad range of relaxation times achieved using more conventional linear topologies (up to ~3 ms), suggests that TPAM is a promising and robust rheological modifier worthy of continued investigation. Our findings enhance understanding of TPAM\u2019s structural and rheological properties under a range of conditions and lay the groundwork for further study of aqueous megasupramolecule dynamics and applications.
", "doi": "10.7907/98fw-fx90", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16506", "collection": "thesis", "collection_id": "16506", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06052024-052757779", "primary_object_url": { "basename": "Gunnarson_Peter_2024.pdf", "content": "final", "filesize": 32177904, "license": "other", "mime_type": "application/pdf", "url": "/16506/1/Gunnarson_Peter_2024.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Autonomous Flow-Based Navigation in Unsteady Underwater Environments", "author": [ { "family_name": "Gunnarson", "given_name": "Peter John", "orcid": "0000-0002-4437-5379", "clpid": "Gunnarson-Peter-John" } ], "thesis_advisor": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "thesis_committee": [ { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Burdick", "given_name": "Joel Wakeman", "orcid": "0000-0002-3091-540X", "clpid": "Burdick-J-W" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Autonomous ocean-exploring robots promise to significantly enhance the rate at which we can explore ocean environments. However, the limited range and speed of existing autonomous underwater vehicles (AUVs) are barriers to comprehensive ocean exploration. To address these limitations, the work in this thesis investigates strategies for improving the capabilities of existing AUVs, such as targeted sampling and efficient navigation through background flows. Inspired by the ability of aquatic animals to navigate via flow sensing, hydrodynamic cues are investigated as a sensory input for accomplishing these feats of autonomous navigation using only onboard sensors. First, reinforcement learning (RL) is investigated as an algorithm for accomplishing efficient point-to-point navigation in simulated cylinder flow. The algorithm entails inputting point measurements of flow quantities such as velocity and vorticity into a deep neural network, which then determines a swimmer's actions. Using point velocity as the sensory input, the RL algorithm achieved a near 100 percent success rate in reaching the target locations while approaching the time-efficiency of optimal navigation trajectories. To test RL and flow-based navigation in a physical setting, we next developed the Caltech autonomous reinforcement learning robot (CARL), a palm-sized underwater robotic platform. As proof-of-concept analogy for tracking hydrothermal vent plumes in the ocean, the robot was tasked with locating the center of turbulent jet flows in a 13,000-liter water tank using data from onboard pressure sensors. Using a navigation policy trained with RL in a simulated flow environment, CARL successfully located the turbulent plumes at more than twice the rate of random searching by detecting mean flow gradients with the onboard pressure sensors. Lastly, combing both flow sensing and efficient navigation, the accelerometer onboard CARL was used to sense and exploit the flow from a passing vortex ring for energy-efficient propulsion. Body acceleration and rotation were shown to be effective methods of indirect flow sensing, which enabled the energy-efficient vortex ring surfing strategy. Throughout this work, efforts are made to understand the governing physics behind the discovered navigation strategies to generalize the results beyond a specific navigation problem, sensor type, or robotic implementation.", "doi": "10.7907/vnh6-3t44", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16435", "collection": "thesis", "collection_id": "16435", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292024-213954509", "primary_object_url": { "basename": "MatthewYao_PhD_Thesis.pdf", "content": "final", "filesize": 13823304, "license": "other", "mime_type": "application/pdf", "url": "/16435/1/MatthewYao_PhD_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Lean Premixed Hydrogen Flames: Turbulence, Chemistry, and Modelling", "author": [ { "family_name": "Yao", "given_name": "Matthew Xuhuai", "orcid": "0000-0001-6141-1477", "clpid": "Yao-Matthew-Xuhuai" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Lean turbulent premixed hydrogen/air flames have substantially increased flame speeds, a behaviour which is attributed to differential diffusion effects. In this thesis, the relationships between turbulence, chemistry, and modelling are studied through direct numerical simulation (DNS) and large eddy simulation (LES).
\r\n\r\nThe effect of turbulence on lean hydrogen combustion is studied through DNS using detailed chemistry and detailed transport. Simulations are conducted at six Karlovitz numbers and four integral length scales. A general expression for the burning efficiency is proposed which depends on the conditional mean chemical source term and gradient of a progress variable. At a fixed Karlovitz number, the normalized turbulent flame speed and area both increase almost linearly with the integral length scale ratio. The effect on the mean source term profile is minimal, indicating that the increase in flame speed can solely be attributed to the increase in flame area. At a fixed integral length scale, both the flame speed and area first increase with Karlovitz number before decreasing. Neglecting Soret diffusion is shown to reduce the flame speed, area, and burning efficiency. At higher Karlovitz numbers, the diffusivity is enhanced due to penetration of turbulence into the reaction zone, significantly dampening differential diffusion effects.
\r\n\r\nThe structure of lean hydrogen flames, namely the species mass fraction dependence on the local temperature, differs significantly from that of unity Lewis number fuels due to thermodiffusive instabilities. When subjected to turbulence, the conditional mean species mass fraction profiles are observed to transition from the laminar mixture-averaged flamelet solution to the unity Lewis number flamelet solution. We assess the impact of Soret diffusion and integral length scales on an effective Lewis number model. The results show that the turbulent flame structure can be mapped onto laminar flamelets via the use of effective Lewis numbers, which are expressed by an a priori Karlovitz number model. Although the flame structure is altered by Soret diffusion, there is still strong agreement with previously derived Karlovitz number models for effective Lewis numbers. To map the turbulent flames onto laminar flames with effective Lewis numbers, the relative impact of Soret diffusion needs to be proportionally reduced.
\r\n\r\nTo assess the LES modelling of lean hydrogen flames, we simulate a low-swirl burner, an alternative means of clean energy generation. The LES modelling of these flows remains challenging because the transition of small-scale instabilities into large-scale turbulent structures cannot be modelled by conventional strategies. Traditional one-equation tabulated chemistry formulations require only a progress variable, and cannot capture differential diffusion and curvature effects. In this work, we study the effects of tabulating different conditional mean source terms. It is shown that tabulating the appropriate conditional mean source term leads to improvements in the flow field prediction, however, key features such as the main recirculation region are not reproduced. Then, a two-equation tabulated chemistry model which accounts for differential diffusion and curvature effects is tested. This model provides the best agreement with experimental results. The work is a first effort in evaluating the performance of the two-equation model in the LES framework.
", "doi": "10.7907/yjzw-vp60", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:15100", "collection": "thesis", "collection_id": "15100", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02042023-015312785", "primary_object_url": { "basename": "Oshima_EK_Dissertation.pdf", "content": "final", "filesize": 77968077, "license": "other", "mime_type": "application/pdf", "url": "/15100/1/Oshima_EK_Dissertation.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Experimental Studies of Flow Control Techniques for Future Aircraft", "author": [ { "family_name": "Oshima", "given_name": "Emile Kazuo", "orcid": "0000-0002-1689-3726", "clpid": "Oshima-Emile-Kazuo" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "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" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Wygnanski", "given_name": "Israel J.", "orcid": "0009-0001-5711-7029", "clpid": "Wygnanski-I-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "From the signing of the Paris Agreement to the COVID-19 outbreak, the past decade has truly challenged the aviation industry to adapt. New technologies need to be developed constantly to meet the increasing commercial and defense demands for more efficient, quiet, safe, and agile aircraft. To keep up with these rapidly changing times, an approach that marries a fundamental understanding of aerodynamics with systems design and optimization is necessary. This thesis explores two promising concepts for controlling flow over next-generation aircraft: active control on a swept wing for airplane applications, and passive control on a rotating blade for drone applications. In each, force measurements are combined with advanced flow visualization techniques to create a research framework that is both data-driven and physics-informed.
\r\n\r\nIn Part I, a comprehensive wind tunnel campaign is carried out on a swept wing model of modular geometry equipped with an array of sweeping jet actuators, which have demonstrated tremendous promise for flow control authority in both laboratory settings and full-scale flight tests. The flow physics and performance of the wing is investigated first without actuation, revealing separation behaviors at both the leading and trailing edges that are crucial to consider when flow control is applied. This paves the way for an optimization study in a newly proposed framework that relies on fluid power coefficients rather than the momentum coefficient that has been the accepted parameter of choice for characterizing blowing systems over the past seven decades of active flow control research.
\r\n\r\nPart II explores the feasibility of a \"prop-shroud\" concept for small-scale aerial vehicles, in which the shroud is directly attached to the blade tips and thus co-rotates with the propeller. Such a configuration has the potential to provide the various aerodynamic and engineering benefits of a shrouded propeller without the associated costs and complexities of its installation. The hover efficiency of a prop-shroud is shown to be comparable to commercially available drone propellers, even without a rigorous optimization of its geometry. The effect of the co-rotating shroud is then analyzed in detail on the time-averaged, phase-averaged, and unsteady features of the flow field. A model based on vortex formation time is developed, laying out a foundation for future research and understanding.
", "doi": "10.7907/fpcj-w268", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15112", "collection": "thesis", "collection_id": "15112", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02272023-230531120", "primary_object_url": { "basename": "Thesis_Draft___Final.pdf", "content": "final", "filesize": 20577292, "license": "other", "mime_type": "application/pdf", "url": "/15112/1/Thesis_Draft___Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Applied Machine Learning for Prediction and Control of Fluid Flows", "author": [ { "family_name": "Renn", "given_name": "Peter Ian James", "orcid": "0000-0002-5735-3873", "clpid": "Renn-Peter-Ian-James" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Anandkumar", "given_name": "Anima", "orcid": "0000-0002-6974-6797", "clpid": "Anandkumar-A" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "Modern aerodynamic technologies such as unmanned aerial systems and horizontal axis wind turbines must regularly contend with forces from highly stochastic and turbulent atmospheric gusts. Conventional methods for modeling and controlling fluid flows are limited in their ability to mitigate these aerodynamic forces in real-time. By applying modern machine learning techniques in an experimental setting, this thesis demonstrates the utility of machine learning in addressing these important problems. We follow two complementary approaches towards this goal.
\r\n\r\nFirst, we find an end-to-end solution for control in a gusty environment with model-free reinforcement learning. We deploy state-of-the-art reinforcement learning algorithms on a generalized aerodynamic test-bed consisting of an airfoil with motorized trailing edge flaps. The system features embedded flow sensors, enabling the inclusion of flow measurements in state observations. We place this system in a highly irregular wake behind a bluff-body, dynamically mounted on elastic bands and therefore free to oscillate, and train reinforcement learning agents to minimize the net lifting force on the system by controlling the position of the trailing edge flaps. We find that model-free reinforcement learning agents can outperform basic linear controllers in this gusty, turbulent environment. We also show that augmenting state observations with flow measurements can lead to more consistent learning of the system dynamics.
\r\n\r\nNext, we explore Fourier neural operators (FNOs) as a method for forecasting the time evolution of turbulent fluid flows. FNOs are capable of learning underlying operator solutions to families of partial differential equations and can be evaluated in just milliseconds. We specifically focus on training FNOs with experimentally measured velocity fields of bluff body wakes in the subcritical regime. To the best of our knowledge, this is the first application of operator learning for fluid mechanics that features experimental measurements. We find that FNOs can accurately predict the evolution of these turbulent wakes even when trained with imperfect measurements. We then show that FNOs can quickly adapt to unseen conditions with minimal data and training through transfer learning. Finally, we consider the performance of FNOs over longer prediction horizons. This approach could enable real-time gust prediction capabilities and monitoring for applied aerodynamic systems.
", "doi": "10.7907/smnv-tz73", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15271", "collection": "thesis", "collection_id": "15271", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012023-233806562", "type": "thesis", "title": "Numerical Analysis of Folding and Deployment Dynamics of Thin Shell Structures with Localized Folds", "author": [ { "family_name": "Canales Escobedo", "given_name": "Fabricio Gianfranco", "orcid": "0000-0001-9071-3263", "clpid": "Canales-Escobedo-Fabricio-Gianfranco" } ], "thesis_advisor": [ { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Schroeder", "given_name": "Peter", "orcid": "0000-0002-0323-7674", "clpid": "Schr\u00f6der-P" }, { "family_name": "Pellegrino", "given_name": "Sergio", "orcid": "0000-0001-9373-3278", "clpid": "Pellegrino-S" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This thesis focuses on the analysis of tape springs folded in the opposite sense and their dynamic deployment, and aims to use methods to reduce the computational cost of the analysis. The tape spring is a thin shell deployable structure that has features in common with other deployable structures. The deployment process of such structures can be difficult to predict, and the use of numerical models can be a more cost-effective alternative to experimental testing. Approaches to reduce the computational cost of the analysis of tape springs are investigated such as adaptive meshing and reduced order models. The thesis also presents an accurate analysis of tape spring deployment and a detailed study of the energies and the physics of the deployment. This is used to investigate the energy leak observed in previous tape spring deployment work. Overall, this thesis contributes to improving the efficiency and accuracy of the analysis of deployable structures, particularly tape springs, which can have significant applications in spacecraft technology.", "doi": "10.7907/gt81-0s18", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" } ]