CaltechTHESIS committee: Erratum
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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenMon, 14 Oct 2024 13:35:47 -0700On the Behavior of Pliable Plate Dynamics in Wind: Application to Vertical Axis Wind Turbines
https://resolver.caltech.edu/CaltechTHESIS:05272014-160129404
Year: 2014
DOI: 10.7907/X7S3-CS74
<p>Numerous studies have shown that flexible materials improve resilience and durability of a structure. Several studies have investigated the behavior of elastic plates under the influence of a free stream, such as studies of the fluttering flag and others of shape reconfiguration, due to a free stream.</p>
<p>The principle engineering contribution of this thesis is the design and development of a vertical axis wind turbine that features pliable blades which undergo various modes of behavior, ultimately leading to rotational propulsion of the turbine. The wind turbine design was tested in a wind tunnel and at the Caltech Laboratory for Optimized Wind Energy. Ultimately, the flexible blade vertical axis wind turbine proved to be an effective way of harnessing the power of the wind.</p>
<p>In addition, this body of work builds on the current knowledge of elastic cantilever plates in a free stream flow by investigating the inverted flag. While previous studies have focused on the fluid structure interaction of a free stream on elastic cantilever plates, none had studied the plate configuration where the trailing edge was clamped, leaving the leading edge free to move. Furthermore, the studies presented in this thesis establish the geometric boundaries of where the large-amplitude flapping occurs.</p>https://resolver.caltech.edu/CaltechTHESIS:05272014-160129404Numerical Methods for Fluid-Structure Interaction, and their Application to Flag Flapping
https://resolver.caltech.edu/CaltechTHESIS:10122017-095438989
Year: 2018
DOI: 10.7907/Z95T3HPB
<p>This thesis is divided into two parts. Part I is devoted to the development of numerical techniques for simulating fluid-structure interaction (FSI) systems and for educing important physical mechanisms that drive these systemsâ€™ behavior; part II discusses the application of many of these techniques to investigate a specific FSI system.</p>
<p>Within part I, we first describe a procedure for accurately computing the stresses on an immersed surface using the immersed-boundary method. This is a key step to simulating FSI problems, as the surface stresses simultaneously dictate the motion of the structure and enforce the no-slip boundary condition on the fluid. At the same time, accurate stress computations are also important for applications involving rigid bodies that are either stationary or moving with prescribed kinematics (e.g., characterizing the performance of wings and aerodynamic bodies in unsteady flows or understanding and controlling flow separation around bluff bodies). Thus, the method is first formulated for the rigid-body prescribed-kinematics case. The procedure described therein is subsequently incorporated into an immersed boundary method for efficiently simulating FSI problems involving arbitrarily large structural motions and rotations.</p>
<p>While these techniques can be used to perform high-fidelity simulations of FSI systems, the resulting data often involves a range of spatial and temporal scales in both the structure and the fluid and are thus typically difficult to interpret directly. The remainder of part I is therefore devoted to extending tools regularly used for understanding complex flows to FSI systems. We focus in particular on the application of global linear stability analysis and snapshot-based data analysis (such as dynamic mode decomposition and proper orthogonal decomposition) to FSI problems. To our knowledge, these techniques had not been applied to deforming-body problems in a manner that that accounts for both the fluid and structure leading up to this work.</p>
<p>Throughout part I, our methods are derived in the context of fairly general FSI systems and are validated using results from the literature for flapping flags in both the conventional configuration (in which the flag is pinned or clamped at its leading edge with respect to the oncoming flow) and the inverted configuration (in which the flag is clamped at its trailing edge). In part II, we apply many of the techniques developed in part I to uncover new physical mechanisms about inverted-flag flapping. We identify the instability-driving mechanism responsible for the initiation of flapping and further characterize the large-amplitude and chaotic flapping regimes that the system undergoes for a range of physical parameters.</p>https://resolver.caltech.edu/CaltechTHESIS:10122017-095438989Towards Single Molecule Imaging Using Nanoelectromechanical Systems
https://resolver.caltech.edu/CaltechTHESIS:05182020-141933604
Year: 2020
DOI: 10.7907/n4ap-7h91
<p>We incorporate nanoelectromechanical systems (NEMS) into a state-of-the-art commercial mass spectrometer (Q Exactive Plus with Orbitrap detection). This unique hybrid instrument is capable of ionizing molecules up to 4.5 MDa in their intact native state, isolating molecules of interest according to their mass-to-charge ratio, performing high resolution mass spectrometry (MS), and delivering those molecules to the NEMS. We use NEMS optimized for detecting the inertial mass of adsorbed species directly, which contrasts with indirect measurements of the mass-to-charge ratio performed with typical instruments. This unique form of mass spectrometry, NEMS-MS, with its single-molecule sensitivity, has promising applications to the fields of proteomics and native mass spectrometry, including deep proteomic profiling, single-cell proteomics, mass spectrometry-based imaging, or identifying viruses in their <i>in vivo</i> state.</p>
<p>We analyze intact <i>E. coli</i> GroEL chaperonin, a noncovalent 801 kDa complex consisting of 14 identical subunits. GroEL was sent to NEMS operated with the first two vibrational modes monitored in real time. Molecules physisorbing to the NEMS cause an abrupt shift in its resonance frequencies. The change in resonance frequencies is used to calculate the mass of each molecule. A mass spectrum is compiled with a main peak of 846 kDa, close to the expected value, and a secondary peak resolved near twice the mass of GroEL.</p>
<p>Measurements are then performed operating the first three modes simultaneously. Using a technique called inertial imaging, frequency shifts are used to calculate the first three mass moments: mass, position, and variance (size). This is used to distinguish between adsorbates arriving in a single, point-like distribution or a more extended distribution, thus demonstrating a rudimentary form of molecular imaging.</p>
<p>Two new theories are presented for analyzing frequency-shift data. The first approach offers a more streamlined approach for calculating the mass moments. This approach is used to improve the mass spectrum of the GroEL calculated using three-mode data, producing a main peak almost fully resolved at 805 kDa. An entirely different approach is presented that allows for obtaining the mass density distribution of an adsorbed molecule (i.e., imaging) with a higher number of modes.</p>https://resolver.caltech.edu/CaltechTHESIS:05182020-141933604Folding and Dynamic Deployment of Ultralight Thin-Shell Space Structures
https://resolver.caltech.edu/CaltechTHESIS:05292023-160132013
Year: 2023
DOI: 10.7907/m7rd-6s86
<p>Thin-shell structures are becoming increasingly popular for space missions due to their high stiffness-to-mass ratio, easy folding and coiling, and self-deployment using stored strain energy. Broadly, two deployment strategies exist: 1) controlled or deterministic, and 2) unconstrained. Controlled deployment involves carefully orchestrated events using control or guidance systems, while in unconstrained deployment, the structure is simply allowed to self-deploy with minimal guidance. Unconstrained deployment offers lighter deployment mechanisms and better packaging efficiency but the unpredictability of this process has been a significant obstacle to its adoption.</p>
<p>This study focuses on demonstrating the predictability of unconstrained dynamic deployment of thin-shell structures, using the Caltech Space Solar Power Project (SSPP) structures as a case study. The Caltech SSPP uses composite triangular rollable and coilable longerons as the primary building blocks to create large bending-stiff structures. The specific objective is to improve the predictability and robustness of the unconstrained dynamic deployment of the Caltech SSPP structures. Deployment is influenced by the initial conditions and the interaction between the structure and the mechanism during the deployment. To understand these effects, high-fidelity numerical simulations are developed and validated against experiments. The study also examines the sensitivity of deployment characteristics to various design parameters and external influences to ensure the robustness of deployment.</p>
<p>This research demonstrates that the interaction between the structure and the deployment mechanism must be minimal to ensure the predictability of deployment, as thin-shell structures can self-deploy using stored strain energy. This study's sensitivity analysis will inform the design of future SSPP deployment mechanisms and structures. Additionally, the numerical simulation techniques developed have broader applicability beyond this specific case study to any deployable thin-shell structure.</p>
<p>Due to the large aspect ratios of thin-shell structures, a very fine finite element mesh is required to model them accurately. A dense finite element mesh is also required to model the contact interactions between the structure and the rigid components of the deployment mechanism. As large spacecraft structures become increasingly complex, full-scale numerical modeling becomes impractical, necessitating the search for more computationally efficient finite element methods. In this study, NURBS-based isogeometric analysis is explored, and it is shown that it is not yet worth switching to NURBS-based elements for the analysis of thin-shell deployable structures. In addition, h-adaptive meshing for quadrilateral shell elements is investigated, and more efficient refinement indicators and solution mapping techniques for nonlinear analyses are proposed and their superior performance is demonstrated using a test case of quasi-static folding of a tape spring.</p>
<p>This thesis fills a gap in the literature on unconstrained dynamic deployment of space structures, providing crucial insights and numerical modeling tools for further research. It establishes a knowledge and resource foundation to advance space structure design and promote more frequent use of unconstrained deployment, marking a pivotal contribution to the field and enabling safe and efficient space structure deployment. Furthermore, the study provides insights into more computationally efficient finite element methods, such as h-adaptive meshing. These insights are broadly applicable and can inform the design of future deployable structures beyond the tested cases.</p>https://resolver.caltech.edu/CaltechTHESIS:05292023-160132013Modal Analysis of Harmonically Forced Turbulent Flows with Application to Jets
https://resolver.caltech.edu/CaltechTHESIS:08072024-203148023
Year: 2025
DOI: 10.7907/e6fe-kz94
<p>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.</p>
<p>To 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.</p>
<p>We 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.</p>https://resolver.caltech.edu/CaltechTHESIS:08072024-203148023