Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

Browse

Subject: search.filters.subject.Hypersonics

Search Results

Now showing 1 - 8 of 8
  • Thumbnail Image
    Item
    Development of an Automated Volume Mesh Generation CFD Framework for Hypersonic Heat Flux Predictions
    (2024) McQuaid, Joel Anthony; Brehm, Christoph; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The workflow of computational fluid dynamics (CFD) solvers traditionally involves a labor-intensive pre-processing stage, which includes case setup and mesh generation, followed by the solver phase and subsequent data post-processing. Particularly in hypersonic applications, mesh generation has predominantly been a manual and cumbersome process, significantly hindering the scalability of large-scale simulations. The time needed for mesh creation escalates with increasing geometric complexity, posing a substantial bottleneck. This PhD research project has developed innovative numerical methods aimed at addressing these challenges to enhance the efficiency and feasibility of complex simulations. This dissertation outlines the creation of the hybrid CHAMPS near body-Cartesian (NBS-Cart) solver framework, designed for automatic volume mesh generation. This new approach has been tested over a wide range of hypersonic heating scenarios and fluid-ablation interaction cases. It integrates a Cartesian grid solver with adaptive mesh refinement to effectively track off-body wake and shock structures, while the NBS component accurately captures strong boundary layer gradients. The efficacy of the Cartesian higher order shock-capturing scheme was studied for a canonical 2D hypersonic cylinder flow and a full 3D Mars Science Lander configuration. This testing confirmed the scheme's ability for efficient shock capturing on non-aligned grids which is desirable for accurate NBS heat flux predictions within the coupled solver paradigm. Furthermore, the solver was integrated with the external KATS material response solver to simulate both steady-state and transient graphite ablation processes. The fluid-ablation coupling approach was validated against existing numerical models and arc-jet test data, showing excellent agreement in predicted surface heat fluxes, thermal responses within materials, and morphological changes resulting from thermo-chemical ablation processes. The final stage of this work focused on the development of low dissipation, higher-order numerical schemes that leverage the NBS structure targeting scale-resolved turbulence flow simulations. A key application involved simulating the Boundary Layer Transition (BOLT-II) flight vehicle at its Mach 6 descent condition. This simulation served as a verification exercise against other CFD codes and existing flight data. The NBS demonstrated its proficiency in accurately capturing the dominant curved shock-induced vortices at the vehicle's leading edge, as well as the outboard cross-flow vortex structures. The heat flux predictions from the NBS aligned closely with published data, underscoring the effectiveness and robustness of the CHAMPS NBS-Cart solver in complex aero-thermodynamic conditions. This newly developed capability provides users with a fully-automated volume mesh generation CFD platform suitable for both low and high-enthalpy hypersonic flight environments. This advancement represents a significant stride towards enhancing CFD workflow automation, facilitating design and production-level simulations for real-world applications. This innovation not only streamlines processes but also increases the accuracy and reliability of simulations in complex aerodynamic scenarios.
  • Thumbnail Image
    Item
    EXCITATION AND PHOTOGRAMMETRY ANALYSIS OF FLUID-STRUCTURAL VIBRATIONS
    (2023) Killian, Matthew Vincent; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The field of fluid-structural interactions (FSI) requires specifically designed measurementsystems that can be used to interpret the results of experiments without interfering with any aspects of the tests. In our hypersonic experiments, the chosen method of measurement is photogrammetry, the processing of 2-dimensional images to obtain 3-dimensional positional information about a structure exposed to a flow. To examine vibrations, the test specimen is painted with evenly spaced markers which are then photographed by a stereo digital image correlation setup of two high-speed cameras. There exists an effective algorithm for processing these images to obtain displacement and deflection data that can in turn be analyzed using spectral proper orthogonal decomposition (SPOD) to find vibrational modes. However, the current method for locating markers within an image is computationally expensive and slow, so a new algorithm was adapted to perform the same task. This adapted method differs from the old method by not being iterative, allowing it to run more quickly as it detects the markers and tracks them between images. We verified the efficacy of this new algorithm with two calibration tests, one with artificial marker images and one with real images of a painted plate translated at known displacements. After characterizing the errors of the method, it was tested on FSI experimental data collected at the NASA Langley Facility in their Mach 10 wind tunnel. The results of these tests showed that the algorithm can be quick and accurate, but it is not robust with regards to non-ideal image conditions. The images obtained in FSI photogrammetry are often not ideal, so this method must be developed further. A mechanism for test specimen excitation was also explored. We evaluated a solenoid-based prototype by performing a modal test on a compliant panel with a vibrometer. The results of this test show that the prototype is effective in producing strong and reliable vibrations in the test panel, and as such this should be developed further for use in hypersonic wind tunnel tests.
  • Thumbnail Image
    Item
    DEVELOPMENT OF TWO-POINT FOCUSED LASER DIFFERENTIAL INTERFEROMETRY FOR APPLICATIONS IN HIGH-SPEED WIND TUNNELS
    (2022) Ceruzzi, Andrew; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Focused laser differential interferometry (FLDI) and its relative two-point focused laser differential interferometry (2pFLDI) are completely non-intrusive (i.e. seedless) optical techniques for measuring density fluctuations and velocity respectively that offer high frequency response (>10MHz). Developed in the 1970s, FLDI is receiving renewed attention today for its potential usefulness in measuring turbulent fluctuations and velocity in hypersonic flows. In the technique, two focused, closely-spaced (~100microns), orthogonally-polarized beams pass through a region of interest and are subsequently combined and focused onto a photodetector. Differences in refractive index between the two focal volumes cause a phase shift, thus interference, between the beams which is measured by the detector. In this way the instrument is sensitive to the gradient in refractive index along a line between the two focal volumes perpendicular to the beams (dn/dx). Since gradients in index of refraction arise from gradients in density (in homogeneous flows), fluctuations in the FLDI signal are proportional to local fluctuations in density. If the fluctuations are due to localized eddies convecting through the FLDI measurement volume, then the cross-correlation of the FLDI signal with a that from a second FLDI instrument located a known distance downstream of the first provides a measure of convection velocity (2pFLDI). The ability to measure density fluctuations and velocity simultaneously and at the same point in the flow is critical because it enables one to relate the temporal scales measured by the instrument to the spatial scales present in the flow. In spite of the technique's age, a unified theory for the FLDI operation and sensitivity limits which is simple and easy to use does not exist so the first objective of this thesis is to develop such a theory. It does so using transfer functions that enable one to isolate the effects of focusing, beam separation, and disturbance frequency on the performance (i.e. sensitivity and spatial resolution) of the instrument. While the transfer functions have been previously proposed by others, an application of these functions which accounts for velocity variation in space (u_c(z)) and frequency (u_c(f)) is unique to this work. The theory is validated via comparison to experimental measurements in a canonical turbulent jet where the distributions of velocity and density fluctuations are well known. Measurements made using different FLDI instruments collapse when the differences between them are accounted for, indicating that the unified theory is correctly capturing the effects of instrument parameters like beam separation and beam diameter. FLDI response to the jet is also modeled by substituting the velocity distribution for a dispersion relation, u_c(f), measured by 2pFLDI. The advantage of the latter procedure is it allows for signal interpretation in flows where historical measurements are unavailable. This is demonstrated by comparing modeled FLDI response to experimental measurements in the flow downstream of a ramp in a small (6.4cm square) Mach 3 wind tunnel. The second objective of this thesis is to demonstrate 2pFLDI in other industrially-relevant flows. To this end, density fluctuations and convection velocities are measured in the near-wall flow in a 61cm square Mach 4 wind tunnel (Ludwieg tube) and in the free-stream flow of a 1.5m diameter Mach 18 tunnel. In each case, a method for estimating the spatiotemporal resolution using transfer functions is demonstrated. The spatiotemporal resolution of the instrument was not well understood prior to this work so quantifying it is an important contribution. Achieving acceptable signal/noise at Mach 18 was difficult because densities were so low. However, convection velocities of ~75-80 of the freestream velocity are measured above 200kHz in two runs. Spatiotemporal analysis suggests these measurements are the result of freestream disturbances; the first measurement of its kind in a Mach 18 flow.
  • Thumbnail Image
    Item
    AERODYNAMIC SEPARATION OF FRAGMENTED BODIES IN HIGH-SPEED FLOW
    (2021) Whalen, Thomas James; Laurence, Stuart; Brehm, Christoph; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Atmospheric entry of meteoroids poses danger to humans in the form of blast-wave overpressure, impact craters, tsunamis, and other assorted threats. The relative risks of each are highly dependent on the details of the unavoidable structural disruption that occurs and the subsequent aerodynamic separation sequence, so accurate prediction of fragment trajectories is required for threat mitigation. However, the physics of aerodynamic separation immediately following meteor fragmentation are virtually uncharacterized, allowing for only low confidence in threat assessment projections. The present work endeavors to constrain the separation behavior of fragmenting bodies by examining the model problem of close-packed sphere clusters and, to a lesser extent, clouds of dusty debris. Free-flight experimentation in UMD HyperTERP, a Mach-6 shock tunnel, is conducted to provide a foundation for both statistical and aerodynamic analyses, while coupled inviscid CFD/FEA provides complementary insight into the mechanisms driving fragment separation. First, computations of equal-sized sphere pairs reveal a previously unidentified phenomenon wherein two bodies in continual mechanical contact oscillate about a stable angle-of-attack equilibrium and achieve anomalously high lateral velocities. Proceeding to higher cluster populations, separation procedure can be divided into two stages: mutual repulsion from a common center and subsequent subcluster interactions dictated by the influence of an upstream body. The degree of repulsion induced by the former demonstrates close correlation with the initial angular position of a fragment, whereas the lateral velocities resulting from the latter appear to be normally distributed about a slightly positive value. The transverse separation characteristics of equal-sphere clusters numbering from 2 to 115 bodies are used to constrain a power-law fit between the lateral extent of a disrupted swarm and its population, providing a significant improvement to existing models of aerodynamic separation following fragmentation. Furthermore, experiments of unequal-sphere clusters, whose compositions are governed by realizations of truncated power laws, reveal a systematic underestimate in the equal-sphere correlation, resulting largely from massive subclusters suppressing high expulsion. A unified model of fragment separation, based on both the aforementioned power-law fit and a combined Rayleigh—exponential distribution, is then proposed. Finally, the dynamics of dusty debris clouds are discussed, with implications for mass depletion and energy deposition of rubble-pile-type impactors highlighted.
  • Thumbnail Image
    Item
    An Experimental Investigation of Hypersonic Boundary-Layer Transition on Sharp and Blunt Slender Cones
    (2019) Kennedy, Richard Edward; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Understanding the instabilities leading to the laminar-to-turbulent transition of a hypersonic boundary layer is a key challenge remaining for the design of efficient hypersonic vehicles. In the present study, experiments are performed in three different facilities at freestream Mach numbers between 6 and 14 to characterize instability mechanisms leading to transition on a 7-degree half-angle slender cone. Second-mode instability waves are visualized using a high-speed schlieren setup with the camera frame rate and spatial resolution optimized to allow individual disturbances to be tracked. In order to facilitate quantitative time-resolved measurements, a method of calibrating the schlieren system and novel image-processing algorithms have been developed. Good agreement is observed between the schlieren measurements, surface pressure measurements, and parabolized stability equation computations of the second-mode most-amplified frequencies and N factors. The high-frequency-resolution schlieren signals enable a bispectral analysis that reveals phase locking of higher harmonic content leading to nonlinear wave development. Individual disturbances are characterized using the schlieren wall-normal information not available from surface measurements. Experiments are also performed to investigate the effect of nose-tip bluntness. For moderate to large bluntness nose tips, second-mode instability waves are no longer visible, and elongated structures associated with nonmodal growth appear in the visualizations. The nonmodal features exhibit strong content between the boundary-layer and entropy-layer edges and are steeply inclined downstream. Simultaneously acquired surface pressure measurements reveal high-frequency pressure oscillations typical of second-mode instability waves associated with the trailing edge of the nonmodal features.
  • Thumbnail Image
    Item
    An Investigation of Flames, Deflagrations, and Detonations in High-Speed Flows
    (2018) Goodwin, Gabriel Benjamin; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A comprehensive understanding of the fundamental physics underlying combustion and detonations in turbulent and high-speed flows is crucial to the design of robust ramjet, scramjet, and detonation engines. This work uses high-fidelity, multidimensional numerical simulations to investigate flame stability and deflagration-to-detonation transition (DDT) mechanisms in supersonic reactive flows. The study consists of four major sections. The first section discusses the acceleration of a flame in a channel with obstacles and its transition from a laminar, expanding flame to a turbulent deflagration and eventual detonation. As the flame accelerates, a highly dynamic, shock-heated region forms ahead of the flame. Shock collisions and reflections focus energy in localized volumes of unburned gas at timescales that are small relative to the acoustic timescale of the unburned gas. The rapid deposition of energy causes the unburned gas to detonate through an energy-focusing mechanism that has elements of both direct initiation and detonation in a gradient of reactivity. The second section describes how the blockage of a channel with regularly spaced obstacles, analogous to the igniter in a detonation engine, affects flame acceleration and turbulence in the region ahead of the accelerating flame. The rate of flame acceleration, time and distance to DDT, and detonation mechanism are compared for channels with high, medium, and low blockage ratios. Stochasticity and uncertainty in the numerical solutions are discussed. In the third section, the stability of premixed flames at high supersonic speeds in a constant-area combustor is investigated. After autoignition of the fuel-oxidizer mixture in the boundary layer at the combustor walls, the flame front eventually becomes unstable due to a Rayleigh-Taylor (RT) instability at the interface between burned and unburned gas. The turbulent flame front transitions to a detonation through the energy-focusing mechanism when a shock passes through the flame and amplifies its energy release. The final section discusses the effect of inflow Mach number in the supersonic combustor on ignition, flame stability, and transition to detonation of a premixed flame. Timescales for growth of the RT instability and detonation initiation increase rapidly with flow speed, but, qualitatively, flame evolution is independent of Mach number.
  • Thumbnail Image
    Item
    Dynamic Force Measurement Capabilities for Hypersonic Wind Tunnel Testing
    (2015) Collopy, Arianne Xaviera; Lee, Sung W.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The object of this research is to characterize a new hybrid force measurement methodology using a piezoelectric balance in parallel with a conventional strain gauge balance, with the goal of enabling wide frequency range measurements for use in hypersonic ground test facilities. This technology, developed in collaboration with the Hypervelocity Wind Tunnel 9 at AEDC in White Oak, Maryland, is expected to provide accurate static and dynamic force and moment measurements on conventional test articles, providing simultaneous force, moment, heat transfer, and pressure measurements for maximal test productivity. A finite element model of the model-balance-sting assembly was developed to perform static and transient simulations and thereby characterize the influence of design parameters such as model weight, stiffness, and placement of load cells. This thesis describes the work done with the computational model, paralleling ongoing laboratory work, which includes development of methodologies for static calibration and dynamic calibration using acceleration compensation.
  • Thumbnail Image
    Item
    ENTROPY CONSIDERATIONS APPLIED TO SHOCK UNSTEADINESS IN HYPERSONIC INLETS.
    (2012) Bussey, Gillian Mary Harding; Lewis, Mark J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The stability of curved or rectangular shocks in hypersonic inlets in reponse to flow perturbations can be determined analytically from the principle of minimum entropy. Unsteady shock wave motion can have a significant effect on the flow in a hypersonic inlet or combustor. According to the principle of minimum entropy, a stable thermodynamic state is one with the lowest entropy gain. A model based on piston theory and its limits has been developed for applying the principle of minimum entropy to quasi-steady flow. Relations are derived for analyzing the time-averaged entropy gain flux across a shock for quasi-steady perturbations in atmospheric conditions and angle as a perturbation in entropy gain flux from the steady state. Initial results from sweeping a wedge at Mach 10 through several degrees in AEDC's Tunnel 9 indicates the bow shock becomes unsteady near the predicted normal Mach number. Several curved shocks of varying curvature are compared to a straight shock with the same mean normal Mach number, pressure ratio, or temperature ratio. The present work provides analysis and guidelines for designing an inlet robust to off- design flight or perturbations in flow conditions an inlet is likely to face. It also suggests that inlets with curved shocks are less robust to off-design flight than those with straight shocks such as rectangular inlets. Relations for evaluating entropy perturbations for highly unsteady flow across a shock and limits on their use were also developed. The normal Mach number at which a shock could be stable to high frequency upstream perturbations increases as the speed of the shock motion increases and slightly decreases as the perturbation size increases. The present work advances the principle of minimum entropy theory by providing additional validity for using the theory for time-varying flows and applying it to shocks, specifically those in inlets. While this analytic tool is applied in the present work for evaluating the stability of shocks in hypersonic inlets, it can be used for an arbitrary application with a shock.