Aerospace Engineering Theses and Dissertations

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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.
GLOBAL ANALYSIS OF TRANSITIONAL HYPERSONIC FLOW OVER CONE AND CONE-FLARE GEOMETRIES
(2024) Sousa, Cole Edward; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Accurately predicting the laminar-to-turbulent boundary-layer transition on hypersonic vehiclesremains one of the principal challenges in characterizing the expected heat loads and skin friction the vehicle will experience in flight. Ground facilities, while incapable of replicating the complete set of flow conditions found at hypersonic flight, play a critical role in providing physical measurements of the transition process. The experimental characterization of hypersonic boundary-layer disturbances, however, has traditionally faced limitations in its ability to provide spatiotemporally dense data sets comparable to those of computational fluid dynamics (CFD) investigations. The present work aims to provide global off-body measurements of hypersonic boundary-layer disturbances at frequencies much greater than that of the fundamental instability, enabling the exploration of nonlinear phenomena and more extensive comparisons between experimental and computational studies. The current methodology utilizes the fact that hypersonic-boundary layer disturbances havebeen observed to propagate at measurable and statistically predictable velocities. Particularly for the second-mode instability, the density gradient fields acquired by a calibrated schlieren system provide an avenue for resolving dense high-frequency spatiotemporal data. Disturbance propagation velocities extracted from the schlieren images are used to conduct a time-interpolation of the disturbances, which transforms spatially-available descriptions of the travelling waveforms into up-sampled temporal signals at specific pixel locations. When performed across the entire schlieren field of view, the resulting time-resolved signals have a new sampling frequency much greater than the original camera frame rate and a spatial density equal to the camera resolution. This enables the spectral analysis of high-frequency disturbances, including superharmonics of the fundamental instability, which are not originally resolvable from raw time series of the video data. The methodology is employed here in three different experimental data sets, comprising a7° half-angle sharp cone at zero incidence in Mach 6 flow, a 7° half-angle sharp cone at variable incidence in Mach 14 flow, and a cone-flare geometry composed of a 5° frustum with compression angles of +5°, +10°, and +15° at zero incidence in Mach 14 flow. A comprehensive global analysis is conducted on the linear and nonlinear development of the second-mode instability waves in each case. Pointwise measures of the autobicoherence are used to identify specific triadic interactions and the locations of their highest levels of quadratic phase coupling. Significant resonance interactions between the second-mode fundamental and harmonic instabilities are found along with interactions between these and the mean flow. Bispectral mode decomposition is employed to educe the flow structures associated with these interactions. A similar analysis is performed for the power spectrum, with power spectral densities computed for each pixel’s timeseries and spectral proper orthogonal decomposition employed to derive the modal structure and energy of the flow at specific frequencies. The instability measurements taken on the cone-flare geometry are the first of their kind atMach 14. The analysis reveals that incoming second-mode waves undergo extended interactions with the shock waves present at the corner, consistently leading to amplification of the waves and accelerating their nonlinear activity. The disturbance energy is also found to strongly radiate along the shock waves, a behavior that appears to be intensified at high Mach numbers. In the case of separated flow at the corner, additional low-frequency disturbances arise along the shear layer. Self-resonance of these disturbances leads to the radiation of elongated structures upstream of reattachment, which extend outward from the shear layer and terminate at the separation shock. This shear-layer disturbance is determined to be dominantly unstable between separation and reattachment but is significantly damped after reattachment.
WAVE INTERACTION WITH INJECTOR FLOWFIELD IN ROTATING DETONATION ENGINE
(2024) Chang, Minwook; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Rotating Detonation Engines (RDE) utilize detonative combustion processes for heat release instead of deflagration, which is more commonly used in conventional combustors. Potential benefits of RDE include pressure gain combustion, efficient energy conversion, and simpler designs that avoid combustion instability problems due to their cyclic nature of operation. It has been observed, however, that RDE operation can become unsteady due to the onset of counter-rotating detonation waves. In addition, the random presence of residual liquid fuel droplets and their unexpected breakup could also affect the periodic operation of RDE. This study aims to better understand the physical mechanisms that destabilize the RDE’s periodic processes and lead to unsteady operation. Specifically, the investigation focuses on understanding physical mechanisms associated with two key off-design scenarios: (i) the onset of counter-rotating detonation waves and their impact on next cycle fuel injection, and (ii) the breakup of liquid fuel droplets by detonation wave and decoupled detonation wave which consists of shock and flame fronts. Experiments using either hydrogen-oxygen or ethylene-oxygen detonation in linear channel simulate an unwrapped RDE combustor process. For the counter-rotating wave study, detonation waves are initiated from both ends of the channel, and complex recovery behavior associated with colliding detonation waves is examined providing insights into RDE slapping mode operation. For the fuel droplet breakup study, 2-mm diameter ethanol and JP-8 droplets are placed on the downstream path of detonation waves and decoupled shock-flame fronts, which propagate at average wave speeds of Mach 7.3, 3.6, and 2.6, respectively. Liquid droplets break up faster when exposed to slower decoupled shock-flame waves compared to faster detonation waves. This unexpected difference is attributed to the initial slip flow Mach number around the droplet, which is subsonic for detonation waves but supersonic for decoupled waves. Research findings suggest that the slip flow Mach number, along with the Weber number, plays a crucial role in RDE fuel droplet breakup.
INVESTIGATING A POSSIBLE LUNAR COLD SPOT FORMATION MECHANISM: MODELING GRANULAR WAVES IN SURFACE REGOLITH USING SOFT SPHERE DISCRETE ELEMENT METHOD
(2024) Frizzell, Eric; Hartzell, Christine; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Lunar Cold Spots (LCS) are recently classified ephemeral features appearing as halos around fresh impact craters on the Moon. LCS can only be observed by examining nighttime surface regolith temperatures; there is no other indicator of their presence. They appear cold as compared to the background, a suspected consequence of reduced thermal inertia regolith (to about 40 cm depth) in the halo. The reduction in thermal inertia implies that there is some event associated with the impact that dilates (‘fluffs up’) surface material over a large radial extent. The LCS halo extends a much greater distance than could be explained by ejecta alone (the average extent is 100 crater radii), with the halo being occasionally punctured by low thermal inertia rays that extend even further. Cold Spots are ubiquitous, occurring across all terrains of the Moon and representing an approximated 1% of total surface area. However, the LCS formation mechanism is unknown. In this work, we use the Soft Sphere Discrete Element Method (SSDEM) to model granular wave propagation in conditions analogous to the lunar surface and near-surface. Our investigated hypothesis is that LCS form as the result of a granular wave propagating radially outward from an impact site. First, we characterize the behavior of near surface grains experiencing a laterally propagating granular wavefront when they are exposed to vacuum and low-gravity. We simulate piston impacts into long channels filled randomly with Hertzian particles and observe the emergence of a solitary wave (SW, packetized energy propagation) which lofts particles in its wake. We term this mechanism SW Induced Dilation (SID) and see that the amount of fluffing that occurs is enhanced with larger piston impact speeds and as the initial compaction (packing fraction) of the bed increases. We initially observed this effect in an idealized scenario and so we undertake a scaling analysis to predict how SID would manifest in lunar conditions. We show that particle forces experienced in a 3D granular wavefront follow the same scaling relationships as in a 1D chain. We then balance 3D wavefront forces with gravitational overburden to determine an equation that predicts lofting depth (depth to which bulk dilation could be expected to occur) as a function of material properties and the magnitude of particle-particle collisional velocities along the channel floor. Our loft depth equation agrees well with simulated results and predicts a depth within the same order of magnitude as the 40 cm LCS halo depth. However, the existence of a surface SW (and thus, dilation) requires external energy input to be driven along the floor of the channel.Second, we consider a possible driver for the surface dilating wave by characterizing the decay of an initial pulse through chains of buried boulders. On the Moon, the surface regolith (a uniform layer of fine, <100 micron sized particles) covers an underlying layer of larger scale ejecta (boulders). In granular assemblies, the decay of a wave is typically governed by a fixed number of particles since energy is lost through collisions. We suspect that meter scale boulders may be able to support energy propagation over the km scale distances of the LCS halo. Through measuring wave decay in scaled up versions of the channels from our initial work, we see that the decay rate in 3D systems is governed by the same concepts as in a 1D chain: particle number governs decay distance, power law decay in time. We use the lofting equation to estimate the requisite collisional velocity (4 m/s) of particles within the buried layer’s wavefront needed to initiate SID in the surface layer. We see this velocity threshold exceed over hundreds of meters, within an order of magnitude of LCS halo scale distances. The results of this thesis point to the following formation mechanism: cold spot regolith is fluffed up by a surface solitary wave driven from below by the decay of impact energy throughboulders at the regolith-megaregolith boundary. However, this hypothesis requires experimental validation. Understanding cold spot formation has implications for both science and engineering efforts on the Moon. Cold spot regolith fluffing represents yet another surface modification processes altering the already complex history of the Moon (and solar system) entrained in the regolith. If our proposed mechanism is proven to be true, cold spots can also reveal details about local subsurface topography such as the size distribution of buried boulders. The location of buried features may be an important consideration for future large scale construction efforts. Finally, our proposed mechanism suggests that high speed impacts on any low-gravity, low-pressure planetary surface covered with regolith and possessing an underlying layer of larger boulders would display cold spots around young craters.
INDOOR TARGET SEARCH, DETECTION, AND INSPECTION WITH AN AUTONOMOUS DRONE
(2024) Ashry, Ahmed; Paley, Derek; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis investigates the deployment of unmanned aerial vehicles (UAVs) in indoor search and rescue (SAR) operations, focusing on enhancing autonomy through the development and integration of advanced technological solutions. The research addresses challenges related to autonomous navigation and target inspection in indoor environments. A key contribution is the development of an autonomous inspection routine that allows UAVs to navigate to and meticulously inspect targets identified by fiducial markers, replacing manual piloted inspection. To enhance the system’s target recognition, a custom-trained object detection model identifies critical markers on targets, operating in real-time on the UAV’s onboard computer. Additionally, the thesis introduces a comprehensive mission framework that manages transitions between coverage and inspection phases, experimentally validated using a quadrotor equipped with onboard sensing and computing across various scenarios. The research also explores integration and critical analysis of state-of-the-art path planning algorithms, enhancing UAV autonomy in cluttered settings. This is supported by evaluations conducted through software-in-the-loop simulations, setting the stage for future real-world applications.
Aero Database Development and Two-Dimensional Hypersonic Trajectory Optization for the High-speed Army Reference Vehicle
(2023) James, Brendan; Brehm, Christoph; Larsson, Johan; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Steady-flow inviscid and simulations of the High-Speed Army Reference Vehicle geometry were performed within the CHAMPS solver framework at Mach numbers of 4, 6, and 8, and an integrated streamline method was used to apply viscous corrections for Reynolds numbers up to 2x10^8. For each flow Mach, angle of attack sweeps from -10° to +10° were used to determine baseline drag, lift, and moment coefficient alpha dependencies. Coefficient values were then interpolated across Mach, alpha, and Reynolds number parameter spaces to construct an aerodynamic force coefficient database for use in two-dimensional flight simulation and trajectory optimization. By simulating flight with a maximum lift-to-drag control input, sample trajectories for determining maximum vehicle range were produced. A proportional-navigation (PN) controller was implemented which allowed for the targeting of specific altitudes throughout the progression of a trajectory. The PN controller and simulation schemes were then utilized in genetic-algorithm optimization to produce trajectory profiles for achieving minimum time-to-target for gliding flight in standard atmospheric conditions. Over the examined range of initial altitudes, Mach numbers, and release angles, the fastest trajectories were consistently shown to be those which achieved or maintained stratospheric altitudes and consequently benefited from significantly reduced drag before performing a nose-over maneuver for an accurate ground strike.
DIRECT FUSION DRIVE BASED ON CENTRIFUGAL MIRROR CONFINEMENT
(2023) Carson, Jerry Lee; Sedwick, Raymond J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A concept for direct fusion drive based on centrifugal mirror confinement of thermonuclear plasmas (DFD-CM) is described. In centrifugal mirrors, electric and magnetic fields are combined to confine the plasma within a rapidly rotating annulus of burning plasma fixed between two mirror magnets. High-energy fusion products leave the reactor core at a rate determined by the velocity of plasma rotation and the strength of the mirrors. Those departing through the aft jet-side mirror deposit their energy in a “warm plasma” which then expands through a magnetic nozzle to deliver jet power in the 100-1000 kW range. Fusion products departing through the forward, power-side mirror are converted to electricity to power the reactor. Moderate thrusts at attractive specific impulses (15000+ seconds) are possible. Findings are presented on centrifugal mirror reactor dynamics in propulsion applications, to include new insights into the relationship between mirror and centrifugal components of plasma confinement. Additionally, analysis is presented on reactor operability limits and characterization of viable configurations based on power density, technology constraints, and the ability to self-power. Physics of the warm plasma are discussed, to include estimates for fusion energy deposition. Finally, considerations for Alfvén’s “frozen-in” theorem relative to fusion plasmas and magnetic nozzle performance will be outlined.Analysis indicates the DFD-CM system can self-power, and would be relatively compact. For the 200 kW delivered jet power system, the volume of burning plasma in the CM fusion reactor is estimated to be on the order of 1 m3. Self-powering in propulsion applications requires DFD-CM reactor operation at M_θ>9. This in turn requires electric fields ranging from 40-90 MV/m, and mirror strengths up to 15T. The main losses in the propulsion system are due to heating and ionizing the propellant. These losses decrease with increasing specific impulse. This work has resulted in four contributions. To start, it is the first analysis of the end-to-end performance of direct fusion drive based on centrifugal mirror confinement of the burning plasma. It demonstrates that the concept is thermodynamically feasible with nominal cycle efficiencies of 50 percent based on fusion energy entering the propulsion system. The second contribution is characterization of CM fusion reactor performance and operability. A particular finding is that self-powering DFD-CM reactors in propulsion applications may need to operate at centrifugal Mach numbers greater than 9, as previously mentioned. The third contribution is the development and preliminary application of a set of engineering models of the reactor, warm plasma, and plasma acceleration and expansion. These models are considered moderate fidelity in that they account for first order effects, as well as salient second order effects. The fourth contribution is identifying the possibility that the burning plasma in the reactor and the warm plasma may be electrically coupled. The nature and implications of any coupling are uncertain, and the current research proceeds assuming that the coupling does not occur. However, the question indicates the need for further research.
A GHOST CELL BASED IMMERSED BOUNDARY METHOD FOR WALL-MODELED LARGE-EDDY SIMULATIONS
(2023) Ganju, Sparsh; Brehm, Christoph; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Turbulence is one of the most important unsolved problems in classical physics as it strongly affects skin friction and heat transfer rates and, thus, the performance of aerospace systems. Due to the high cost associated with flight and wind tunnel tests, there exists an ever-increasing demand for computational tools for engineering analysis. Hence, accurately predicting turbulent flows around aerospace vehicles is crucial for lowering safety factors, achieving optimal performance, and for the development of novel designs. However, simulating turbulent flows around complex geometries is a challenging task, especially for off-design conditions, where the flow may not be fully attached to the vehicle surface, e.g., close to stall conditions. A key challenge is that resolving all relevant turbulent scales is computationally intractable in a day-to-day engineering environment even with currently available computing hardware. For these complex turbulent flows, higher-fidelity simulation approaches, such as large eddy simulation (LES), are employed as they are able to provide the desired accuracy. In recent years, with the advancement of simulation methodologies and the accessibility of modern computer hardware, there has been a growing consensus within the Computational Fluid Dynamics (CFD) community that higher-fidelity turbulent flow simulations (including DES and wall-modeled LES, or WMLES) will be used more frequently complimenting lower-fidelity CFD approaches, such as Reynolds Averaged Navier-Stokes (RANS) approaches, which are generally accurate enough at more benign flow conditions. While conventional wall-resolved LES approaches are still significantly more expensive than RANS-based methods, WMLES can provide accurate turbulent flow predictions at a fraction of the cost compared to direct numerical simulations (DNS) or wall-resolved large-eddy simulations (WRLES). This is achieved by modeling only the large-scale flow structures as in conventional LES and in the vicinity of the wall, where most of the grid points are concentrated in WRLES, a wall model is utilized to account for the very small near wall flow structures. All aforementioned CFD approaches, i.e. RANS, LES, and DNS, rely on the creation of high-quality body-fitted grids that can be a time-consuming task depending on the geometric complexity of the fluid system being analyzed. The use of immersed boundary methods (IBM) on Cartesian grids can completely eliminate the manual mesh generation process, thus, reducing the overall time-to-solution. This dissertation assesses the viability of an unconventional simulation approach eliminating the time-consuming mesh generation approach by combining the IBM with WMLES. It was shown that in cases where the Cartesian grid is not aligned with the immersed boundary, the details of the numerical scheme, \textit{i.e.}, the order of accuracy, boundary treatment, \textit{etc.} play an important role in obtaining accurate solutions. Schemes with higher orders of accuracy lead to reduced numerical errors in the vicinity of the boundary providing better solutions. Furthermore, a physics informed boundary operator was used to provide a better representation of the mean flow in the near-wall region by using the gradient from the wall-model solution. A series of test cases are used to demonstrate the capabilities of this method for real-world applications and provide comparisons with conventional body-fitted results.
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.
Hover Performance of a Two-Bladed Model Rotor in Confined Areas
(2023) Shenk, Cole; Tritschler, John; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The present study investigated the performance of a teetering rotor hovering in ground effect inside of confined areas. A teetering rotor with blades 1:13.24 the scale of OH-58C blades was positioned over a six degree-of-freedom motion platform. Hover power and thrust were measured for collective blade pitches of 0◦, 4◦, 8◦, and 10◦ at 45 heights ranging from hub heights, z, of 0.8-3.0R. This process was completed in the presence of four wall orientations including parallel, L-shape, and U-shape configurations spaced 1.5R from the vertical axis of the rotor hub as well as a U-shaped configuration spaced 2.36R from the rotor hub axis to match full-scale flight tests. Configurations with spacing of 1.5R were tested for wall heights of 0.25, 0.5, 1.0, and 1.5R. The configuration scaled to flight test conditions was tested at wall heights equal to 0.61, 1.08, 1.56 and 2.08R. Rotor performance measurements were also collected out of ground effect at blade collective pitches of 0◦ and 4-10◦ in order to make comparisons between in-ground-effect and out-of-ground effect hover performance. Hover within confined areas was found to have up to a 20% increase in power required compared to hover out of ground effect. In all but four configurations, the variation in the magnitude of the maximum penalty across a blade loading range from 0.06 to 0.1 was greater than or equal to 1%, so the results were repeatable and consistent. When wall heights were greater than or equal to 1.0R, the maximum power penalty typically occurred at a hub height between 83% and 90% of the wall height. Penalties associated with the confined area appeared to be influenced by changes in blade loading, wall orientation, wall height, and wall configuration.
The Effect of Confined Areas on Helicopter Performance
(2023) Black, Dylan; Tritschler, John; Milluzzo, Joseph; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Flight test performance of an OH-58C helicopter hovering in confined areas is discussed using a combination of pilot-recorded data cards and instrumentation data time histories. The test includes an investigation of the effects of wall height and blade loading on hover performance in close proximity to a three-walled structure forming a confined space. Hover performance for a range of altitudes far from the confined area, at the edge of the confined area, and at the center of the confined area are discussed. Significant performance penalties (i.e., up to 20% greater than the power required to hover out of ground effect) were observed at various positions within the confined space. Additionally, the pilots reported uncommanded vehicle excursions at some positions within the confined area, necessitating increased control inceptor activity. These observations are indicative of unique aerodynamic interactions that affect helicopter performance and handling qualities within confined areas.
Experimental Investigation of Boundary-Layer Transition on a Slender Cone at Mach 4
(2023) Jones, Benjamin Rhys; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Boundary-layer transition over a 5 degree half-angle straight cone model was examined in University of Maryland's Multiphase flow Investigations Tunnel (MIST), a Mach 4 Ludweig tube. In order to prepare for future studies with particle-laden flow, this study was conducted to characterize the boundary-layer of the cone under dry conditions. Furthermore, both first-mode (Tollmien-Schlichting) and second-mode (Mack) boundary-layer instabilities are expected at Mach 4 (Mack), with the former being not widely studied. The boundary-layer on the top surface of the cone was visualized using high-speed Schlieren and analyzed using image and signal processing techniques. Experimentation was conducted over a range of unit Reynolds numbers from 31.3-40.3 10^6 m^-1 in order to vary the transition location. Power Spectral Density (PSD) and Spectral Proper Orthogonal Decomposition (SPOD) revealed the most coherent wave packets propagating within the boundary layer at frequencies ranging from 10 to 40 kHz, frequencies that are consistent with the first mode. Frequencies between 110-140 kHz contained additional content consistent with the first mode but less coherent. The presence of these structures was verified with a bandpass filter allowing 10-40 kHz and a highpass filter allowing >100 kHz each separately applied to the original footage. Future work is planned to compare the results of this paper with multi-phase flow experiments conducted with the same model at the same freestream conditions.
FAST FEASIBLE MOTION PLANNING WITHOUT TWO-POINT BOUNDARY VALUE SOLUTION
(2023) Nayak, Sharan Harish; Otte, Michael; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Autonomous robotic systems have seen extensive deployment across domains such as manufacturing, industrial inspection, transportation, and planetary surface exploration. A crucial requirement for these systems is navigating from an initial to a final position, while avoiding potential collisions with obstacles en route. This challenging task of devising collision-free trajectories, formally termed as motion planning, is of prime importance in robotics. Traditional motion planning approaches encounter scalability challenges when planning in higher-dimensional state-spaces. Moreover, they rarely consider robot dynamics during the planning process. To address these limitations, a class of probabilistic planning methods called Sampling-Based Motion Planning (SBMP) has gained prominence. SBMP strategies exploit probabilistic techniques to construct motion planning solutions. In this dissertation, our focus turns towards feasible SBMP algorithms that prioritize rapidly discovering solutions while considering robot kinematics and dynamics. These algorithms find utility in quickly solving complex problems (e.g., Alpha puzzle) where obtaining any feasible solution is considered as an achievement. Furthermore, they find practical use in computationally constrained systems and in seeding time-consuming optimal solutions. However, many existing feasible SBMP approaches assume the ability to find precise trajectories that exactly connect two states in a robot's state space. This challenge is framed as the Two-Point Boundary Value Problem (TPBVP). But finding closed-form solutions for the TPBVP is difficult, and numerical approaches are computationally expensive and prone to precision and stability issues. Given these complexities, this dissertation's primary focus resides in the development of SBMP algorithms for different scenarios where solving the TPBVP is challenging. Our work addresses four distinct scenarios -- two for single-agent systems and two for multi-agent systems. The first single-agent scenario involves quickly finding a feasible path from the start to goal state, using bidirectional search strategies for fast solution discovery. The second scenario focuses on performing prompt motion replanning when a vehicle's dynamical constraints change mid-mission. We leverage the heuristic information from the original search tree constructed using the vehicle's nominal dynamics to speed up the replanning process. Both these two scenarios unfold in static environments with pre-known obstacles. Transitioning to multi-agent systems, we address the feasible multi-robot motion planning problem where a robot team is guided towards predefined targets in a partially-known environment. We employ a dynamic roadmap updated from the current known environment to accelerate agent planning. Lastly, we explore the problem of multi-robot exploration in a completely unknown environment applied to the CADRE mission. We demonstrate how our proposed bidirectional search strategies can facilitate efficient exploration for robots with significant dynamics. The effectiveness of our algorithms is validated through extensive simulation and real-world experiments.
Physics and Modelling of Compressible Turbulent Boundary Layer
(2023) Lee, Hanju; Martin, Pino; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Key findings from a research study that focuses on understanding the effect of Mach number, Reynolds number and wall temperature on compressible turbulent boundary layers (CTBL) in the hypersonic regime are presented in this dissertation. The study utilizes a comprehensive CTBL database developed using an in-house direct numerical simulation (DNS) code at the CRoCCo laboratory. The database encompasses a range of semi-local Reynolds numbers (800 to 34,000) and Mach numbers up to 12, incorporating wall-cooling. The effects of density and viscosity fluctuations on the total stress balance are identified and used to create a new mean velocity transformation for compressible boundary layers. The role, significance and physical mechanisms connecting density and viscosity fluctuations to the momentum balance and to the viscous, turbulent and total stresses are presented, allowing the creation of generalized formulations. We identify the significant properties that thus-far have been neglected in the derivation of velocity transformations: (1) the Mach-invariance of the near-wall momentum balance for the generalized total stress, and (2) the Mach-invariance of the relative contributions from the generalized viscous and Reynolds stresses to the total stress. The proposed velocity transformation integrates both properties into a single transformation equation and successfully demonstrates a collapsing of all currently considered compressible cases onto the incompressible law of the wall, within the bounds of reported slope and intercept for incompressible data. Based on the physics embedded in the two scaling properties, the success of the newly proposed transformation is attributed to considering the effects of the viscous stress and turbulent stresses as well as mean and fluctuating density viscosity in a single transformation form. The Reynolds number trends of large turbulent structures in compressible turbulent boundary layers are investigated using the pre-multiplied energy spectra based on the density corrected fluctuating streamwise velocity signal. Results demonstrate the existence of friction as well as semi-local Reynolds number trend associated with large-scale structures, similar to trends observable in incompressible turbulent boundary layers (ITBL). In particular, the behavior of turbulence in the inner layer is seen to exhibit dependence based on both definitions of Reynolds numbers. On the contrary, the strength of large turbulent structures is seen to be only dependent on friction Reynolds number. This result directly contrasts with the observation of the near-wall turbulent intensity peak increasing with semi-local Reynolds number. The discrepancy is mended with a suggestion that the large turbulent scales in the log layer of which the strength increases with friction Reynolds number, are modified through the changes in local fluid properties such that the scale interaction near the wall increases as semi-local Reynolds number. In another words, closer to the wall, the CTBL flow behaves like a semi-local Reynolds number flow, while closer to the freestream, it behaves like a friction Reynolds number flow. Furthermore, the present study examines the Reynolds number dependence of the length scale between small and large turbulent scales. The analysis highlights the inadequacy of using a univariable wavelength based on viscous, semi-local or outer length scales to differentiate small and large scales. Based on this, the use of Reynolds number-dependent length scales is recommended. Overall, the study provides valuable insights into the Reynolds number trends of large turbulent structures in CTBL, emphasizing the influence of both semi-local Reynolds number and friction Reynolds number on turbulence characteristics.
Aeroacoustic Implications of Installed Propeller Interactional Aerodynamics and Transient Propeller Motions
(2023) Herath Mudiyanselage, Anjanaka Dilhara Jayasundara; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The emergence of advanced air mobility and sustainable aviation concepts have revived the interest in propeller-driven aircraft. A number of electric vertical take-off and landing (eVTOL) aircraft have been developed to cater to the demands of urban air mobility (UAM) and significant advancements have been made in unmanned aerial vehicles (UAV) equipped with vertical take-off and landing capabilities. However, the community acceptance of these new aircraft configurations highly depends on having a low noise footprint as they will operate in dense urban environments. Propeller noise is considered the major source of noise in these aircraft with the introduction of electric propulsion and it can significantly increase with the effects of installation and transient propeller motions. This study aims to comprehend the complex aerodynamic interactions within such aircraft that result from propeller installation and contribute to the generation of high noise levels. To understand the physics of propeller installation, a wingtip-mounted propeller was analyzed at several angles of attack using computational fluid dynamics (CFD) based on Reynolds-averaged Navier-Stokes (RANS) equations and computational aeroacoustics based on the Ffowcs Williams - Hawkings equation. The aeroacoustic implications of the propeller axis inclination and the propeller-wing aerodynamic interaction were studied in-depth. The propeller-wing interaction leads to a significant increase in propeller noise (~20 to 30 dB increase along the rotational axis) and causes the wing to generate a loading noise in the same order of magnitude as the propeller noise. To extrapolate the understanding of installation effects to a full aircraft, the aeroacoustic characteristics of a quadrotor biplane tailsitter were analyzed in both hover and forward flight focusing on the rotor-rotor and rotor-airframe aerodynamic interaction. The rotor-rotor interaction was found to be a significant source of loading noise in hover but having the fuselage as a physical barrier between the rotors largely reduces its effect. The airframe loading noise and rotor broadband noise are equally dominant as the rotor tonal noise when the aircraft is in forward flight. Moreover, the study evaluated the effectiveness of rotor synchrophasing in reducing the aircraft noise footprint and it showed promising results in hover, causing a reduction of aircraft noise by more than 10 dB. Furthermore, an efficient computational aeroacoustics framework was developed to facilitate the computations, ensuring optimal utilization of the computational resources. The CPU and GPU parallelization and other optimization techniques were able to achieve a 98% reduction in computation time for an isolated propeller case. This enabled the rapid aeroacoustic computations of periodic and non-periodic problems. This was used to analyze the aeroacoustics of an isolated propeller undergoing a transition from hover to forward flight. The aerodynamic and acoustic results of the unsteady case were compared with quasi-steady cases performed at intermediate tilt angles. The quasi-steady CFD simulations predicted the unsteady transition aeroacoustics with reasonable accuracy. A tilting quasi-steady approach was proposed to better capture the aerodynamics and acoustics of the unsteady transition.
Experimental Evaluation of Circulation Control Aerodynamics on a Cylindrical Body
(1987) Ngo, Hieu Thien; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
In this study, an experimental investigation is conducted on a two-dimensional circulation control cylinder with blowing taking place from a single spanwise slot to determine its aerodynamic characteristics. The results include detailed pressure distributions (both chordwise and spanwise) for a range of momentum coefficients and slot locations. The measured results showed that the lift coefficients up to 4.8 were produced at momentum coefficients of 0.14 in a turbulent flow condition. The experimental results of lift coeffficients Were correlated satisfactorily with analytical results. The surface flow patterns were observed using the oil and smoke techniques. Also flow field surveys of the model Were obtained using total pressure, yaw and pitch probes. A color video display technique was used to present the results of the flow field surveys. Based on this evidence, a flow field model of the circulation control cylinder is presented.
Fault Detection and Emergency Path Planning for Fixed Wing UAVs
(2023) Gomez Quezada, Ruth; Xu, Huan; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The implementation of Uncrewed Aerial Vehicles (UAVs) for civilian and military purposes requires safety protocols. Control surface failures are among the most common issues in fixed-wing UAVs. This study presents two heuristic-based algorithms developed to detect such faults. The Mahalanobis Distance Fault Detection Algorithm (MDFDA) employs the Mahalanobis distance to identify anomalies in UAV states. On the other hand, the Fuzzy Logic Fault Detection Algorithm (FLFDA) uses fuzzy logic to identify jammed control surfaces. In the event of a fault, an emergency path planning algorithm is initiated. This algorithm leverages terrain and population data to pinpoint safe landing zones. However, the type of fault the system encounters will impact the aircraft’s ability to reach these safe zones. In this study, unreachable zones are delineated based on the specific control surface fault detected. These zones are areas where the aircraft cannot land or traverse due to the fault. By employing the proposed protocol, the aircraft can detect a fault during mid-flight, select a safe landing zone within its reachable range, and mitigate the effects of the control surface fault. This approach enhances the chances of preserving the aircraft and ensures the safety of the surrounding population.
AN INVESTIGATION OF THE OBSERVABILITY OF PLASMA SOLITONS GENERATED BY CUBESATS USING INCOHERENT SCATTER RADAR ARRAYS
(2023) Wilson, Connor MacNaughton; Hartzell, Christine M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Conventional ground-based observation methods, such as tracking radars, are unable to reliably detect and track subcentimeter orbital debris. This debris poses a risk to crewed and robotic spacecraft as it is capable of penetrating structures and damaging instruments. Tracking this debris reliably would allow for improved mitigation maneuvers to reduce mission risk. Based on recent publications, an alternate detection method could involve sensing plasma density solitary waves. These waves, henceforth “solitons,” are predicted to be produced by the interaction of the electrical charge on the lethal nontrackable debris with the local ionospheric plasma. This body of work seeks to test the feasibility of detecting the predicted soliton generated by defunct 1U CubeSats because they are already tracked using conventional methods and are predicted to produce solitons. Ground-based observers, such as incoherent scatter radars operated by the European Incoherent Scatter Scientific Association, may be able to detect the presence of solitons created by a CubeSat traveling through the ionosphere. This may be accomplished by observing the variation in electron density of the measurements near the time when the CubeSat passes through the radar beam. Testing this hypothesis initially involves several 1U CubeSats that are propagated from their last time of observation until they overfly an incoherent scatter radar site during its operating cycle. In this observation window, the hypothesized soliton is modeled for each target of opportunity. The modeled solitons will travel with the CubeSats and are effectively pinned to them. These pinned solitons are compared with electron density measurements from the radar station. The variance in the measurements makes it unlikely to confirm an observation without a filter, but the apparently uncorrelated variance of the plasma density measurements is on the same order of magnitude as the one-dimensional soliton disturbances induced by the CubeSat. Based on this, a one-dimensional linear Kalman filter is implemented to look for positively correlated deviations of the electron density estimates when the debris is passing through the radar beam; three out of the five test targets are positively correlated and a fourth is nearly correlated. Simulating the pinned solitons across a variation of times and altitudes indicates that there are better times of day and year to look for evidence of these solitons; the largest possible soliton for this data set is 140% the magnitude of the mean electron density. Further analysis of this best case scenario determines that it is unlikely any measurements could detect a deviation in electron density due to a soliton because of the size of the range gate and beam width relative to the signal size and strength. The range gate and beam width would need to be on the order of 1 meter as opposed to the 700 meter range gate in the test case. The sample rate would need to be less than 10 µs instead of 1 minute. Given this conclusion, the experimental evidence implies that there may be another factor causing the correlation algorithm to function as initially intended; perhaps the soliton is larger than predicted or the CubeSat itself is being detected in the measurement.
NEUTRON SHIELDING DESIGN FOR CENTRIFUGALLY CONFINED SPACE PROPULSION SYSTEM
(2023) Parsons, Jennifer; Sedwick, Raymond J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis presents a preliminary neutron shielding design for the HTS coils of a centrifugally confined fusion space propulsion system, which is a promising technology for future space travel. The design process involved a comprehensive study of neutron transport, material selection, and shielding optimization using MCNP and MATLAB simulations. First, the neutron attenuating properties of reflector, moderator, and absorber candidate materials were compared in MCNP. The thickness and composition of the shield were optimized from the resulting MCNP data. Next, two overall reactor and shielding geometry models were developed in MATLAB to estimate the total mass of the HTS shielding for both coils. The first model assumed a point neutron source and uniform thickness across the surface area of the shield. The second model improved upon the first by considering a source distribution and the varying distance between the source and surface of the shield. Both D-T and D-D fuel cases were run with the model and the resulting mass estimates were used to compare the specific mass to the state-of-the-art technology.
Effect of Sloped Terrain on In-Ground-Effect Hover Performance for an Isolated Rotor
(2023) Prewitt, Jack; Tritschler, John; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The present work conducted performance testing using a laboratory-scale isolated rotor operating over a ground plane mounted to a six degree-of-freedom motion platform to simulate in-ground-effect operations over sloped terrain. The rotor utilized a pair of 1:13.24 scale OH-58C blades, and performance measurements were collected using a six-axis load cell to which the rotor was mounted. Seven ground plane angles ranging from 0–18 deg, five collective blade pitches ranging from 0–8 deg, and 15 hub heights ranging from a nondimensional hub height, z/R, of 0.6 to 2.0 were tested. Additionally, the rotor was operated out-of-ground-effect for collective blade pitches ranging from 0–12 deg in increments of 1 deg in order to compare in-ground-effect and out-of-ground-effect hover performance. In-ground-effect hover over sloped terrain was found to have over a 7% reduction in performance as compared to hover over level terrain at low hub heights and large ground plane angles. In-ground-effect hover over sloped terrain was also found to require 2% more power than hover out-of-ground-effect at high hub heights and large ground plane angles. Finally, a semi-empirical model for hover performanceover sloped terrain was developed on the basis of the classic Cheeseman and Bennett ground effect model for level terrain. The coefficients obtained from this model were found to change in a consistent manner as both ground plane angle and blade loading coefficient changed, which suggests that the model could be used for future performance predictions for hover over sloped terrain.

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