# Aerospace Engineering Theses and Dissertations

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Item 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.Item 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.Item 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.Item 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.Item 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.Item 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.Item 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.Item 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.Item 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.Item 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.Item 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.Item Examining the Passive Stiffness Workspace Using Variable Stiffness Robots(2023) Feinberg, Evan Harris; Akin, David L; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Passive stiffness control is a method for managing contact forces and dynamics between a robotic manipulator and its environment. Compliance control is typically implemented in redundant serial manipulators using a force torque sensor and active software algorithms. However, time delays in these algorithms can cause large impulse forces between the manipulator and its environment. For applications with limited computation power, large time delays, and low damping, such as In-space Servicing Assembly and Manufacturing (ISAM) and Active Debris Removal (ADR), these effects can cause a manipulator to push away or tip off the target, preventing successful capture. This thesis examines the implementation of passive stiffness control in a redundant serial manipulator using Variable Stiffness Actuators (VSA). Unlike traditional robot actuators, VSAs have an adjustable stiffness element in series with the primary joint position/control motor to generate varying end-effector position and stiffness. These adjustable springs act as low pass force filters to increase the actuator robustness against external loads at the cost of positioning accuracy. Different optimization algorithms are used to vary the VSA joint stiffness to achieve a desired Cartesian stiffness matrix. However, there are severe limitations with the passive joint to Cartesian stiffness mapping performance over the whole robotic workspace due to the significant kinematic configuration dependence. Given this limitation, this work attempts to answer for a single, well-defined task, are there regions of the workspace where a prescribed level of passive stiffness realization can be achieved? Or for a mobile robot, can we plan trajectories within a region of the workspace to improve realization performance? This is done by first examining the implementation of three passive stiffness realization methods, each with increasing performance. Next, the idea of Successful Task/Stiffness Trajectories and Success Task/Stiffness Regions are introduced as a way to examine the workspace dependency of the passive stiffness realization. Finally, the application of passive stiffness control for ISAM and ADR applications is studied, and unique design objectives for the manipulator are proposed.Item A Meta-Learning based Aerodynamic Analysis Framework for Wind Turbine Design Applications(2023) Marepally, Koushik; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Design testing and analysis is a major bottleneck in the design process of wind turbine applications, mainly due to the computational cost of analysis tools like computational fluid dynamics (CFD). Furthermore, the accuracy of the state-of-the-art turbulence models is low in flows with high adverse pressure gradients such as airfoils operating at high angles of attack. This study aims to develop an aerodynamic analysis framework for wind turbine airfoils with both improved cost and improved accuracy to use in design applications. An artificial neural network-based data-driven surrogate model is developed to predict the aerodynamic performance quantities of lift coefficient, lift-to-drag ratio, and pitching moment coefficient for wind turbine airfoils. An efficient geometric space exploration strategy is used to generate a representative database of wind turbine airfoils and their corresponding performance quantities. The developed surrogate model shows a uniform accuracy across a wide range of wind turbine airfoil geometries, with an L2 error estimates of 0.03 in lift coefficient, 0.4 in lift-to-drag ratio, and 0.003 in pitching moment coefficient. These errors correspond to less than 2% magnitudes of the corresponding performance quantities at the design point. With a benefit of more than six orders of magnitude in computational cost compared to CFD, the surrogate model has the capabilities to be embedded in uncertainty quantification (UQ) and multidisciplinary design analysis and optimization (MDAO) frameworks. To reduce the model development cost, various parameter space exploration and reduction strategies are tested to benchmark the impact of reducing the training data on the accuracy of the surrogate model. With uniform data puncturing style, the accuracy level of the surrogate model is maintained even with up to a 50% reduction in the training data. The propagation of uncertainty from the geometric parameters of the airfoils to the airfoil performance quantities is quantified using the surrogate model coupled with a Monte-Carlo-based UQ framework. The performance quantities show an uncertainty of about 3% of their magnitude for a 5% geometric uncertainty near the operational angle of attack and more than 10% magnitude of uncertainty near the stall angle of attack. Secondly, field inversion machine learning (FIML) methodology is applied on multiple airfoils to arrive at a model consistent correction to the turbulence model for improved airfoil stall predictions. The corrected turbulence model shows a consistent improvement of the stall lift predictions with an improvement in stall angle of attack by more than 35% and stall lift coefficient by more than 40%. Besides the lift coefficient, the corrected turbulence model predicts the surface pressure and flow separation point more accurately. A meta-learning model is developed using the corrected turbulence model on the database of wind turbine airfoils, which is both computationally inexpensive and closer to the experimental data. The model is integrated with an evolutionary optimization framework and tested on various airfoil design problems, including airfoil drag minimization by 5% and stall delay by 1 degree.Item Flight Dynamics of a Coaxial Helicopter Hovering on Mars(2023) Greenbaum, Eric Ryan; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis develops a fundamental understanding of the flight dynamics and stability of a coaxial helicopter hovering on Mars. A simple flight dynamics solver is built to identify and investigate unique characteristics of Martian hover and validated with Ingenuity chamber test data from NASA JPL. The impact of two key parameters that are unique to the Mars helicopter — an unusually high flap frequency and an unusually low Lock number, are systematically studied. The rotor pitch and roll damping derivatives flip sign on Mars. The hover roots are more unstable, and the impact of wake curvature and inflow gradients on these roots appear unusually high. Even though the simplicity of the solver provided enough latitude to insert correction factors to replicate test data, the underlying causes remain unknown. These and other interesting gaps remain subject for the future researcher. The world of Mars flight dynamics has only been explored for two years and for fifty flights of Ingenuity. Larger, more capable aircraft envisioned for future science missions will encounter this new world and require that these gaps be resolved.Item Experimental Measurements of Vortex Breakdown with non-Isothermal Inflow(2023) Krupnik, Assaf; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Vortex breakdown occurs in many flow applications such as weather, aerodynamics, swirl combustors, and more, introducing unsteadiness that is often undesired. While vortex breakdown has been previously investigated extensively, researchers have struggled to efficiently characterize these flows due to the flow sensitivity to intrusive probes, and there is no real consensus regarding the process and reason of its formation. Recently, researchers have discovered the blue whirl, a silent and efficient flame which they believe is a mode of vortex breakdown, and suggested mechanisms of stabilization and prevention of it through temperature control. This thesis uses modern time-resolved measurement techniques to investigate breakdown at smaller temporal and spatial scales than previously researched, and shows the effect of heat addition at the inflow of a vortex generator on the onset and behaviour of breakdown modes. Smoke flow visualization experiments were preformed to identify heating and swirl rate effects on vortex flow, and the resulting swirl angle of the flow was measured. Decreasing the incoming swirl was found to delay or fully suppress the formation of a breakdown bubble. Increasing the inlet temperature had a similar effect due to the buoyancy effects on the flow increasing the axial velocity thus reducing the effective swirl. 3D Particle Tracking Velocimetry was used to obtain time-resolved flow fields of the vortex flow and breakdown with and without heating. Individual velocity profiles and velocity fields are presented, showing that flow behavior is dependent on much smaller scales than previously researched. Proper Orthogonal Decomposition is used to isolate energetically dominant modes and determine whether higher order modes are significant to the flow.Item A Time Parallel Approach to Numerical Simulation of Asymptotically Stable Dynamical Systems with Application to CFD Models of Helicopter Rotors(2023) Silbaugh, Benjamin Scott; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modern High Performance Computing (HPC) machines are distributed memoryclusters, consisting of multi-core compute nodes. Engineering simulation and analysis tools must employ efficient parallel algorithms in order to fully utilize the compute capability of modern HPC machines. The trend in Computational Fluid Dynamcis (CFD) has been to construct parallel solution algorithms based on some form of spatial domain decomposition. This approach has been shown to be a success for many practical applications. However, as one attempts to utlize more compute cores, limitations in strong scalability are inevitably reached due to a diminishing compute workload per compute core and either fixed or increasing communication cost. Furthermore, spatial domain decomposition approaches cannot be easily applied to mid-fidelity structural dynamics or rigid body dynamics models. A significant majority of industrial fluid and structural dynamic models utilize some form of time marching. Thus, if the domain decomposition strategy may be extended to include the temporal dimension, additional opportunity for increased parallelism may be realized. A new form of periodic multiple shooting is proposed that ismatrix-free and may be applied to high-fidelity multiphysics models or other high dimensional systems. The proposed methodology is formulated entirely in the time domain. Therefore, existing time-domain simulation tools may utilize the proposed approach to achieve a high degree of distributed memory parallelism without requiring any reformulation. Furthermore, the proposed methodology may be combined with conventional space domain decomposition techniques and other forms of data parallelism to achieve maximal performance on modern HPC architectures. The proposed algorithm retains the iterative shoot-correct approach of conventational periodic shooting methods. However, the correction stage is formulated using a hierarchical evaluation strategy combined with an Arnoldi subspace approximation to eliminate the need for explicit formulation of Jacobian matricies. The local convergence of the proposed method is formally proven for the case of an asyptotically stable dynamical system. The proposed method is numerically tested for a 2D limit cycle problem, a rigid blade helicoper rotor model with quasi-steady aerodynamics and autopilot trim, and an OVERSET CFD model of a helicopter rotor with prescribed elastic blade motions. The method is observed to be convergent in all test cases and found to exhibit good scalability. The proposed periodic multiple shooting method is a practical means of reducingtime-to-solution for numerical simulations of asymptotically stable periodic systems on distributed memory parallel computers. Furthermore, the proposed method may be used to enhance the parallel scalability of OVERSET CFD models of helicopter rotors in steady periodic flight.Item An Assessment of Aerogravity-Assisted Trajectories for Aerocapture at the Ice Giants(2023) Zimmerman, Grace Katherine; Hartzell, Christine; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Ice Giants, Neptune and Uranus, are two candidates for an aerocapture maneuver, in which a spacecraft is captured into a bound orbit through a single atmospheric pass. Another aeroassisted maneuver, the aerogravity-assist (AGA), uses an atmospheric pass to increase the turn angle about a planet, thus enabling large changes in a spacecraft's heliocentric velocity. Both maneuvers require high arrival velocities and thermal protection system technologies; thus, it may be beneficial to execute both maneuvers on a single mission. To investigate the possible benefit of an AGA trajectory for setting up an aerocapture maneuver at the Ice Giants, a two-layer optimization approach has been employed to investigate the trajectory space. As an AGA maneuver relies on minimal planetocentric velocity loss due to drag, previous studies have focused on AGAs with high lift-to-drag ratio (L/D) vehicles, despite all heritage interplanetary vehicles being low-L/D. Due to the technology barrier for high-L/D interplanetary vehicles, the present study uses lower L/D vehicles, from low-L/D heritage vehicles to mid-L/D optimally-shaped vehicles. Feasible trajectories are identified for both Uranus and Neptune that increase the number of feasible launches as compared to traditional gravity-assisted (GA) trajectories. In addition, the study identifies a new family of periodic high-altitude AGA trajectories to Uranus that are feasible using heritage vehicles. For Uranus, trajectories are identified that enable aerocapture within current heat shield technology constraints using a vehicle with L/D = 1.25.Item SATELLITE SERVICING AS A MEANS TO INCREASE SPACE MISSION RESILIENCE IN LOW EARTH ORBIT: A PARAMETRIC ARCHITECTURE ANALYSIS(2023) Gabriel, Jonathon Loegan; Akin, David L; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Satellite servicing and associated capabilities have the potential to establish a new space mission design and operation paradigm throughout Earth orbit. By integrating fundamental elements of the logistics chains that complex engineered systems enjoy on Earth’s surface to Earth’s orbit, the feasible domain for space missions will significantly expand. Some estimates suggest that the burgeoning satellite servicing industry could generate over 14 billion United States Dollars in revenue over the next decade, driven in large part by growing demand from satellite operators in Low Earth Orbit. However, despite significant economic development and the modern shift of commercial and government space industry focus to Low Earth Orbit, the study of satellite servicing architecture design in this context requires more analysis to mature. Existing satellite servicing mission design literature generally investigates the system design problem as an economic feasibility analysis or system optimization problem. A subpopulation of this literature introduces novel design metrics to the system design process, such as mission flexibility, bringing significant utility to mission designers. In recent years, mission resilience has proven to be a space mission design metric of significant interest to a diverse set of stakeholders such as the United States Department of Defense. Despite its rapidly expanding use, resilience in the context of space mission design has been described primarily qualitatively, limiting its engineering use. Satellite servicing, as with other applications of engineering resilience techniques, aims to integrate capabilities into a complex system that enables a response to system degradation in a favorable manner. This thesis develops a robust simulation framework to parametrically investigate the Low Earth Orbit satellite servicing system design space in the context of scenarios of interest, such as the potentially degrading events of solar storms, orbital debris collisions, and natural satellite failures. A focus will be placed on quantifying the effects on system resilience that satellite servicing can afford Low Earth Orbit constellations. First-order space mission design parameters will be parametrically investigated using the developed analysis and simulation framework. Through leveraging the Earth’s J2 perturbation to help route servicer satellites efficiently throughout a constellation of modeled customer satellites, it will be shown that the integration of satellite servicing capabilities into Low Earth Orbit constellations can significantly increase system resilience inside the performance constraints of existing space vehicles. Satellite Servicing system design strategies will be presented that can be employed to increase mission resilience and feasibility.Item A Time Parallel Approach to Numerical Simulation of Asymptotically Stable Dynamical Systems with Application to CFD Models of Helicopter Rotors(2023) Silbaugh, Benjamin Scott; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modern High Performance Computing (HPC) machines are distributed memoryclusters, consisting of multi-core compute nodes. Engineering simulation and analysis tools must employ efficient parallel algorithms in order to fully utilize the compute capability of modern HPC machines. The trend in Computational Fluid Dynamcis (CFD) has been to construct parallel solution algorithms based on some form of spatial domain decomposition. This approach has been shown to be a success for many practical applications. However, as one attempts to utlize more compute cores, limitations in strong scalability are inevitably reached due to a diminishing compute workload per compute core and either fixed or increasing communication cost. Furthermore, spatial domain decomposition approaches cannot be easily applied to mid-fidelity structural dynamics or rigid body dynamics models. A significant majority of industrial fluid and structural dynamic models utilize some form of time marching. Thus, if the domain decomposition strategy may be extended to include the temporal dimension, additional opportunity for increased parallelism may be realized. A new form of periodic multiple shooting is proposed that ismatrix-free and may be applied to high-fidelity multiphysics models or other high dimensional systems. The proposed methodology is formulated entirely in the time domain. Therefore, existing time-domain simulation tools may utilize the proposed approach to achieve a high degree of distributed memory parallelism without requiring any reformulation. Furthermore, the proposed methodology may be combined with conventional space domain decomposition techniques and other forms of data parallelism to achieve maximal performance on modern HPC architectures. The proposed algorithm retains the iterative shoot-correct approach of conventational periodic shooting methods. However, the correction stage is formulated using a hierarchical evaluation strategy combined with an Arnoldi subspace approximation to eliminate the need for explicit formulation of Jacobian matricies. The local convergence of the proposed method is formally proven for the case of an asyptotically stable dynamical system. The proposed method is numerically tested for a 2D limit cycle problem, a rigid blade helicoper rotor model with quasi-steady aerodynamics and autopilot trim, and an OVERSET CFD model of a helicopter rotor with prescribed elastic blade motions. The method is observed to be convergent in all test cases and found to exhibit good scalability. The proposed periodic multiple shooting method is a practical means of reducingtime-to-solution for numerical simulations of asymptotically stable periodic systems on distributed memory parallel computers. Furthermore, the proposed method may be used to enhance the parallel scalability of OVERSET CFD models of helicopter rotors in steady periodic flight.Item Dynamics and Control of Bioinspired Swimming, Schooling, and Pursuit(2023) Thompson, Anthony Allan; Paley, Derek A; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the benefits of the behaviors of aquatic animals can improve the capabilities of robotic systems. Aquatic species such as the zebrafish swim with discrete motions that alternate between perception and action while avoiding predators and swimming in schools, and other species such as the lionfish use their pectoral fins to herd and trap prey. This work seeks to model these bioinspired behaviors (i.e., schooling, swimming with intermittent sensing and actuation, and pursuit and evasion in a structured environment) and enhance our understanding of their benefits. A hybrid dynamic model is derived with two phases; namely a burst phase during which each particle applies a control input and a coast phase during which each particle performs state estimation. This model provides a way to investigate how having non-overlapping sensing and control affects a multi-agent system's ability to achieve collective behavior such as steering to some desired direction. By evaluating the stability properties of the equilibrium points for the collective behavior, investigators can determine parameter values that exhibit exponentially stable behavior. Aside from swimming intermittently, fish also need to avoid predators. Inspired by observations of predation attempts by lionfish (Pterois sp.), a pursuit-evasion game is derived in a bounded environment to study the interaction of an advanced predator and an intermittently steering prey. The predator tracks the prey with a pure-pursuit strategy while using a bioinspired tactic to minimize the evader's escape routes, i.e, to trap the prey. Specifically, the predator employs symmetric appendages inspired by the large pectoral fins of lionfish, but this expansion increases its drag. The prey employs a bioinspired randomly-directed escape strategy to avoid capture and collisions with the boundary known as the protean strategy. This game investigates the predator's trade-off of minimizing the work to capture the prey and minimizing the prey's escape routes. Using the predator's expected work to capture as a cost function determines when the predator should expand its appendages as a function of the relative distance to the evader and the evader's proximity to the boundary. Prey fish also swim in schools to protect themselves from predators. To drive a school of fish robots into a parallel formation, a nonlinear steering controller is derived and implemented on a robotic fish platform. These robotic fish are actuated with an internal reaction wheel driven by a DC motor. Implementation of the proposed parallel formation control law on an actual school of soft robotic fish is described, including system identification experiments to identify motor dynamics and the design of a motor torque-tracking controller to follow the formation torque control. Experimental results demonstrate a school of four robotic fish achieving parallel formations starting from random initial conditions.