Aerospace Engineering Theses and Dissertations

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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.
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.
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.
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.
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.

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