Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item Dynamics, Estimation, and Control for Stabilizing the Attitude and Shape of a Flexible Spacecraft(2024) Merrill, Curtis; Paley, Derek A; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Advances in technology have enabled the development of large spacecraft structures such as solar sails, expansive antennas, and large solar arrays. A critical design constraint for these structures is mass, necessitating lightweight construction which, in turn, increases structural flexibility. This flexibility poses significant challenges resulting from structural deformations and vibrations that complicate attitude control and can degrade the performance and lifespan of the spacecraft. The goal of this research is to develop estimation and control strategies to mitigate the effects of spacecraft flexibility.A flexible spacecraft model is derived using a hub and appendage framework. In this model one or more flexible appendages attach to a central rigid hub. The model represents the appendages as a discretized set of flexibly connected elements called panels. Stiff springs connect the panels, and the dynamic model of the system’s internal forces and moments uses coordinates in the hub’s reference frame. Reaction wheels on the hub perform attitude control, while distributed pairs of magnetic torque rods on the appendage influence its shape. Initially, the model restricts flexibility to one direction, resulting in a planar model. A Lyapunov-based control design provides a feedback law for the reaction wheel and torque rods in the planar model. Numerical simulations demonstrate that the proposed controller meets the control objectives and compares favorably to other controllers. An Extended Kalman Filter is applied to the system to perform state estimation and output feedback control, which performs at nearly the same level as state feedback control. The modeling framework and flexibility are extended to three dimensions. The development of a control law for the magnetic torque rods considers the attitude control of a single panel using two magnetic torque rods. Due to the system being underactuated, the attitude error is defined in terms of the reduced-attitude representation. Lyapunov analysis yields a control law that stabilizes the reduced attitude and angular velocity of a rigid panel using only two magnetic torque rods. Numerical simulations validate the control law’s performance for a single panel. This control law is then applied to the flexible appendage to stabilize its shape. Numerical simulations show that this implementation of shape control significantly reduces structural deformations and dampens structural oscillations compared to scenarios without shape control. To perform state estimation of the high-dimensional flexible spacecraft model, dynamic mode decomposition generates a reduced order model that is linear with respect to the evolution of the resulting modes. A Kalman filter estimates the mode amplitudes of the reduced order model from a limited set of measurements, enabling the reconstruction of the entire system state. The optimization of the number and placement of sensors maximizes the observability of the observer. Numerical simulations demonstrate that this framework yields accurate state estimates with reduced computational cost.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.Item INVESTIGATION OF COMPOUND ROTORCRAFT AEROMECHANICS THROUGH WIND-TUNNEL TESTING AND ANALYSIS(2022) Maurya, Shashank; Datta, Anubhav; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The aeromechanics of a slowed-rotor compound rotorcraft is investigated through wind-tunnel testing and comprehensive analysis. The emphasis is on a lift-offset wing compound with a hingeless rotor configuration. A new Maryland Compound Rig is developed and instrumented for wind-tunnel testing and an in-house rotor comprehensive code is modified and expanded for compound rotorcraft analysis. The compound rig consists of a lift compound model and a propeller model. The lift compound model consists of an interchangeable hub (articulated or hingeless), a fuselage, a half-wing of 70% rotor radius on the retreating side. The wing has a dedicated load cell and multiple attachment points relative to the rotor hub (16%R, 24%R, and 32%R and 5%R aft of the hub). The rotor diameter is 5.7-ft. The rotor has four blades with NACA 0012 airfoils with no twist and no taper. The wing incidence angle is variable between 0 to 12 degrees. The wing has a linearly varying thickness with symmetric airfoils NACA 0015 at the tip and NACA 0020 at the root. Sensors can measure rotor hub forces and moments, wing root forces and moments, blade pitch angles, structural loads (flap bending moment, lagbending moment, and torsional moment) at 25%R, pitch link loads, and hub vibratory loads. Wind tunnel tests are conducted up to advance ratio 0.7 for lift compound with half-wing at wing incidence angles of 4 and 8 degrees and compared with an isolated rotor. Hover tests are conducted up to tip Mach number of 0.5 to measure download penalty with the wing at various positions. The University of Maryland Advanced Rotorcraft Code (UMARC) is modified for compound rotorcraft analysis code. Aerodynamic models for the wing and the propeller are integrated. A recently developed Maryland Free Wake model is integrated, which can model the wake interaction between unequal and inharmonic speed rotor, wing, and propeller. The analysis is then validated with the test data. The validated analysis is used to analyze the US Army hypothetical full-scale aircraft. The compound rotorcraft is categorized into multiple configurations in a systematic manner to find the extreme limits of speed and efficiency of each. The key conclusions are: 1) slowing the rotor or compounding the configuration provide no benefit individually; they must be accomplished together, 2) Half-Wing is more beneficial if a lift-offset hingeless rotor is used, 3) hover download penalty is only 3% of net thrust, and this penalty can be predicted satisfactorily by free wake, 4) the main rotor wake interaction is more pronounced on the wing and less on the propeller, 5) the validated analysis indicates a speed of 240 knots may be possible with 20% RPM reduction along with a wing and propeller, if structural weights allow, and 6) the oscillatory and vibratory lag moments and in-plane hub loads may be significantly reduced by compounding.Item A MODERN AEROMECHANICAL ANALYSIS OF HINGELESS HUB TILTROTORS WITH MODEL- AND FULL-SCALE WIND TUNNEL VALIDATION(2022) Gul, Seyhan; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A new aeromechanics solver was developed, verified, and validated systematically to explore how whirl flutter might be eliminated to achieve significantly higher cruise speeds with future tiltrotor aircraft. The hub explored is hingeless, more advanced than the gimballed hub of current generation tiltrotors. The major finding is that whirl flutter is not the barrier at all for hingeless hubs, instead air resonance, which is another fascinating instability particular to soft in-plane rotors. A possible design change to achieve high cruise speeds with thin, low-profile wings is blade tip sweep. The key mechanism is the aerodynamic center shift. The trade-off is the increase in blade and control system loads. A fundamental understanding of the physics for soft in-plane hingeless hub stability was provided. The induced flow model showed no effect on high-speed stability, as the wake is quickly washed away and insignificant for airplane mode flight. Predictions in powered mode are necessary. At least the first rotor flap, lag, and torsion modes need to be included. Rotor aerodynamics should use airfoil tables; wing aerodynamics is not essential for air resonance. Periodic solution before stability analysis is necessary for powered mode flight. Details of the mathematical model were reported. The solver was built to study high-speed stability of hingeless hub tiltrotors; hence the verification and validation cases were chosen accordingly. The stability predictions were verified with U.S. Army's CAMRAD II and RCAS results that were obtained for hypothetical wing/pylon and rotor models. Soft in-plane, stiff in-plane, hyper-stiff in-plane, and rigid rotors were studied with a simple and a generic wing/pylon model. A total of nine cases were investigated. A satisfactory agreement was achieved. Validation was carried out with Boeing Model 222 test data from 1972. This rotor utilized a soft in-plane hingeless hub. Good agreement was observed for performance predictions. Trends for the oscillatory blade loads were captured, but differences in the magnitudes are present. The agreement between the stability predictions and test data was good for low speeds, but some offset in the damping levels was observed for high speeds. U.S. Army also published stability predictions for this rotor, which agreed well with the present predictions. A further parametric validation study was carried out using the University of Maryland's Maryland Tiltrotor Rig test data. This is a brand new rig that was first tested for stability in October – November 2021. Eight different configurations were tested. Baseline data is gimbal-free, freewheeling mode, wing fairings on with straight and swept-tip blades. Gimbal-locked, powered mode, and wing fairings off data was also collected, all with straight and swept-tip blades. Wing beam mode damping showed good agreement with the test data. Wing chord mode damping was generally under-predicted. The trends for this mode for the gimbal-locked, straight blade configurations (freewheeling and powered) were not captured by the analysis. Swept-tip blades showed an increase in wing chord mode damping for gimbal-locked, freewheeling configuration. Locking the gimbal increased wing chord damping, which was picked up by the analysis. Powered mode also increased the wing chord damping compared to freewheeling mode, but the analysis did not predict this behavior. Wing beam mode damping test data showed an increase at high speeds due to wing aerodynamics, and the analysis agreed.Item Unsteady Shock Propagation in a Constant-Area Thermally Choked Flow : Numerical and Experimental Investigations(2021) Pee, Xiu Yi; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The development of scramjet propulsion systems for sustained hypersonic air-breathing flight remains an area of active research. Inlet unstart is one of the main transient phenomena affecting scramjet operation, whereby the inlet shock system rapidly diverges from its design condition. One cause of unstart is heat release above the Rayleigh limit, which causes the formation of an unsteady, upstream-propagating shock system. This thesis aims to investigate the detailed behavior of unsteady shock-system propagation in a constant-area duct with heat release, representative of a simple scramjet combustor, from both numerical and experimental perspectives. A one-dimensional, unsteady method of characteristics scheme is developed to study the shock-system propagation in an idealized manner. Connected-pipe experiments of the flow configuration are developed and conducted in a small-scale shock tunnel with injection of gaseous Hydrogen into a 40% Oxygen freestream using both porthole and slot injectors. Schlieren imaging and pressure sensor measurements are used to provide shock-speed measurements and reveal the development and propgation of the unsteady shock system. Numerical studies predict the slowing of the shock as it propagates upstream, in close agreement with calculations made using a previously proposed quasi-steady model, except in cases of larger heat-release ratios or rapid changes in heat-release distributions which are unlikely to occur in practical combustors. The extension of the model to include a finite-rate chemical model is presented and discussed. Experimental results show that the flow phenomena associated with combustion, even for simple geometries, are highly complex and dynamic. For equivalence ratios of approximately 0.5, a close-coupled flow regime between the normal shock and the heat release in the post-shock flow is observed, which transitions to a loosely coupled regime further upstream; this latter regime was seen throughout the shock-train development at lower equivalence ratios. Although it is generally observed that the shock speed decreases with upstream propagation in accordance with a simplified analysis, in some cases the shock is observed to re-accelerate mid-combustor at a location corresponding to mean-flow structures. Schlieren velocimetry enabled measurement of the mean velocity distributions at different injection conditions, showing that slot injection results in lower velocities in the lower-wall region at low equivalence ratios compared to porthole injection.Item CHARACTERIZATION AND ANALYSIS OF FLUIDIC ARTIFICIAL MUSCLES(2021) Chambers, Jonathan Michael; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluidic artificial muscles (FAMs) are a form of soft actuator that have been applied to an expanding number of applications, due to their unique characteristics such as low weight, simple construction, inherent compliance, and high specific force and specific work capabilities. With energy sourced from a pressurized fluid, contractile FAMs provide a uniaxial contractile force, while their morphing geometry allows them to contract in length. In a design environment where actuators have tight spatial requirements and must provide precise force and position control, it is becoming more important than ever to have accurate mathematical representations of FAM actuation behavior and geometric characteristics to ensure their successful implementation. However, geometric models and force analyses for FAMs are still relatively crude. Geometric models of FAMs assume a cylindrical geometry, the accuracy of which is suspect because there are no documented methods for effectively measuring FAM shape. Actuation force analyses are also relatively inaccurate unless they are adjusted to fit to experimental response data. Research has continually pursued methods of improving the predictive performance of these analyses by investigating the complex working mechanisms of FAMs. This research improves these analyses by first, making improvements to the experimental characterization of a FAM's actuation response, and then using the more comprehensive data results to test long-held modeling assumptions. A quantitative method of measuring FAM geometry is developed that provides 0.004 in/pixel resolution measurements throughout a characterization test. These measurements are then used to test common assumptions that serve as sources of uncertainty: the cylindrical approximation of FAM geometry, and assumption that the FAM's braid is inelastic. Once these sources of modeling error are removed, the model's performance is then tested for potential improvements. Results from this research showed that the cylindrical approximation of the FAM's geometry resulted in overestimations of the FAM's average diameter by 4.7%, and underestimations of the FAM's force by as much as 37%. The inelastic braid assumption resulted in a maximum 4% underestimation of average diameter and a subsequent 5% overestimation in force, while the use of softer braid materials was found to have the potential for much larger effects (30% underestimation in diameter, 70% overestimation in force). With subsequent adjustments made to the force model, the model was able to achieve a fit with a mean error of only 2.8 lbf (0.3% of maximum force). This research demonstrates improvements to the characterization of a FAM's actuation response, and the use of this new data to improve the fidelity of existing FAM models. The demonstrated characterization methods can be used to clearly define a FAM's geometry to aid in the effective design and implementation of a FAM-actuated mechanism, or to serve as a foundation for further investigation into the working mechanisms and development of FAMs.Item CONTROLLER SYNTHESIS AND FORMAL BEHAVIOR INFERENCE IN AUTONOMOUS SYSTEMS(2021) Carrillo, Estefany; Xu, Huan; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Autonomous systems are widely used in crucial applications such as surveillance,defense, reghting, and search & rescue operations. Many of these application require systems to satisfy user-dened requirements describing the desired system behavior. Given high-level requirements, we are interested in the design of controllers that guarantee the compliance of these requirements by the system. However, ensuring that these systems satisfy a given set of requirements is challenging for many reasons, one of which is the large computational cost incurred by having to account for all possible system behaviors and environment conditions. These computational diculties are exacerbated when systems are required to satisfy requirements involving large numbers of tasks emerging from dynamic environments. In addition to computational diculties, scalability issues also arise when dealing with multi-agent applications, in which agents require coordination and communication to satisfy mission requirements. This dissertation is an eort towards addressing the computational and scalability challenges of designing controllers from highlevel requirements by employing reactive synthesis, a formal methods approach, and combining it with other decision-making processes that handle coordination among agents to alleviate the load on reactive synthesis. The proposed framework results in a more scalable solution with lower computational costs while guaranteeing that high-level requirements are met. The practicality of the proposed framework is demonstrated through various types of multi-agent applications including reghting, re monitoring, rescue, search & rescue and ship protection scenarios. Our approach incorporates methodology from computer science and control, including reactive synthesis of discrete systems, metareasoning, reachability analysis and inverse reinforcement learning. This thesis consists of two key parts: reactive synthesis from linear temporal logic specications and specication inference from demonstrations of formal behavior. First, we introduce the reactive synthesis problem for which the desired system behavior species the method by which a multi-agent system solves the problem of decentralized task allocation depending on communication availability conditions. Second, we present the synthesis problem formulated to obtain a high-level mission planner and controller for managing a team of agents ghting a wildre. Third, we present a framework for inferring linear temporal logic specications that succinctly convey and explain the observed behavior. The gained knowledge is leveraged to improve motion prediction for agents behaving according to the learned specication. The eectiveness of the inference process and motion prediction framework are demonstrated through a scenario in which humans practice social norms commonly seen in pedestrian settings.Item Modeling Viscoelastic Behavior Using Flexible Multibody Dynamics Formulations(2020) Nemani, Nishant; Bauchau, Olivier Prof.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Viscoelastic behavior is frequently observed in dynamical flexible multibody systems. In the simplest form it is manifested in one dimensional revolute and prismatic joints. Beyond which more complex force elements such as six degree of freedom flexible joints can also be found. Finally, beams, plates and shells are found to exhibit viscoelastic behavior too. In the past extensive work has been done on analyzing the dynamic response of three dimensional beams by performing cross-sectional analysis through finite element methods and subsequently solving the reduced beam problem. The approach is particularly relevant for the analysis of complex cross sections and helps improve computational efficiency significantly. A formulation which incorporates a viscoelastic model of the generalized Maxwell type with a solution of the three dimensional beam theory which gives an exact solution of static three dimensional elasticity problems is presented. Multiple examples incorporating the use of the aforementioned model in the context of viscoelastic beams and joints are presented. Shortcomings of the Kelvin-Voigt model, which is often used for flexible multibody systems, are underlined.Item Overcoming Local Minima Through Viscoelastic Fluid-Inspired Swarm Behavior(2020) McGuire, Loy James; Otte, Michael W; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)My paper discusses a novel swarm robotic algorithm inspired by the open channel siphon phenomena displayed in certain viscoelastic fluids. This siphoning ability enables the algorithm to mitigate the trapping effects of local minima, which are known to affect physicomimetics-based potential field control methods. Once a robot senses the goal, local communication between robots is used to propagate path-to-goal gradient information through the swarm's communication graph. This information is used to augment each agent's local potential field, reducing the local minima trap and often eliminating it. In this paper real world experiments using the Georgia Tech Miniature Autonomous Blimp (GT-MAB) aerial robotic platforms as well as mass Monte Carlo test simulations conducted in the Simulating Collaborative Robots in Massive Multi-Agent Game Execution (SCRIMMAGE) simulator are presented. Comparisons between the resultant behaviors and potential field based swarm behaviors that both do, and do not incorporate local minima fixes were assessed. These experiments and simulations demonstrate that this method is an effective solution to susceptibility to local minima for potential field approaches for controlling swarms.Item BARELY IMPLICIT CORRECTION ALGORITHM FOR LOW-MACH-NUMBER FLOWS AND ITS APPLICATION TO VORTEX BREAKDOWN UNDERGOING HEAT ADDITION AND EXTRACTION(2020) Zhang, Xiao; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents a new Barely Implicit Correction (BIC) algorithm combined with a modified flux-corrected transport (FCT) algorithm for the simulation of three-dimensional (3D), low-Mach-number flows and then proceeds to apply it to the study of vortex breakdown undergoing heat addition and heat extraction. This new algorithm is based on the original, introduced by a prior work in 1987, which was a solution procedure including an explicit predictor step to solve the convective portion of the Navier-Stokes equations and an implicit corrector step to remove the acoustic limit on the integration time-step. The explicit predictor uses the flux-corrected transport (FCT) algorithm while the implicit corrector solves an elliptic equation for a pressure correction to equilibrate acoustic waves. This thesis introduces a procedure for stabilizing and implementing FCT for 3D flows and extends BIC for 3D with physical diffusion processes. A new filter is introduced to further stabilize the algorithm and the solution procedure is clarified for the inclusion of the diffusion fluxes. The new BIC-FCT algorithm is examined in four test problems with successively increased difficulty. The test problems culminate with calculations of vortex breakdown in 3D swirling flows. All the test problems demonstrate that the algorithm is able to predict accurate and robust solutions using time steps varying from near the explicit stability limit to tens and hundreds of times larger. Excellent agreement is also obtained when compared with results from other algorithms. The algorithm is then used to study how vortex breakdown is affected when heat is extracted from or added to the flow. Two heat release rates are applied to a flow with a bubble mode of breakdown upstream and double-helix mode downstream. The simulations show that heat release causes the double-helix structure to become narrower. With more heat release, the double-helix mode transitions to a columnar vortex. In addition, a lower heat extraction rate causes the columnar vortex to first transition to a spiral mode and then to a double-helix mode. With a higher heat extraction rate, the columnar vortex transitions to a double-helix mode, bypassing the spiral mode. Further investigation show that the density gradient formed by heat addition and extraction is the dominant effect in the transitions. The transition is promoted by changes in viscosity due to temperature changes from heat addition and extraction. The new algorithm presented in this thesis provides a new way to calculate low-Mach-number flows. Such vortex breakdown simulations with heat changes serve as a base for understanding the dynamics of a precessing vortex core in swirl combustors and other vortex flows with changes in heat input.