Aerospace Engineering Theses and Dissertations

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    Separation of an Ellipsoidal Body from a Two-Dimensional Ramp in Hypersonic Flow with Kestrel Validation
    (2022) Brent, Denikka Lynette; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This study investigated the separation dynamics of an ellipsoidal body shedding from a two-dimensional ramp using experimental and computational methods. The main objective was to assess the fidelity of computational simulations of a complex, interacting flow configuration via comparison with experimental data. Experimental data was generated by the HyperTERP shock tunnel at the University of Maryland. Ellipsoids were stationed on a 10° ramp with varying initial positions and were then exposed to Mach 6 flow, allowing them to fly freely in response to the aerodynamic forces experienced. Experimental results revealed three trajectory behaviors that were dependent upon the initial shock impingement location: expulsion to surfing, surfing, and direct entrainment. These behaviors were consistent with earlier sphere experiments, but the introduction of pitch resulted in somewhat more complex dynamics. Numerical simulations were performed with CREATE-AV Kestrel, the fixed-wing multiphysics tool developed by the Computational Research and Engineering Acquisition Tools and Environments (CREATE) Program. Computational results exhibited discrepancies primarily in terms of the velocity and acceleration values when compared to the experimental results. The sensitivity of the initial conditions caused unsteadiness at the start of the solution, and potentially propagated errors in velocity and acceleration downstream. Despite these initial errors, however, the computational simulations showed a comparable trajectory to those of the experimental results.
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    (2022) Neisess, Christoph; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Currently, limitations exist on collecting aero-optic wavefront data in a wind tunnel dueto the types of sources available to illuminate the flow-field for the sensing optics. Collimated laser sources are commonly used, but are limited by the ability to place hardware to steer the beam towards the sensing optics. Laser-Induced Breakdown (LIB) sparks have also been tested, but create additional measurement errors due to variations in their size and position with each laser pulse. In this work, a new approach using a Femtosecond Laser-Induced Breakdown (FSLIB) spark is evaluated as a possible solution to the problems faced by nanosecond LIB sparks, namely the significant amount of spark size and position variation present in the latter. The FSLIB spark was imaged with a camera in order to study the amount of pulse-to-pulse position and size change present in its generation. Additionally, the FS-LIB spark was used to collect aero-optic data in conjunction with a Shack-Hartmann style wavefront sensor on a Mach 2.8 flowfield. The results of this analysis indicated that the FS-LIB spark experiences significantly less pulse-to-pulse variation in its size and position than a nanosecond LIB spark. In addition, the wavefront data collected with the FS-LIB spark compared favorably to data collected with a more conventional collimated laser beam for illumination. This indicates that the FS-LIB spark is a promising alternative to the use of collimated sources in aero-optic data collection.
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    Mitigation of transverse gusts via open- and closed-loop pitching maneuvers
    (2022) Sedky, Girguis; Jones, Anya R.; Lagor, Francis D.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Unsteady flow conditions present significant challenges to stable flight, and gust rejectionremains a concern for flight control in many modern flight environments. Examples of gustdominated flight conditions include flight in stormy conditions, aircraft takeoff and landing in strong crosswinds or ship air wakes, and micro air vehicles in strong shear flow engendered by urban settings and complex terrain. Improving flight stability during gust encounters relies on an improved understanding of the flow physics and the development of effective mitigation control strategies. To this end, the present work seeks to (1) improve our understanding of the unsteady flow physics of a pitching wing encountering a transverse gust and (2) develop and characterize successful open- and closed-loop control strategies to mitigate aerodynamic lift transients induced by the gust using wing pitching input. Classic unsteady aerodynamic theory was used to construct the open-loop pitch maneuvers and tune the closed-loop controller for closed-loop control. The dynamical systems treatment of the problem during control design revealed several important physical features important to vehicle control. Two sets of wing-gust encounter experiments were conducted using a flat-plate wing model in a water towing tank. The transverse gust was generated in the center of the towing tank using a recirculating water jet. Data was acquired using a combination of Particle Image Velocimetry (PIV), force, and torque measurements. In the first set of experiments, the constructed openloop pitch maneuvers were implemented as open-loop kinematics in the water towing tank. This study revealed several findings regarding the change in the flow topology due to pitch actuation, the necessity of modeling added mass for open-loop pitch maneuver construction, and the increase in the pitching moment transients due pitch control. This study also demonstrated how lift-mitigating pitching maneuvers minimized the disturbance to the gust’s flow field, thereby reducing the momentum exchange between the gust and the wing. The second set of experiments implemented a proportional control strategy based on classic unsteady aerodynamic theory using a pitch acceleration input and real-time force measurements. The closed-loop control experiments spanned upwards and downwards gusts of various strengths and lift tracking at pre- and post-stall angles of attack. The controller yielded an average rejection performance of 80% without a priori knowledge of gust strength or onset time and for various aerodynamic conditions. Reasons for the controller’s success include using lift measurements directly in control feedback, aerodynamic models that capture the salient physics in the control design process, and wing pitching as input. Simultaneous time-resolved PIV and force measurements were used to discover and understand the flow physics underlying the lift transients and how applying closed-loop control mitigated those transients.
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    Solid Oxide Fuel Cell and Gas Turbine Hybrid Cycles for Aerospace Power and Propulsion
    (2022) Pratt, Lucas Merritt; Cadou, Christopher P.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Hybrid propulsion systems combining gas turbine and solid oxide fuel cells (GT/SOFCs) have the potential to substantially reduce carbon emissions from 737-class aircraft. Many turbine/fuel cell hybrid cycles have been proposed for ground-based energy conversion at the utility scale, and some work has investigated small-scale (<500 kW) fuel cell-based energy conversion systems for aircraft (mostly auxiliary power units). However there is relatively little known about large hybridengine/fuel cell systems capable of providing main propulsive power in large (i.e. 737-class) aircraft. This work takes several important steps toward filling this gap. First, it develops an analytical model of a GT/SOFC system that provides insight into the trends and tradeoffs associated with varying design parameters across a wide design space. Key insights that emerged from this modeling effort are: a)Increasing the fraction of fuel processed by the fuel cell always increases effciency. b) A tradeoff between fuel cell effciency and specific power determines the optimum range of the vehicle. This tradeoff is heavily influenced by the polarization curveof the SOFC. This optimum operating point is different from the maximum power point. c) The GT/SOFC could be used to increase the cycle’s flow specific work, enabling a smaller core to drive the same size fan. This premise is investigated in more detail later in the thesis. d) The fraction of fuel processed by the fuel cell is limited by the ability to cool it. An analytical expression for this limit is derived but in general the maximum power output of the fuel cell is limited to less than half of the total system power output for most hybridization schemes. Second, this work develops an improved thermodynamic model of the hybrid turbine and fuel cell system. The model accounts for off-design performance of the turbomachinery as well as suffcient details of the transport and electrochemistryin the fuel cell to predict the effect of specific design changes (physical dimensions, flow rates, pressure, temperature, etc.) and operating conditions on power output, energy conversion effciency, and system mass. The model is implemented using a NASA-developed tool called Numerical Propulsion System Simulation (NPSS) that is emerging as a standard in modern engine development. While third-party NPSS fuel cell modules are available, they are not suitable for fuel cell design because key performance parameters like utilization, effciency, and specific power are inputs. Our module predicts fuel cell performance from its geometric attributes (channel length, width, height, number) and electrochemical attributes (i.e. temperature, pressure and composition effects on the polarization curve). Such capability is computationally expensive but essential for predicting GT/SOFC performance over varying flight conditions. This work implemented a) ’guardrails’ to prevent solver divergence due to self-reinforcing high or low temperatures, b) an adaptive Newtonsolver damping scheme to improve convergence, c) an electrochemical performance map to find close initial conditions, and d) the option for methane as an additional fuel, amongst other alterations. Taken together, these changes reduced execution time from weeks to hours and greatly improved stability making the thermodynamic model a much more useful tool for design and analysis. Third, the NPSS system model is used to assess the viability of two possible hybridization schemes. The first is a ‘parallel’ hybrid system where an SOFC powers an electric motor that assists the turbine in driving the main fan. The second is a ‘turboelectric’ hybrid system where all of the propulsive power is provided electrically by a fuel cell working in tandem with a mechanical generator attached to the gas turbine. The results show that a parallel hybrid can reduce fuel consumption by 27%, but requires a reformer/fuel cell that achieves > 1kW/kg to achieve range parity with a conventionally-powered B737. This occurs because the thermodynamic effciency of the system increases by 10% and the propulsive effciency increases by 10% due to the higher bypass ratio made possible by the increase in flow specific work associated with hybridization. The turboelectric system reduces fuel consumption by 12% when 25% of power is generated by the SOFC, but requires a reformer/fuel cell that achieves > 1.2kW/kg to achieve range parity with a conventionally-powered B737. This higher specific power requirement occurs because the gas turbine operates at a lower OPR = 15 vs. OPR = 24 to enable recuperation via a heat exchanger. The heat exchanger also improves the thermodynamic performance of both the Brayton cycle and the SOFC (by reducing preheating requirements) even at 30% effectiveness, but adds mass and complexity. Fourth, this work investigates the potential impacts of introducing the fuel cell exhaust—which is hot and contains large amounts of water and combustible reformate—on the Brayton cycle. The system modeling efforts show that the fuelcell exhaust can constitute up to 70% of the total mass flow rate through the system and up to 50% of the total net heat release. Therefore, the effect of the fuel cell exhaust on the operation of the main combustor is expected to be substantial both for integration with traditionally injected fuels, and influencing trades for the SOFC subsystem design choices that affect that exhaust (e.g. fuel utilization). Subsequent chemical kinetic simulations implemented in Cantera show that SOFC exhaust adiabatic flame temperatures can reach as high as 2200K, laminar flame speeds may vary by as much as 500% across a range of fuel utilization targets, ignition delay times with hydrocarbon/air mixtures can reach the millisecond range, and mixed SOFC exhaust can achieve extinction strain rates of over 300,000/s in pressures reasonable for gas turbines. These results suggest that aircraft GT/SOFCs may also require new combustor designs for effective hybridization.
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    (2022) Shastry, Abhishek; Datta, Anubhav; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The objective of this dissertation is to develop and demonstrate autonomous ship-board landing with computer vision. The problem is hard primarily due to the unpredictable stochastic nature of deck motion. The work involves a fundamental understanding of how vision works, what are needed to implement it, how it interacts with aircraft controls, the necessary and sufficient hardware, and software, how it differs from human vision, its limits, and finally the avenues of growth in the context of aircraft landing. The ship-deck motion dataset is provided by the U.S. Navy. This data is analyzed to gain fundamental understanding and is then used to replicate stochastic deck motion in a laboratory setting on a six degrees of freedom motion platform, also called Stewart platform. The method uses a shaping filter derived from the dataset to excite the platform. An autonomous quadrotor UAV aircraft is designed and fabricated for experimental testing of vision-based landing methods. The entire structure, avionics architecture, and flight controls for the aircraft are completely developed in-house. This provides the flexibility and fundamental understanding needed for this research. A fiducial-based vision system is first designed for detection and tracking of ship-deck. This is then utilized to design a tracking controller with the best possible bandwidth to track the deck with minimum error. Systematic experiments are conducted with static, sinusoidal, and stochastic motions to quantify the tracking performance. A feature-based vision system is designed next. Simple experiments are used to quantitatively and qualitatively evaluate the superior robustness of feature-based vision under various degraded visual conditions. This includes: (1) partial occlusion, (2) illumination variation, (3) glare, and (4) water distortion. The weight and power penalty for using feature-based vision are also determined. The results show that it is possible to autonomously land on ship-deck using computer vision alone. An autonomous aircraft can be constructed with only an IMU and a Visual Odometry software running on stereo camera. The aircraft then only needs a monocular, global shutter, high frame rate camera as an extra sensor to detect ship-deck and estimate its relative position. The relative velocity however needs to be derived using Kalman filter on the position signal. For the filter, knowledge of disturbance/motion spectrum is not needed, but a white noise disturbance model is sufficient. For control, a minimum bandwidth of 0.15 Hz is required. For vision, a fiducial is not needed. A feature-rich landing area is all that is required. The limits of the algorithm are set by occlusion(80\% tolerable), illumination (20,000 lux-0.01 lux), angle of landing (up to 45 degrees), 2D nature of features, and motion blur. Future research should extend the capability to 3D features and use of event-based cameras. Feature-based vision is more versatile and human-like than fiducial-based, but at the cost of 20 times higher computing power which is increasingly possible with modern processors. The goal is not an imitation of nature but derive inspiration from it and overcome its limitations. The feature-based landing opens a window towards emulating the best of human training and cognition, without its burden of latency, fatigue, and divided attention.