Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
Browse
253 results
Search Results
Item NUMERICAL SIMULATION OF THE BLUE WHIRL: A REACTING VORTEX BREAKDOWN PHENOMENON(2019) Chung, Joseph Dong il; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The blue whirl is a small, stable, spinning blue flame that evolved spontaneously in recent laboratory experiments while studying turbulent, sooty fire whirls. It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a new potential way for low-emission combustion. This thesis uses numerical simulations to present, for the first time, the flame and flow structure of the blue whirl. These simulations show that the blue whirl is composed of three different flames - a diffusion flame and a premixed rich and lean flame - all of which meet in a fourth structure, a triple flame which appears as a whirling blue ring. The results also show that the flow structure emerges as the result of vortex breakdown, a fluid instability which occurs in swirling flows. This thesis also presents the development and testing of the numerical algorithms used in the simulation of the blue whirl. This work is a critical step forward in understanding how to use this new form of clean combustion.Item The Effects of Model Scaling on Sediment Transport in Brownout(2013) Glucksman-Glaser, Mark Samuel; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The phenomenon of “brownout” is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.Item A Magnetorheological Energy Absorber for Enhanced Crashworthiness in Drop-Induced Impacts(2019) Pierce, Rebecca; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis uses a multidisciplinary approach to investigate the enhanced crashworthiness of a magnetorheological energy absorber (MREA). Magnetorheological (MR) fluids have been considered for use in crashworthiness applications because they can be modified to adjust for parameters such as an occupant’s weight or the impact velocity of a crash. This study first reviews an existing soft landing control algorithm for an MREA vertically stroking crew seat and applies it to several crash scenarios. The combined addition of a bumper and optimized yield force is found to successfully reduce the jerk at the end of the MREA stroke without introducing new discontinuities in the acceleration profile. Secondly, this study explores the use of mesocarbon microbeads (MCMBs) in MR fluids. The MCMBs are found to increase the yield force produced in an MR damper. An endurance study further reveals the durability of the yield force-enhancing effect up to 100,000 cycles.Item Guided Lamb Wave Structural Health Monitoring Techniques for Aircraft Applications(2019) McCullum, Jacob Ryan; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Growing aerospace industry interest in structural health monitoring (SHM) has led to the development of many damage detection and localization techniques which make use of guided Lamb Waves (GLW). To continue this growth, further development of these techniques is necessary with an industry-focused mindset through studies with realistic, complex aircraft structures. The present study applies GLW techniques to two aircraft structures and examines the feasibility of their use for practical SHM applications. Particularly this work focuses on evaluating the effects of complex structural features found in aircraft, examining the human interaction with GLW techniques, and enhancing GLW techniques using nontraditional dual PZT transducers. Several damage case studies are performed showing that damage can be detected and located, and limitations to the techniques are characterized. Moreover, the use of dual PZT transducers shows improvements to damage localization techniques which potentially enable greater flexibility for aircraft applications.Item An Experimental Investigation of Hypersonic Boundary-Layer Transition on Sharp and Blunt Slender Cones(2019) Kennedy, Richard Edward; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the instabilities leading to the laminar-to-turbulent transition of a hypersonic boundary layer is a key challenge remaining for the design of efficient hypersonic vehicles. In the present study, experiments are performed in three different facilities at freestream Mach numbers between 6 and 14 to characterize instability mechanisms leading to transition on a 7-degree half-angle slender cone. Second-mode instability waves are visualized using a high-speed schlieren setup with the camera frame rate and spatial resolution optimized to allow individual disturbances to be tracked. In order to facilitate quantitative time-resolved measurements, a method of calibrating the schlieren system and novel image-processing algorithms have been developed. Good agreement is observed between the schlieren measurements, surface pressure measurements, and parabolized stability equation computations of the second-mode most-amplified frequencies and N factors. The high-frequency-resolution schlieren signals enable a bispectral analysis that reveals phase locking of higher harmonic content leading to nonlinear wave development. Individual disturbances are characterized using the schlieren wall-normal information not available from surface measurements. Experiments are also performed to investigate the effect of nose-tip bluntness. For moderate to large bluntness nose tips, second-mode instability waves are no longer visible, and elongated structures associated with nonmodal growth appear in the visualizations. The nonmodal features exhibit strong content between the boundary-layer and entropy-layer edges and are steeply inclined downstream. Simultaneously acquired surface pressure measurements reveal high-frequency pressure oscillations typical of second-mode instability waves associated with the trailing edge of the nonmodal features.Item Evolutionary Spacecraft Design Using a Generalized Component-Resource Model(2019) Marcus, Matthew Leo; Sedwick, Raymond J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A new framework is proposed for modeling complex multidisciplinary systems as a collection of components and resource flows between them. The framework is developed for modeling and optimizing conceptual spacecraft designs. Its goal is to remain sufficiently general to address any space mission without modification of the developed model or code. Spacecraft are modeled as a collection of components and the resources that flow between them. New missions can be considered and capabilities added by simply adding components and resources. Constraints can be imposed on a component basis or system-wide, and are based on the flow of the resources within the system. Additionally, the proposed component-resource model and framework can address many complex systems engineering problems beyond spacecraft design by a similar implementation. Design optimization is performed by a genetic algorithm utilizing a variable length genome. This allows the algorithm to represent the variable number of components that could be present in a system design, enabling a more open-ended design capability than previous frameworks of this nature. Systems are evaluated through a user-defined simulation, and results can be presented in any trade space of interest based on the designs' performance in the simulation. We apply the framework to the design of a simple Earth orbiting, data gathering mission, as well as to the design of low Earth orbit active debris removal spacecraft constellations.Item INTERPOLATION OF RIGID-BODY MOTION AND GALERKIN METHODS FOR FLEXIBLE MULTIBODY DYNAMICS(2019) Han, Shilei; Bauchau, Olivier A.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Traditionally, flexible multibody dynamics problems are formulated as initial value problems: initial states of the system are given and solving for the equations of motion yields the dynamic response. Many practical problems, however, are boundary rather than initial value problems; two-point and periodic boundary problems, in particular, are quite common. For instance, the trajectory optimization of robotic arms and spacecrafts is formulated as a two-point boundary value problem; determination of the periodic dynamic response of helicopter and wind turbine blades is formulated as a periodic boundary value problem; the analysis of the stability of these periodic solutions is another important of problem. The objective of this thesis is to develop a unified solution procedure for both initial and boundary value problems. Galerkin methods provide a suitable framework for the development of such solvers. Galerkin methods require interpolation schemes that approximate the unknown rigid-body motion fields. Novel interpolation schemes for rigid-body motions are proposed based on minimization of eighted distance measures of rigid-body motions. Based on the proposed interpolation schemes, a unified continuous/discontinuous Galerkin solver is developed for the formulation of geometrically exact beams, for the determination of solutions of initial and periodic boundary value problems, for the stability analysis of periodic solutions, and for the optimal control/optimization problems of flexible multibody systems.Item Gas Turbine / Solid Oxide Fuel Cell Hybrids: Investigation of Aerodynamic Challenges and Progress Towards a Bench-Scale Demonstrator(2019) Pratt, Lucas Merritt; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modern aircraft are becoming more electric making the efficiency of on-board electric power generation more important than ever before. Previous work has shown that integrated gas turbine and solid oxide fuel cell systems (GT-SOFCs) can be more efficient alternatives to shaft-driven mechanical generators. This work advances the GT-SOFC concept in three areas: 1) It develops an improved model of additional aerodynamic losses in nacelle-based installations and shows that external aerodynamic drag is an important factor that must be accounted for in those scenarios. Additionally, this work furthers the development of a lab-scale prototype GT-SOFC demonstrator system by 2) characterizing the performance of a commercial off-the-shelf (COTS) SOFC auxiliary power unit that will become part of the prototype; and 3) combining a scaled-down SOFC subsystem model with an existing thermodynamic model of a small COTS gas turbine to create an initial design for the prototype.Item Characterization and Modeling of Brushless DC Motors and Electronic Speed Controllers with a Dynamometer(2019) Brown, Robert; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The global drone market is expected to grow from $4.9 billion to $14.3 billion within the next decade, indicating a heavy demand for high performance electric aircraft. Modern drones are propelled with brushless DC (BLDC) motors and electronic speed controllers (ESCs). However, a current lack of information concerning the performance and efficiency of BLDC motors and ESCs prevents their use in rigorous aircraft design. Low cost hobby ESCs and BLDCs are typically used in research aircraft, but few technical details are released by their manufacturers. To better understand these devices, a custom dynamometer was constructed to study the performance of ESCs and BLDC motors. By properly recording the DC, AC, and mechanical power, information on peak efficiency and performance for the ESCs and BLDC motors are determined experimentally. Motors between 920 KV to 2500 KV were tested with 18 A, 30 A, and 40 A ESCs. A combination of these tests were carried out at 7.2 V, 11.1 V, and 14.8 V DC to explore trade offs in the design process. While typically neglected in formal analysis, this work seeks to better understand the power loss mechanisms in ESCs, as it was found that ESCs could have efficiencies as low as 65%, reducing the overall efficiency of the system considerably. This custom dynamometer features a load varying device, power analyzers, and a unique two DAQ setup to properly capture the high frequency electrical signals of BLDC motors. From the sets of experimentally recorded motor and ESC tests, a novel analytical model is developed to predict the performance of ESCs and BLDC motors. At the heart of this modeling effort is describing the 3 phase AC circuit as a single equivalent circuit, which encapsulating the motor’s performance. This work is critical in the design process, as properly sizing ESCs, motors, and rotors for an electric aircraft can improve aircraft endurance and range. Performance metrics are extracted from experimental results and are fit into the analytical model. Predictions for the system’s mechanical power, AC power, and DC power agree well with experimental results, demonstrating applicability of the robust model.Item Active Spanwise Lift Control: A Distributed Parameter Approach(2019) Dias, Joaquim; Hubbard, James E; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Structural load alleviation has been a very active research topic since the 1950s for many reasons. By mitigating the effect of gusts on the wing, the maximum loads can be effectively reduced. This capability would lead to substantial benefits, such as reduced structural weight, better fuel burn performance, and improved passenger ride comfort. Instead of controlling the structural response, however, it can be argued that the aerodynamic behavior of the wing should be primarily controlled. Since the gust loads are caused by disturbances in the lift distribution, it is possible to mitigate the gust loads by controlling the shape of the lift distribution profile along the span. In contrast to previous approaches, this research builds on concepts from Distributed Parameter Systems (DPS), which is indeed the case of aerodynamic surfaces. The unsteady aerodynamic behavior of the 3D flow around a wing is modeled using two approaches: the Unsteady Lifting-Line Theory (ULLT) and the Unsteady Vortex-Lattice Method (UVLM). Then, modal identification techniques are used to identify spanwise aerodynamic mode shapes in terms of local lift coefficient along the span. These shapes provide an optimal basis for model order reduction and also for spatial control. The lift distribution is decomposed as a linear superposition of these shapes, with each weighted by a shape coefficient. By controlling a set of shape coefficients, the overall lift profile can be effectively controlled. In this work, the shape control problem is addressed using a Linear Quadratic Tracker to dynamically follow any desired reference lift profile. The gust alleviation problem is investigated using a similar controller with a special observer, able to decouple the state estimation from the gust input.