Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Mechanisms of Vortex-Induced Particle Transport from a Mobile Bed below a Hovering Rotor(2013) Reel, Jaime Lynn; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A study has been conducted to examine rotor-generated blade tip vortices that pass near to a ground plane covered with mobile sediment particles and to explore whether they induce a pressure field that may affect the problem of rotor-induced dust fields. It was hypothesized that fluctuating pressures lower than ambient at the ground could potentially affect the processes of sediment particle mobilization and uplift into the flow. To investigate the relationship between the vortex wake characteristics and the motion of the mobilized sediment particles, single-phase and dual-phase (particle) flow experiments were conducted using a small laboratory-scale rotor hovering overing a ground plane. Time-resolved particle image velocimetry was used to quantify the flow velocities in the rotor wake and near the ground plane, and particle tracking velocimetry was used to quantify the particle velocities. Measurements were also made of the unsteady pressure over the ground plane using pressure transducers that were sensitive enough to resolve the small induced pressures. Time-histories of the measured responses showed significant pressure fluctuations occurred before, during, and after the rotor wake impinged upon the ground. While it was not possible to separate out the effects of pressure forces from other forces acting on the particles, the present work has shown good evidence of vortex-induced pressure effects on the particles in that particle trajectories significantly deviated from the directions of the surrounding flow in the immediate presence of the vortices. The characteristics of the pressure responses produced at the ground by vortices passing nearby was also predicted using a model based on unsteady potential flow theory, and was used to help interpret the measurements. The vortex strength (circulation), height of the vortex above the ground, and the vortex convection velocity, were all shown to affect the pressures at the ground and were likely to affect particle motion.Item A GPU-ACCELERATED, HYBRID FVM-RANS METHODOLOGY FOR MODELING ROTORCRAFT BROWNOUT(2013) Thomas, Sebastian; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A numerically effecient, hybrid Eulerian- Lagrangian methodology has been developed to help better understand the complicated two- phase flowfield encountered in rotorcraft brownout environments. The problem of brownout occurs when rotorcraft operate close to surfaces covered with loose particles such as sand, dust or snow. These particles can get entrained, in large quantities, into the rotor wake leading to a potentially hazardous degradation of the pilots visibility. It is believed that a computationally efficient model of this phenomena, validated against available experimental measurements, can be a used as a valuable tool to reveal the underlying physics of rotorcraft brownout. The present work involved the design, development and validation of a hybrid solver for the purpose of modeling brownout-like environments. The proposed methodology combines the numerical efficiency of a free-vortex method with the relatively high-fidelity of a 3D, time-accurate, Reynolds- averaged, Navier-Stokes (RANS) solver. For dual-phase simulations, this hybrid method can be unidirectionally coupled with a sediment tracking algorithm to study cloud development. In the past, large clusters of CPUs have been the standard approach for large simulations involving the numerical solution of PDEs. In recent years, however, an emerging trend is the use of Graphics Processing Units (GPUs), once used only for graphics rendering, to perform scientific computing. These platforms deliver superior computing power and memory bandwidth compared to traditional CPUs and their prowess continues to grow rapidly with each passing generation. CFD simulations have been ported successfully onto GPU platforms in the past. However, the nature of GPU architecture has restricted the set of algorithms that exhibit significant speedups on these platforms - GPUs are optimized for operations where a massively large number of threads, relative to the problem size, are working in parallel, executing identical instructions on disparate datasets. For this reason, most implementations in the scientific literature involve the use of explicit algorithms for time-stepping, reconstruction, etc. To overcome the difficulty associated with implicit methods, the current work proposes a multi-granular approach to reduce performance penalties typically encountered with such schemes. To explore the use of GPUs for RANS simulations, a 3D, time- accurate, implicit, structured, compressible, viscous, turbulent, finite-volume RANS solver was designed and developed in CUDA-C. During the development phase, various strategies for performance optimization were used to make the implementation better suited to the GPU architecture. Validation and verification of the GPU-based solver was performed for both canonical and realistic bench-mark problems on a variety of GPU platforms. In these test- cases, a performance assessment of the GPU-RANS solver indicated that it was between one and two orders of magnitude faster than equivalent single CPU core computations ( as high as 50X for fine-grain computations on the latest platforms). For simulations involving implicit methods, a multi-granular technique was used that sought to exploit the intermediate coarse- grain parallelism inherent in families of line- parallel methods like Alternating Direction Implicit (ADI) schemes coupled with con- servative variable parallelism. This approach had the dual effect of reducing memory bandwidth usage as well as increasing GPU occupancy leading to significant performance gains. The multi-granular approach for implicit methods used in this work has demonstrated speedups that are close to 50% of those expected with purely explicit methods. The validated GPU-RANS solver was then coupled with GPU-based free-vortex and sediment tracking methods to model single and dual-phase, model- scale brownout environments. A qualitative and quantitative validation of the methodology was performed by comparing predictions with available measurements, including flowfield measurements and observations of particle transport mechanisms that have been made with laboratory-scale rotor/jet configurations in ground effect. In particular, dual-phase simulations were able to resolve key transport phenomena in the dispersed phase such as creep, vortex trapping and sediment wave formation. Furthermore, these simulations were demonstrated to be computationally more efficient than equivalent computations on a cluster of traditional CPUs - a model-scale brownout simulation using the hybrid approach on a single GTX Titan now takes 1.25 hours per revolution compared to 6 hours per revolution on 32 Intel Xeon cores.Item Rotorcraft Brownout Mitigation Through Flight path Optimization Using a High Fidelity Rotorcraft Simulation Model(2012) Alfred, Jillian; Celi, Roberto; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Brownout conditions often occur during approach, landing, and take off in a desert environment and involve the entrainment and mobilization of loose sediment and dust into the rotor flow field. For this research, a high fidelity flight dynamics model is used to perform a study on brownout mitigation through operational means of flight path. In order for the high fidelity simulation to model an approach profile, a method for following specific profiles was developed. An optimization study was then performed using this flight dynamics model in a comprehensive brownout simulation. The optimization found a local shallow optimum approach and a global steep optimum approach minimized the intensity of the resulting brownout clouds. These results were consistent previous mitigation studies and operational methods. The results also demonstrated that the addition of a full rotorcraft model into the brownout simulation changed the characteristics of the velocity flow field, and hence changing the character of the brownout cloud that was produced.Item Investigation into the Effects of Aeolian Scaling Parameters on Sediment Mobilization below a Hovering Rotor(2011) Baharani, Ajay Kumar; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flow visualization and particle image velocimetry (PIV) experiments were conducted on a small-scale rotor hovering over a ground plane covered with a mobile sediment bed to help understand the effects of certain selected scaling parameters on the processes of sediment mobilization, entrainment, and uplift as induced by the rotor. Flow visualization using high-speed videography was used to study the rotor flow structures, their evolution in the rotor wake, and their interaction with the ground plane. Time-resolved PIV measurements of the rotor wake flow at the sediment bed quantified the flow velocities where most of the sediment mobilization was observed to occur. Dual-phase PIV experiments were conducted using ten different sediment samples of known characteristics to vary the values of five of the similarity parameters: 1. Particle diameter-to-rotor radius ratio, 2. Particle-to-fluid density ratio, 3. Ratio of characteristic flow (or wind) speed to particle terminal speed, 4. Densimetric Froude number, and 5. Threshold friction velocity ratio. The particle-to-fluid density ratio was shown to have the greatest effect on the resulting two-phase flow, followed by the threshold friction velocity ratio. The flow was also sensitive to changes in the particle diameter-to-rotor radius ratio. Changes in the densimetric Froude number and ratio of the characteristic flow speed to particle terminal speed also showed good correlations to observations of the quantity of uplifted particles. The effects of the passage of the tip vortex near the bed was shown to increase the shear stresses on the bed, which was observed to be closely correlated to an increase in the quantity of entrained sediment particles if the threshold conditions for particle mobility was exceeded. The observations and results were used to make recommendations regarding scaling on dual-phase experiments to better simulate the problem of rotorcraft brownout in the laboratory environment.