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
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Item An Investigation of Flames, Deflagrations, and Detonations in High-Speed Flows(2018) Goodwin, Gabriel Benjamin; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A comprehensive understanding of the fundamental physics underlying combustion and detonations in turbulent and high-speed flows is crucial to the design of robust ramjet, scramjet, and detonation engines. This work uses high-fidelity, multidimensional numerical simulations to investigate flame stability and deflagration-to-detonation transition (DDT) mechanisms in supersonic reactive flows. The study consists of four major sections. The first section discusses the acceleration of a flame in a channel with obstacles and its transition from a laminar, expanding flame to a turbulent deflagration and eventual detonation. As the flame accelerates, a highly dynamic, shock-heated region forms ahead of the flame. Shock collisions and reflections focus energy in localized volumes of unburned gas at timescales that are small relative to the acoustic timescale of the unburned gas. The rapid deposition of energy causes the unburned gas to detonate through an energy-focusing mechanism that has elements of both direct initiation and detonation in a gradient of reactivity. The second section describes how the blockage of a channel with regularly spaced obstacles, analogous to the igniter in a detonation engine, affects flame acceleration and turbulence in the region ahead of the accelerating flame. The rate of flame acceleration, time and distance to DDT, and detonation mechanism are compared for channels with high, medium, and low blockage ratios. Stochasticity and uncertainty in the numerical solutions are discussed. In the third section, the stability of premixed flames at high supersonic speeds in a constant-area combustor is investigated. After autoignition of the fuel-oxidizer mixture in the boundary layer at the combustor walls, the flame front eventually becomes unstable due to a Rayleigh-Taylor (RT) instability at the interface between burned and unburned gas. The turbulent flame front transitions to a detonation through the energy-focusing mechanism when a shock passes through the flame and amplifies its energy release. The final section discusses the effect of inflow Mach number in the supersonic combustor on ignition, flame stability, and transition to detonation of a premixed flame. Timescales for growth of the RT instability and detonation initiation increase rapidly with flow speed, but, qualitatively, flame evolution is independent of Mach number.Item Slot Film Cooling: A Comprehensive Experimental Characterization(2016) Raffan Montoya, Fernando; Marshall, Andre W; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)When components of a propulsion system are exposed to elevated flow temperatures there is a risk for catastrophic failure if the components are not properly protected from the thermal loads. Among several strategies, slot film cooling is one of the most commonly used, yet poorly understood active cooling techniques. Tangential injection of a relatively cool fluid layer protects the surface(s) in question, but the turbulent mixing between the hot mainstream and cooler film along with the presence of the wall presents an inherently complex problem where kinematics, thermal transport and multimodal heat transfer are coupled. Furthermore, new propulsion designs rely heavily on CFD analysis to verify their viability. These CFD models require validation of their results, and the current literature does not provide a comprehensive data set for film cooling that meets all the demands for proper validation, namely a comprehensive (kinematic, thermal and boundary condition data) data set obtained over a wide range of conditions. This body of work aims at solving the fundamental issue of validation by providing high quality comprehensive film cooling data (kinematics, thermal mixing, heat transfer). 3 distinct velocity ratios (VR=uc/u∞) are examined corresponding to wall-wake (VR~0.5), min-shear (VR ~ 1.0), and wall-jet (VR~2.0) type flows at injection, while the temperature ratio TR= T∞/Tc is approximately 1.5 for all cases. Turbulence intensities at injection are 2-4% for the mainstream (urms/u∞, vrms/u∞,), and on the order of 8-10% for the coolant (urms/uc, vrms/uc,). A special emphasis is placed on inlet characterization, since inlet data in the literature is often incomplete or is of relatively low quality for CFD development. The data reveals that min-shear injection provides the best performance, followed by the wall-jet. The wall-wake case is comparably poor in performance. The comprehensive data suggests that this relative performance is due to the mixing strength of each case, as well as the location of regions of strong mixing with respect to the wall. Kinematic and thermal data show that strong mixing occurs in the wall-jet away from the wall (y/s>1), while strong mixing in the wall-wake occurs much closer to the wall (y/s<1). Min-shear cases exhibit noticeably weaker mixing confined to about y/s=1. Additionally to these general observations, the experimental data obtained in this work is analyzed to reveal scaling laws for the inlets, near-wall scaling, detecting and characterizing coherent structures in the flow as well as to provide data reduction strategies for comparison to CFD models (RANS and LES).Item MEASUREMENTS OF THE TWO-PHASE VORTICAL FLOW AND TURBULENCE CHARACTERISTICS BELOW A ROTOR(2014) Rauleder, Juergen; Leishman, Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Time-resolved particle image and particle tracking velocimetry measurements were made in the particle-laden turbulent flow environment below a rotor hovering over a mobile sediment bed. The results were also compared to the near-wall flow produced by a nominally equivalent two-dimensional wall jet. The objective of the work was to understand the fluid dynamic mechanisms of how the mean flow, stochastic turbulence, and concentrated vorticity produced by the rotor affected the mobilization and pickup of particles from the sediment bed. Another objective was to better understand the assumptions that would be required for the development of models that are more applicable to rotor-induced particle mobilization. It was shown that the mean flow in the boundary layer at the ground below the rotor was similar to that of a wall jet. However, the instantaneous flow field and turbulence characteristics between these two flows were significantly different. Mobilized particles of 45--63 micron diameter (with a particle Reynolds number of less than 30 and a Stokes number of about 60) were individually identified and tracked, with the objective of relating any changes in the temporal evolution of the vortical flow and turbulence characteristics of the carrier flow phase to its coupling to the dispersed particle phase. The processes of particle mobilization and pickup from the bed were found to correlate to the Reynolds stresses and discrete turbulence events, respectively. The mean flow and turbulence characteristics were modified by the presence of particles in the near-wall region, showing clear evidence of two-way coupling between the phases of the resulting two-phase flow. Specifically, it was shown that the uplifted particles altered the carrier flow near the sediment bed, leading to an earlier distortion of the flow induced by the blade tip vortices and to the accelerated diffusion of the vorticity that they contained. The uplifted particles were also seen to modify the overall turbulence field, and when sufficient particle concentrations built up, the particles began to attenuate the turbulence levels. Even in regions with lower particle concentrations, the turbulence was found to be attenuated by the indirect action of the particles because of the distortions to the tip vortices, which were otherwise a significant source of turbulence production. After the tip vortices had diffused further downstream from the rotor, the uplifted particles were also found to increase the anisotropy of the resulting turbulence in the flow.Item CONTRIBUTIONS TOWARDS THE UNDERSTANDING OF ROTOR-INDUCED DUST PARTICLE MOBILIZATION AND TRANSPORT(2014) Sydney, Anish Joshua; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To better understand the problem of rotor-induced particle motion and rotorcraft brownout, time-resolved, dual-phase particle image velocimetry and particle tracking velocimetry measurements were made in the flow produced by a small laboratory rotor that was hovering over a ground plane covered with a mobile sediment bed. To investigate the three-dimensionality of the wake and resultant particle field, flow measurements were made in vertical and horizontal planes around the rotor and near the ground. The primary goals of the work were to: 1. Characterize the fundamental flow physics of a rotor wake interacting with a sediment bed; 2. Investigate how rotor operating parameters, such as the disk loading, blade loading coefficient, and wake shedding frequency affected the mobilization, uplift and overall development of the particle field; 3. Examine the effects of placing a body between the rotor and the ground to understand how the interactions of the rotor wake with the body affected the transport of particles from the bed. The results showed that the rotor wake was very three-dimensional, with highly non-uniform velocities near the ground that resulted in the radially asymmetric mobilization, uplift and suspension of particles. The tip vortices were found to be the primary contributor to the uplift of particles, with the aperiodic variations in their trajectories near the ground causing intermittent particle mobilization events. These effects were caused, in part, by wave-like displacements that developed along the lengths of the tip vortices, which caused some parts of the filaments to convect closer to the ground than other parts and so uplift discrete bursts or plumes of particles. The quantity and distribution of uplifted particles were shown to be affected by the operating condition of the rotor, with the overall complexity of the rotor wake generally resulting in the formation of a highly three-dimensional and time-varying particle field. The rotor operating parameters were shown to interdependently alter the characteristics of the groundwash flow and the tip vortices produced by the rotor. Stronger wake vortices that impinged on the bed generally uplifted more particles, however, higher near-wall flow velocities over the bed also convected particles further downstream before they could be suspended. The near-wall flow developments were further complicated by the interaction of the rotor wake with a body, which significantly distorted the development of the rotor wake at the ground, the resulting near-wall flow velocities generally being lower in magnitude. The degree of wake distortion, however, was found to be sensitive to the cross-sectional shape of the body. In cases where there was direct impingement of the tip vortices on the body surfaces, the distortions to the wake caused lower near-wall flow velocities but still contained vortices that were able to suspend sediment particles radially closer to the rotor compared to the isolated rotor case.Item Spray Droplet Generation in Breaking Water Waves(2013) Towle, David; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents the results of an experiment in which the generation of spray droplets in two mechanically generated plunging breaking water waves was investigated. In both breaking waves, the wind velocity at the free surface was zero and droplets were produced only by means of plunging jet impact, free surface turbulence and bubble bursting. The free surface was kept free of particulates and surfactants for each wave in one set of measurements, while in a second set of experiments, using only the higher amplitude wave, the tank water was mixed with a soluble surfactant. The experiment was able to deliver information on the diameter distributions of droplets produced in both waves. Also, the positional and time dependence of droplet production throughout the stages of wave breaking was investigated. Ultimately, it was determined that under these conditions droplet frequency is influenced primarily the impact of the plunging jet with the front face of the wave. Higher amplitude waves generally lead to markedly higher amounts of droplets produced and a shift towards higher diameters. The addition of surfactant reduced the diameter and suppressed the production of spray droplets. The positional distribution of droplet production within the wave was also altered significantly by the presence of surfactant: droplets were more evenly produced throughout the breaking zone in the wave with surfactant, whereas the wave generated in clean water had a much smaller region of significant droplet production.Item Motion of elastic capsules in microfluidic channels(2010) Kuriakose, Shugi; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Capsule flow dynamics in microchannels plays a significant role in complex biological phenomena, such as the microcirculation, and in engineering applications, such as in microfluidic devices for drug delivery and cell sorting. In this thesis, we investigate the motion of elastic capsules in wall-bounded flows by extending the Membrane Spectral Boundary Element method developed by Dodson and Dimitrakopoulos for free-suspended flows. First, a validation study of the method is performed for the axisymmetric capsule motion in a cylindrical channel. For a capsule moving along the centerline of a cylindrical channel, our computational model successfully reproduced the parachute shape observed in earlier experimental and computational studies. Next, we investigate the flow dynamics of a strain-hardening Skalak capsule moving along the centerline in a square and a rectangular channel. We examine how the capillary number and capsule size influence the deformation and physical properties of the capsule. For large capsules in a square channel, our investigation reveals that the steady-state capsule shape is non-axisymmetric. The capsule assumes a shape similar to the channel's cross-section i.e. a square shape with rounded edges. Buckling of the capsule's upstream end resulting in a negative edge curvature is observed at higher capillary numbers and for large capsule sizes. For the largest capsules studied, we also observe the development of dimples at the capsule's lateral surface. A comparative study of capsule motion and deformation in cylindrical and square channels shows that the capsule deformation in a cylindrical channel is similar to that in a square channel at a larger capillary number. In a rectangular channel, we observe a three-dimensional (i.e. non-axisymmetric) deformation of the capsule at high capillary numbers resulting in dimpling of the capsule's upstream end at steady state. We also consider the transient motion of a capsule in a converging square microchannel and investigate the influence of viscosity ratio, capillary number and capsule size on the evolution of capsule properties. As the capsule moves through the converging region a fluctuation in the geometric and physical properties of the capsule is observed.