UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

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 given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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2013 - 20173

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    Lateral Capsule Migration in Microfluidic Channels
    (2017) Wang, Yiyang; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A capsule motion inside a microfluidic channel has attracted a lot of attention in recent decades owing to its important applications in industrial, pharmaceutical and physiological systems such as in cell sorting, targeted drug delivery and blood flow. In this dissertation, we computationally investigate an elastic capsule's lateral migration inside a constricted microfluidic device under Stokes flow conditions. We use the Membrane Spectral Boundary Element (MSBE) method to determine the capsule dynamics due to its high computational accuracy and versatility in dealing with complex solid geometries. In the bounded Poiseuille flow of the microfluidic constriction, a capsule, placed initially off-centered will migrate away from the wall and move toward the channel centerline. The capsule's lateral migration behavior is caused by the combination of the wall effects due to the existence of the channel boundary, the shear gradient generated by the non-linear velocity distribution of the flow, and the lift force created by the capsule deformation. We use a constricted device instead of a straight channel to do the simulations, because the capsule's lateral migration in a straight channel is too slow to be observed easily, while the existence of the converging connection of the constricted device increases the capsule's lateral velocity and thus facilitates its migration. The main goal of our research is to investigate the effects of the capsule's physical properties on its lateral migration behavior. We released various deformable capsules at different initial positions, membrane hardness, viscosity ratios, and capsule volumes inside the constricted channel and computed their deformation behavior and migration trajectories. Our results show that changing a capsule's viscosity ratio or the membrane hardness does not strongly affect the capsule's lateral migration due to the capsule's weak inner circulation. On the other hand, changing the capsule's initial position and capsule volume strongly affect its migration trajectories. Thus soft particles with different sizes can be separated and identified.
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    Magnetorheological fluid dynamics for high speed energy absorbers
    (2017) Sherman, Stephen Gilman; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Fluids with a controllable yield stress allow rapid variations in viscous force in response to an externally applied field. These fluids are used in adaptive energy dissipating devices, which have a controllable force response, reducing shock and vibration loads on occupants and structures. This thesis investigates the physics of these fluids at high speeds and shear rates, through particle modeling and fluid dynamics. The focus is on the experimentally observed reduction in controllable force at high speeds seen in magnetorheological (MR) fluid, a suspension of magnetizable particles that develop a yield stress when a magnetic field is applied. After ruling out particle dynamic effects, this dissertation takes the first rigorous look at the fluid dynamics of a controllable yield stress fluid entering an active region. A simplified model of the flow is developed and, using computational fluid dynamics to inform a control volume analysis, we show that the reduction in high speed controllable force is caused by fluid dynamics. The control volume analysis provides a rigorous criteria for the onset of high speed force effects, based purely on nondimensional fluid quantities. Fits for pressure loss in the simplified flow are constructed, allowing yield force prediction in arbitrary flow mode geometries. The fits are experimentally validated by accurately predicting yield force in all of the known high speed devices. These results should enable the design of a next generation of high performance adaptive energy absorbers.
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    Compact-Reconstruction Weighted Essentially Non-Oscillatory Schemes for Hyperbolic Conservation Laws
    (2013) Ghosh, Debojyoti; Baeder, James D; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A new class of non-linear compact interpolation schemes is introduced in this dissertation that have a high spectral resolution and are non-oscillatory across discontinuities. The Compact-Reconstruction Weighted Essentially Non-Oscillatory (CRWENO) schemes use a solution-dependent combination of lower-order compact schemes to yield a high-order accurate, non-oscillatory scheme. Fifth-order accurate CRWENO schemes are constructed and their numerical properties are analyzed. These schemes have lower absolute errors and higher spectral resolution than the WENO scheme of the same order. The schemes are applied to scalar conservation laws and the Euler equations of fluid dynamics. The order of convergence and the higher accuracy of the CRWENO schemes are verified for smooth solutions. Significant improvements are observed in the resolution of discontinuities and extrema as well as the preservation of flow features over large convection distances. The computational cost of the CRWENO schemes is assessed and the reduced error in the solution outweighs the additional expense of the implicit scheme, thus resulting in higher numerical efficiency. This conclusion extends to the reconstruction of conserved and primitive variables for the Euler equations, but not to the characteristic-based reconstruction. Further improvements are observed in the accuracy and resolution of the schemes with alternative formulations for the non-linear weights. The CRWENO schemes are integrated into a structured, finite-volume Navier-Stokes solver and applied to problems of practical relevance. Steady and unsteady flows around airfoils are solved to validate the scheme for curvi-linear grids, as well as overset grids with relative motion. The steady flow around a three-dimensional wing and the unsteady flow around a full-scale rotor are solved. It is observed that though lower-order schemes suffice for the accurate prediction of aerodynamic forces, the CRWENO scheme yields improved resolution of near-blade and wake flow features, including boundary and shear layers, and shed vortices. The high spectral resolution, coupled with the non-oscillatory behavior, indicate their suitability for the direct numerical simulation of compressible turbulent flows. Canonical flow problems -- the decay of isotropic turbulence and the shock-turbulence interaction -- are solved. The CRWENO schemes show an improved resolution of the higher wavenumbers and the small-length-scale flow features that are characteristic of turbulent flows. Overall, the CRWENO schemes show significant improvements in resolving and preserving flow features over a large range of length scales due to the higher spectral resolution and lower dissipation and dispersion errors, compared to the WENO schemes. Thus, these schemes are a viable alternative for the numerical simulation of compressible, turbulent flows.