The prevalence of cardiovascular disease (CVD) has increased dramatically due in part to the increased rates of obesity in North America. Atherosclerosis, the most prevalent type of CVD, is a progressive disease characterized by the build-up of plaque within the arteries. The plaque development leads to the narrowing of arteries, referred to as stenosis, and restricts crucial blood flow to the organs of the body. This condition is often treated by the implantation of a stent, a wire mesh scaffold device placed in the region of an atherosclerotic plaque after balloon angioplasty. The stent was developed to improve the clinical outcome of angioplasty procedures by mitigating the effects of elastic recoil by the vessel wall and maintaining vessel patency after angioplasty. Since the introduction of stents as a treatment option over a decade ago, in-stent restenosis (ISR) has been an iatrogenic outcome and remains an unsolved limitation of the interventional treatment device, resulting in stent failure and additional surgical procedures to restore blood flow. Many improvements have been made in stent design in order to reduce the likelihood of ISR, but none have eliminated the problem. Endothelial cells lining vessel walls transduce local hemodynamic loading in the stent vicinity, such as wall shear stress magnitude (WSS), into biochemical signals that lead to the progression of ISR. Hence, resolving the hemodynamics in the vicinity of the stent is crucial to reducing the rates of stent failure. The objective of the study is to address the problem of ISR by clearly elucidating the flow physics induced by stent implantation, accounting in particular for vessel curvature, by first considering idealized stent models, then progressing to an actual stent model. Stent designs are typically based upon data originating solely from studies of flow in straight vessels, which, once optimized for this configuration, may lead to suboptimal performance when placed in tortuous vessels. Previous stent studies have almost categorically neglected the effects of curvature on the flow physics, despite the fact that even extremely mild curvature changes the axial WSSM distribution within the vessel and induces the development of secondary flows, which alters the advection of chemicals released into the lumen. Using computational fluid dynamics (CFD) techniques, this study seeks to (i) determine the impact of stent strut amplitude and frequency on primary and secondary flow structures; (ii) determine the significance of the stent strut shape in the size of the stagnation zone; (iii) evaluate flow behavior in the transition region from smooth walled to stented vessel; and (iv) examine the collection of these effects in a full stent model geometry in a curved tube. This study takes a systematic approach, dissecting the impact of the stent first into simplified foundational components, then investigating each component and finally synthesizing the components into a full stent model with the long-term goal of optimizing stent design to reduce the rate of restenosis. As well, the study findings can aid in understanding the signal transduction mechanisms of the endothelial cells, which play a role in the development of ISR, and reduce the cardiovascular disease mortality rate by improving the clinical outcome of treatment procedures. Further, the study findings contribute to the fundamental understanding of flow in curved pipes with wall protrusions, the impact of the choice of the constitutive model of the fluid, and the hemodynamic environment in the vicinity of the stent.

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The use of mechanically stabilized earth (MSE) retaining walls has become quite prevalent in highway embankment applications. A design criterion for these walls was originally established by the Federal Highway Administration (FHWA) and has been modified on a state by state basis. Recently, the Texas Department of Transportation (TxDOT) has recorded several wall failures mostly due to excessive settlement and lateral wall deformation and wanted to evaluate the current state design guidelines for regionally available backfill materials. Prior to numerical modeling simulations, material parameters of regionally available backfill needed to be evaluated. As the state guidelines require 85-percent of the wall backfill material to be above the No. 4 sieve, large scale triaxial testing was an option to evaluate strength and volume change parameters. This research used cylindrical specimen 6-inches in diameter and 12- inches in height in a large scale triaxial apparatus. Three types of backfill material were tested and specimens were mixed and compacted in 4 different gradations for each material type. Each gradation was tested at confining stresses corresponding to wall heights of 10, 15, and 20 feet for a total of 36 tests. Basic material parameters such as unit weight and friction angle were evaluated directly from testing, while more complex material parameters were selected from the data for use in the Duncan-Chang elastic constitutive model. This method utilizes hyperbolic curve fitting of both strength and volumetric test data to define soil behavior parameters which include the following: modulus number (K), modulus exponent (n), initial tangent modulus (Ei), failure ratio (Rf ), initial Poisson’s Ratio (νi), and Poisson’s Ratio Parameters G, F, and d. Friction angles from triaxial testing ranged from 32 to 53 degrees having some uncertainty due to inconsistent compaction. The variation in sand and fine size particles in the backfill tended to reduce friction angles, except in the case of Type-B material where density increased due to the high percentage of sand and fines. Duncan-Chang parameters fit reasonably well with experimental data for strength barring some experimental errors. Volumetric parameters were inconclusive due to inconsistent compaction and membrane leakage. Additional testing is needed to provide more sound volumetric data.

Wall-bounded turbulent flows are prevalent in engineering and industrial applications. Walls greatly affect turbulent characteristics in many ways including production and propagation of turbulent stresses. While computational fluid dynamics can be used as an important design tool, its use is hindered due to the fine-mesh requirements in the near-wall region to capture all of the pertinent turbulent data. To resolve all relevant scales of motion, the number of grid points scales with Reynolds number as N ≈ Re9/4, making it nearly impossible to solve real engineering problems, most of which feature high Reynolds numbers. A method to help alleviate the resolution requirements is the use of wall models. This method allows for a coarser mesh to be used in which the near-wall region is modeled and the first grid point is placed in the log-law region. The shear stress at the wall is correlated with the velocity at a point outside the near-wall region, drastically reducing the number of elements required and reducing the computational time and cost of the simulation. The goal of this study was to test the speed increase and element reduction capabilities of combining a wall function solution with the massively-parallel, spectral element solver, Nek5000, and verify the method using a turbulent channel simulation. The first grid point is placed at y+ = 100, in the log-law region, for Reτ = 950 and the sub-grid scales are modeled using a dynamic Smagorinski model. The results are then compared to a DNS performed by Jimenez and Hoyas for model verification.

We study current-induced domain wall dynamics in a thin ferromagnetic nanowire. We derive the effective equations of domain wall motion, which depend on the wire geometry and material parameters. We describe the procedure to determine these parameters by all-electric measurements of the time-dependent voltage induced by the domain wall motion. We provide an analytical expression for the time variation of this voltage. Furthermore, we show that the measurement of the proposed effects is within reach with current experimental techniques. We also show that a certain resonant time-dependent current moving a domain wall can significantly reduce the Joule heating in the wire, and thus it can lead to a novel proposal for the most energy efficient memory devices. We discuss how Gilbert damping, non-adiabatic spin transfer torque, and the presence of Dzyaloshinskii-Moriya interaction can effect this power optimization. Furthermore, we propose a new nanodot magnetic device. We derive a specific time-dependent current that is needed to switch the magnetization of the nanodot the most efficiently.

The transitional Reynolds number and range for macrochannel flows are around 1200 and 1000 respectively. Several studies have shown that for microchannel flows, the transitional Reynolds number drops to around 500 and the transitional range around 300. This thesis provides some data on the laminar to turbulent transition for nano-channel flows through molecular dynamic simulations. The Lennard-Jones potential is used for fluid-fluid and wall-fluid interactions, and a non-linear spring potential is used for wall-wall interactions. The mixing is characterised by averaging the maximum transverse movement for all fluid molecules. Six nano-separations were simulated and the flow and mixing behaviours examined. The results show that the transitional Reynolds number and the range increase with increasing diameter. The effects of wall properties on flow regimes were also investigated using molecular dynamic simulations. The results show that the transitional Reynolds number and range increased with increasing wall density. For increasing wall interaction strength, the effect on the transitional Reynolds number was inconclusive. However, the transitional range increased. With an increase in wall wettability, which corresponds to an increase in hydrophilicity, there was an increase in both the transition Reynolds number and range. The wall roughness was modeled as sinusoidal. For an increase in the amplitude of the wall roughness, there was a decrease in the transitional Reynolds number. The effect on the transitional range was inconclusive. For an increase in the period of the wall roughness, both the transitional Reynolds number and range increased. Errors involved in MD simulations arise from several sources. They include the size of the time-step, thermostat model, wall model, and viscosity calculation method. No experimental results at the nano-scale are available for direct comparison however they provide a basis for future work. As computation power improves, MD simulations at higher Reynolds numbers may be compared with experimental results of flows through microchannels. Also, as laboratory technology and measurement accuracy improves, experimental results of flows through nano-channels may be conducted and used for comparison. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1522637 / Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 2010

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 78-82). Also available in electronic version.

Honors Project--Smith College, Northampton, Mass., 2009. / Includes bibliographical references (p. 24-25).

Based on the author's Thesis (Doctoral), University of Durham (England), 2003. / Includes bibliographical references (p. 203-222) and index.