Search results
(1 - 3 of 3)
- Title
- COMPUTATIONAL ACCESS FLOW REDUCTION EFFECT ON WALL SHEAR STRESS IN BRACHIOCEPHALIC FISTULAE
- Creator
- Wlodarczyk, Marta P.
- Date
- 2017, 2017-05
- Description
-
The population of patients with end stage renal disease (ESRD) is growing at a rate higher than the global population. The only viable...
Show moreThe population of patients with end stage renal disease (ESRD) is growing at a rate higher than the global population. The only viable treatment for these patients is a kidney transplant. However, in the absence of a suitable kidney donor, renal patients are left with hemodialysis as a renal replacement therapy. Hemodialysis is facilitated through arteriovenous fistula (AVF), and the particular interest in this investigation is brachiocephalic fistula (BCF). The survival of dialysis patients depends on maintaining patency of fistula over a prolonged period of time. The extreme hemodynamic environment that is created by BCF triggers the onset of neointimal hyperplasia (NH) in most renal dialysis patients, which leads to access failure via stenosis. This is because the hemodynamics in AVF are well outside the normal physiological range. Computational fluid dynamics (CFD) along with shape optimization allows for the study of the hemodynamic parameters such as wall shear stress that have been shown to be detrimental in the future occurrence of cephalic arch stenosis. In this study, CFD modeling and identification of hemodynamic patterns was possible in three dimensions due to advanced post processing of IVUS patient-specific geometries. A method utilizing 3D CFD and shape optimization has been developed to implement Miller’s banding method used in clinical practice to evaluate its impact on WSS and onset of neointimal hyperplasia. The level of banding represented by a constriction is in fact a patient specific value and is not a trivial solution of minimum flow rate necessary to conduct hemodialysis; hence suggesting that even restoring the inlet velocity to the velocity pre-fistula creation might not reduce incidence of cephalic arch stenosis. The findings of this study support the previous hypothesis that non-homeostatic WSS distributions trigger neointimal hyperplasia and resulting venous stenosis. The important outcome is that the presented computational framework allows for evaluation of Miller's banding method for reducing the blood flow rate via surgical constriction and identification of a patient-specific banding level that restores the WSS to the normal physiological range.
M.S. in Mechanical and Aerospace Engineering, May 2017
Show less
- Title
- A 4-Phase Flow Model for Methane Production from an Unconsolidated Hydrate Reservoir
- Creator
- Hinz, Deniz
- Date
- 2019
- Description
-
Natural gas from hydrates is extremely abundant as an energy resource; US resource-grade hydrate deposits are estimated to be over 20 times...
Show moreNatural gas from hydrates is extremely abundant as an energy resource; US resource-grade hydrate deposits are estimated to be over 20 times the domestic proved natural gas resources, at approximately 7000 trillion cubic feet (tcf). The theoretical potential of hydrates is immense, but production testing and research remain lacking, which has led to the development of numerous hydrate production numerical simulators for consolidated porous media hydrate reservoirs. However, due to the onset of unconsolidated flow behavior upon significant hydrate dissociation, numerical models haven’t agreed well with the experimental data from the Mallik production tests. Hydrate contributes substantially to the strength of the sediment matrix, such that hydrate-bearing sediment ultimately falls apart exhibiting 4-phase unconsolidated flow behavior of gas, water, hydrate, and sand. In order to better capture the multiphase flow characteristics of gas, water, hydrate, and sand in an unconsolidated gas hydrate reservoir, we have developed a novel 4-phase flow model coupled with numerical simulation of the Mallik 2007/2008 production tests. The model is able to capture the coupled 4-phase hydrodynamics, mass transfer, and heat transfer physics inherent to the unconsolidated hydrate reservoir. Solid deformation is modeled by extending multiphase and granular flow theory to hydrate-bearing sediment. Constitutive models for the solid viscosity and solid pressure are developed to model the change in strength of the sediment as hydrate dissociates and the solid deforms. The solid viscosity is a composite of frictional contributions from the solid normal stress and cohesive contributions from the hydrate. The interphase momentum exchange between the fluid phases (gas and water) and solid phases (hydrate and sand) modeled based on a volume-averaged approach that considers the formation and closure of high-permeability volumes due to dilation and compaction of hydrate-bearing sediment as it deforms. By considering the deformation of solids and the subsequent effect on the permeability, the 4-phase simulations showed good agreement with the experimental data from the Mallik 2007/2008 production phases. The 4-phase modeling approach serves as a proof of concept for the application of granular flow theory to hydrate-bearing sediment. An unconsolidated hydrate reservoir with sustained sand production essentially behaves like a naturally fracking reservoir, exhibiting a dramatic increase in permeability induced solely by depressurization. Conversely, preventing sand production with a sand screen ultimately leads to significant throttling of the gas production rate due to the compaction and accumulation of sand at the sand screen.
Show less
- Title
- COMPUTATIONAL STUDIES OF HEAT TRANSFER IN TURBULENT WAVY CHANNEL FLOWS
- Creator
- Dzubur, Amar
- Date
- 2018, 2018-05
- Description
-
Heat transfer is studied in fully-developed turbulent flows through channels with various geometries using Direct Numerical Simulations (DNS)....
Show moreHeat transfer is studied in fully-developed turbulent flows through channels with various geometries using Direct Numerical Simulations (DNS). Channels where a sinusoidal wave is mapped along the wall in either the streamwise direction or spanwise direction are studied, and comparisons to a simple channel with flat walls (rectangular channel) are provided. The fluid flow velocities fi elds, and pressure fi elds are analyzed along with the vorticity generated in the flow, and are utilized in tandem with the Nusselt number calculated along the heat transfer boundaries, to derive a clearer description of the heat transfer performance of the various geometries. The geometries that have a sinusoidal wave mapped along the spanwise direction and not along the streamwise direction showed the poorest heat transfer performance, as exhibited by the lowest average Nusselt number. The performance of two channels, with an in-phase and out of phase sinusoidal wave mapped along the streamwise direction exhibited heat transfer performance signifi cantly higher than that shown by the rectangular channel, which served as baseline. The heat transfer differences can be largely attributed to the vorticity generation and superior fluid mixing that is generated by the periodic streamwise mapped sinusoid. Streamwise sinusoidal channels exhibit Nusselt numbers that are more than three times greater than the spanwise mapped sinusoid, and almost three times greater than that of the rectangular channel. It is shown that the difference among an in-phase and out of phase wave mapping exists, but is found to be minimal. Further exploration regarding potential geometries with various phase shifts, non-rounded corners, and longer simulation times would be beneficial.
M.S. in Mechanical and Aerospace Engineering, May 2018
Show less