Computational fluid dynamics and finite element analysis were employed to study vestibular hair cell bundle mechanics under physiologic stimulus conditions. CFD was performed using ANSYS CFX and FEA utilized a custom MATLAB model. Nine varieties of hair cell bundles were modeled using tip-forcing only (commonly used experimentally), fluid-flow only (physiologic for free-standing bundles), and combined loading (physiologic for bundles with tip attachments) conditions to determine how the bundles behaved in each case. The bundles differed in the heights of their components, their length and width, and their number of steriocilia. Tip links were modeled to determine ion-channel opening behavior. Results show that positive pressures, negative pressures, and shear stresses on the exterior of the bundles are of comparable magnitude. Under combined loading, some bundles experienced very high suction pressures on their interior. The bundles with tall steriocilia are hindered by the endolymph while those with short steriocilia and much taller kinocilia are assisted by the fluid flow. Each bundle type has a different range over which it is most sensitive so that the bundles cumulatively cover a very large range of stimuli; the order in which bundles respond from smallest stimulus magnitude to largest is free-standing extrastriolar bundles, attached striolar bundles, attached extrastriolar bundles, and free-standing extrastriolar bundles. A short examination of off-axis loading shows that the prevailing theory suggesting that bundle response is proportional to the cosine of the angle between the stimulus direction and the bundle's direction of maximum excitation is incorrect. / Ph. D.

One of the largest sources of error in a CFD simulation is the discretization error. One of the least computationally expensive ways of reducing the discretization error in a simulation is by performing mesh adaptation. In this work, the mesh adaptation processes are driven by the truncation error, which is the local source of the discretization error. Because this work is focused on methods for structured grids, r-adaptation is used as opposed to h-adaptation. A new method for performing the r-adaptation based on an optimization process is developed and presented here. This optimization process was applied to simple 1D and 2D Euler problems as a method of testing the approach. The mesh optimization approach is compared to the more common equidistribution approach to determine which produces more accurate results as well as the costs associated with each. It is found that the optimization process is able to reduce the truncation error than equidistribution. However, in the 2D cases optimization does not reduce the discretization error sufficiently to warrant the significant costs of the approach. This indicates that the much cheaper equidistribution process provides a cost-effective manner to reduce the discretization error in the solution. Further, equidistribution is able to achieve the bulk of the potential reductions in discretization error possible through r-adaptation. This work also develops a new framework for reducing the cost of performing truncation error based r-adaptation. This new framework also addresses some of the issues associated with r-adaptation. In this framework, adaptation is performed on a coarse mesh where it is faster to perform, creating a mapping function for this mesh, and finally evaluating this mapping at a fine enough mesh to meet the error target. The framework is used for 2D Euler and 2D laminar Navier-Stokes problems and shown to be the most cost-effective way to meet a desired error target. Finally, the multi-mesh CFD method is introduced and applied to a wide variety of problems from quasi-1D nozzle to 2D laminar and turbulent boundary layers. The multi-mesh method allows the system of equations to be solved on a system of meshes. With this method, each equation is solved on a mesh that is adapted specifically for it, meaning that more accurate solutions for each equation can be obtained. This work shows that, for certain problems, the multi-mesh approach is able to achieve more accurate results in less time compared to using a single mesh. / Doctor of Philosophy / Computational fluid dynamics (CFD) describes a method of numerically solving equations that attempt to model the behavior of a fluid. As computers have become cheaper and more powerful and the software has become more capable, CFD has become an integral part of the engineering process. One of the goals of the field is to be able to bring these higher fidelity simulations into the design loop earlier. Ideally, using CFD earlier in the design process would allow design engineers to create new innovative designs with less programmatic risk. Likewise, it is also becoming necessary to use these CFD tools later in the final design process to replace some physical experiments which can be expensive, unsafe, or infeasible to run. Both of these goals require the CFD codes to meet the accuracy requirements for the results as fast as possible. This work discusses several different methods for improving the accuracy of the simulations as well as ways of obtaining these more accurate results for the cheapest cost. In CFD, the governing equations modeling the flow behavior are solved on a computer. As a result, these continuous differential equations must be approximated as a system of discrete equations, so that they can be solved on a computer. These approximations result in discretization error, the difference between the exact solutions to the discrete and continuous equations, which is typically the largest type of numerical error in a CFD solution. The source of the discretization error is the truncation error, which is composed of the terms left out of the approximations made when discretizing the continuous equations. Thus, if the truncation error can be reduced, the discretization error in the solution should also be reduced. In this work, several different ways of reducing this truncation error through mesh adaptation are discussed, including the use of optimization methods. These mesh optimization methods are compared to a more common way of performing adaptation, namely equidistribution. It is determined that equidistribution is able to reduce the discretization error by a similar amount while being significantly faster than mesh optimization. This work also presents a framework for making the adaptation process faster overall by performing the adaptation on a coarse mesh and then refining the mesh enough to meet the error tolerance for the application. This framework was the cheapest method investigated to meet a given error target. This work also introduces a new technique called multi-mesh CFD, which allows each equation (conservation of mass, momentum, energy, etc.) to be solved on a separate mesh. This allows each equation to be solved on a mesh that is specifically adapted for it, resulting in a more accurate solution. Here, it is shown that, for certain problems, the multi-mesh technique is able to obtain a solution with lower error than only using a single mesh. This work also shows that these more accurate results can be obtained in less time using multiple meshes than on a single mesh.

In light of rising global temperatures and energy needs, nuclear power is uniquely positioned to offer carbon-free and reliable electricity. In many markets, nuclear power faces strong headwinds due to competition with other fuel sources and prohibitively high capital costs. Small Modular Reactors (SMRs), such as the proposed Advanced Fast Reactor (AFR) 100, have gained popularity in recent years as they promise economies of scale, reduced capital costs, and flexibility of deployment. Fast sodium reactors commonly feature an upper plenum with a large inventory of sodium. When temperatures change due to transients, stratification can occur. It is important to understand the stratification behavior of these large volumes because stratification can counteract natural circulation and fatigue materials. This work features steady-state and transient simulations of thermal stratification and natural circulation of liquid sodium in a simple rectangular slice using a commercial CFD code (ANSYS FLUENT). Different inlet velocities and their effect on stratification are investigated by changing the inlet geometry. Stratification was observed in the two cases with the lowest inlet velocities. An approach for tracking the stratification interface was developed that focuses on temperature gradients rather than differences. Other authors have developed correlations to predict stratification in three dimensional enclosures. However, these correlations predict stratified conditions for all simulations even the ones that did not stratify. The previous models are modified to reflect the two-dimensional nature of the flow in the enclosure. The results align more closely with the simulations and correctly predict stratification in the investigated cases. / Master of Science

Eugene Clement created a design method for swept-back cambered step hulls with deadrise. The cambered step is designed to carry 90% of the planing vessels weight with the remaining 10% being support by a stern mounted hydrofoil. The method requires multiple design iterations in order to achieve an optimal design. Clement stated that the method was not suitable for cambered planing surfaces with high deadrise angles greater than 15 degrees. The goal of this thesis is to create a design procedure for swept-back cambered planing surfaces with high deadrise angles that does not require multiple iterations to obtain an optimal design. Computational fluid dynamics (CFD) program STAR CCM+ is used to generate a database for performance characteristics for a wide range of designs varying deadrise angle, load requirements, trim angle, and different camber values. The simulations are first validated with experimental data for two different cambered steps designed by Stefano Brizzolara and tested in the tow tank at the United States Naval Academy. A series of validation studies utilizing fixed and overset meshes led to a final simulation set up with an overset mesh that allowed for accurate prediction of drag, trim moment, wetted keel length, and the wake profile aft of the cambered planing surface. The database is fitted such that the final equations for optimal design values such as camber, trim angle, drag (shear and pressure), wetted keel length, wetted surface area, and trim moment are in terms of deadrise angle and lift. The optimized design equations are validated with CFD simulation. / Master of Science / Eugene Clement developed a new design method to improve the performance of ultra-fast planing crafts. A planing craft uses the force generated from the flow of water over the bottom to lift the vessel without the use of the static buoyancy force that classic boat designs rely on. Clement wanted to improve the performance of the planing vessel by reducing the total drag force caused by the flow of water on the bottom of the vessel. Clement's design method involves reducing the wetted surface area which reduces drag. Reducing the wetted surface area would normally cause the lifting force on the vessel to reduce, but with the addition of curvature in the smaller wetted surface area, the lifting force would remain the same. Clement's new design method requires multiple iterations to obtain an optimal design. The method limits the angle of the vessels bottom relative to horizontal to under 15 degree. The goal of this thesis is to create a new design method for planing vessels with bottoms that have an incline of 15 degrees or more relative to horizontal. The design method is created using Computational Fluid Dynamics (CFD) solver to model the planing surface moving through water. The CFD solver is validated with experimental test performed at the United States Naval Academy. The improved design method uses equations that can predict the forces and other design characteristics based on the desired vessel weight and seakeeping requirements.

Conventional understanding of quality of energy suggests that heat is a low grade form of energy. Hence converting this energy into useful form of work was assumed difficult. However, this understanding was challenged by researchers over the last few decades. With advances in solar, thermal and geothermal energy harvesting, they believed that these sources of energy had great potential to operate as dependable avenues for electrical power. In recent times, waste heat from automobiles, oil and gas and manufacturing industries were employed to harness power. Statistics show that US alone has a potential of generating 120,000 GWh/year of electricity from oil , gas and manufacturing industries, while automobiles can contribute upto 15,900 GWh/year. Thermoelectric generators (TEGs) can be employed to capture some of this otherwise wasted heat and to convert this heat into useful electrical energy. This field of research as compared to gas turbine industry has emerged recently over past 30 decades. Researchers have shown that efficiency of these TEGs modules can be improved by integrating heat transfer augmentation features on the hot side of these modules. Gas turbines employ advanced technologies for internal and external cooling. These technologies have applications over wide range of applications, one of which is thermoelectricity. Hence, making use of gas turbine technologies in thermoelectrics would surely improve the efficiency of existing TEGs. This study makes an effort to develop innovative technologies for gas turbine as well as thermoelectric applications. The first part of the study analyzes heat transfer augmentation from four different configurations for low aspect ratio channels and the second part deal with characterizing improvement in efficiency of TEGs due to the heat transfer augmentation techniques. / Master of Science

Computational fluid dynamics (CFD) has proven to be an invaluable tool for both engineering design and analysis. As the performance of engineering devices become more reliant upon the accuracy of CFD simulations, it is necessary to not only quantify and but also to reduce the numerical error present in a solution. Discretization error is often the primary source of numerical error. Discretization error is introduced locally into the solution by truncation error. Truncation error represents the higher order terms in an infinite series which are truncated during the discretization of the continuous governing equations of a model. Discretization error can be reduced through uniform grid refinement but is often impractical for typical engineering problems. Grid adaptation provides an efficient means for improving solution accuracy without the exponential increase in computational time associated with uniform grid refinement. Solution accuracy can be improved through local grid refinement, often referred to as h-adaptation, or by node relocation in the computational domain, often referred to as r-adaptation. The goal of this work is to examine the effectiveness of several r-adaptation techniques for reducing discretization error. A framework for geometry preservation is presented, and truncation error is used to drive adaptation. Sample problems include both subsonic and supersonic inviscid flows. Discretization error reductions of up to an order of magnitude are achieved on adapted grids. / Master of Science

The research conducted was a feasibility study using Lithium Fluoride-Beryllium Fluoride (LiF-BeF2) or FLiBe as a coolant with a natural uranium neutron spallation source applied to an accelerator driven sub-critical molten salt reactor. The study utilized two different software tools, MCNPX 2.6 and FLUENT 12.1. MCNPX was used to determine the neutronics and heat deposited in the spallation target structure while FLUENT was used to determine the feasibility of cooling the target structure with FLiBe. Several target structures were analyzed using a variety of plates and large cylinders of natural uranium with a proton beam incident on a Hastelloy-N window. The supporting structures were created from Hastelloy-N due to their anti-corrosive properties of molten salts such as FLiBe and their resistance to neutron damage. The final design chosen was a "Sandwich" design utilizing a section of thick plates followed by several smaller plates then finally a section of thick plates to stop any protons from irradiating the bottom of the target support structure or the containment vessel of the reactor. Utilizing a proton beam with 0.81 MW of proton beam power at 1.35 mA with proton kinetic energies of 600 MeV, the total heat generated in the spallation target was about 0.9 MW due to fissions in the natural uranium. Additionally, the neutrons produced from the final design of the spallation target were approximately 1.25x1018 neutrons per second which were mainly fast neutrons. The use of a natural uranium target proved to be very promising. However, cooling the target using FLiBe would require further optimization or investigation into alternate coolants. Specifically, the final design developed using FLiBe as a coolant was not practically feasible due to the hydraulic forces resulting from the high flow rates necessary to keep the natural uranium target structures cooled. The primary reason for the lack of a feasible solution was the FLiBe as a coolant; FLiBe is unable to pull enough heat generated in the target out of the target structure. Due to the high energy density of a natural uranium spallation target structure, a more effective method of cooling will be required to avoid high hydraulic forces, such as a liquid metal coolant like lead-bismuth eutectic. / Master of Science

A computational study was performed on a vaned radial diffuser using the MEFP (The Moore Elliptic Flow Program) flow code. The vaned diffuser studied by Dalbert et al. was chosen as a test case for this thesis. The geometry and inlet conditions were established from this study. The performance of the computational diffuser was compared to the test case diffuser. The CFD analysis was able to demonstrate the 3D flow within the diffuser. An inlet conditions analysis was performed to establish the boundary conditions at the diffuser inlet. The given inlet flow angles were reduced in order to match the specified mass flow rate. The inlet static pressure was held constant over the height of the diffuser. The diffuser was broken down into its subcomponents to study the effects of each component on the overall performance of the diffuser. The diffuser inlet region, which comprises the vaneless and semi-vaneless spaces, contains the greatest losses, 56%, but the highest static pressure rise, 54%. The performance at the throat was also evaluated and the blockage and pressure recovery were calculated. The results show the static pressure comparison for the computational study and the test case. The overall pressure rise of the computational study was in good agreement with the measured pressure rise. The static pressure and total pressure loss distributions in the inlet region, at the throat, and in the exit region of the diffuser were also analyzed. The flow development was presented for the entire diffuser. The 3D flow calculations were able to illustrate a leading edge recirculation at the hub, caused by an inlet skew and high losses at the hub, and the secondary flows in the diffuser convected the high losses. The study presented in this thesis demonstrated the flow development in a vaned diffuser and its subcomponents. The performance was evaluated by calculating the static pressure rise, total pressure losses, and throat blockage. It also demonstrated current CFD capabilities for diffusers using steady 3D flow analysis. / Master of Science

Bubble columns and airlift reactors were modeled numerically to better understand the hydrodynamics and analyze the mixing characteristics for each configuration. An Eulerian-Eulerian approach was used to model air as the dispersed phase within a continuous phase of water using the commercial software FLUENT. The Schiller-Naumann drag model was employed along with virtual mass and the standard k-e turbulence model. The equations were discretized using the QUICK scheme and solved with the SIMPLE coupling algorithm. The flow regimes of a bubble column were investigated by varying the column diameter and the inlet gas velocity using two-dimensional simulations. The typical characteristics of a homogeneous, slug, and heterogeneous flow were shown by examining gas holdup. The flow field predicted using two-dimensional simulations of the airlift reactor showed a regular oscillation of the gas flow due to recirculation from the downcomer and connectors, whereas the bubble column oscillations were random and resulted in gas flow through the center of the column. The profiles of gas holdup, gas velocity, and liquid velocity showed that the airlift reactor flow was asymmetric and the bubble column flow was symmetric about the vertical axis of the column. The average gas holdup in a 10.2 cm diameter bubble column was calculated and the results for the two-dimensional simulation of varying inlet gas velocities were similar to published experimental results. The average gas holdup in the airlift reactor for the three-dimensional simulations compared well with the experiments, and the two-dimensional simulations underpredicted the average gas holdup. / Master of Science

The transport of particulates within both a fully developed and developing pin fin arrays is explored using computational fluid dynamics (CFD) simulations. The simulations are carried out using the LES solver, GenIDLEST, for the fluid (carrier) phase and a Langragian approach for the particle (dispersed) phase. A grid independency study and validation case versus relevant experiments are given to lend confidence to the numerical simulations. Various Stokes numbers (0.78, 3.1 and 19.5) are explored as well as three nondimensional particle softening temperatures (θ_ST = 0, 0.37 and 0.67). The deposition is shown to increase with decreasing particle Stokes number and thus decreasing size from 0.005% for St_p = 19.5 to 13.4% for St_p = 0.78 and is almost completely concentrated on the channel walls (99.6% - 100%). The erosion potential is shown to increase with Stokes number and is highest on the pin faces. As is to be expected, the deposition increases with decreasing softening temperature from 13.4% at θ_ST = 0.67 to 79% for θ_ST =0. Overall, the channel walls of the array show the greatest potential for deposition. On the other hand, the pin faces show the greatest potential for erosion. Similarly, the higher Stokes number particles have more erosion potential while the lower Stokes number particles have a higher potential for erosion. / Master of Science