R-3 Repository :: Browsing by Author "Houchens, Brent C."

Browsing by Author "Houchens, Brent C."

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A Refined Parallel Simulation of Crossflow Membrane Filtration
(2011) Boyle, Paul Martin; Houchens, Brent C.
This work builds upon the previous research carried out in the development of a simulation that incorporated a dynamically-updating velocity profile and electric interactions between particles with a Force Bias Monte Carlo method. Surface roughness of the membranes is added to this work, by fixing particles to the membrane surface. Additionally, the previous electric interactions are verified through the addition of an allrange solution to the calculation of the electrostatic double layer potential between two particles. Numerous numerical refinements are made to the simulation in order to ensure accuracy and confirm that previous results using single-precision variables are accurate when compared to double-precision work. Finally, the method by which the particles move within a Monte Carlo step was altered in order to implement a different data handling structure for the parallel environment. This new data handling structure greatly reduces the runtime while providing a more realistic movement scheme for the particles. Additionally, this data handling scheme offers the possibility of using a variety ofn-body algorithms that could, in the future, improve the speed of the simulation in cases with very high particle counts.
Coupling a dynamically updating velocity profile and electric field interactions with force bias Monte Carlo methods to simulate colloidal fouling in membrane filtration
(2009) Boyle, Paul Martin; Houchens, Brent C.
Work has been completed in the modeling of pressure-driven channel flow with particulate volume fractions ranging from one to ten percent. Transport of particles is influenced by Brownian and shear-induced diffusion, and convection due to the axial crossflow. The particles in the simulation are also subject to electrostatic double layer repulsion and van der Waals attraction both between particles and between the particles and channel surfaces. These effects are modeled using Hydrodynamic Force Bias Monte Carlo (HFBMC) simulations to predict the deposition of the particles on the channel surfaces. Hydrodynamics and the change in particle potential determine the probability that a proposed, random move of a particle will be accepted. These discrete particle effects are coupled to the continuum flow via an apparent local viscosity, yielding a dynamically updating quasi-steady-state velocity profile. Results of this study indicate particles subject to combined hydrodynamic and electric effects reach a highly stable steady-state condition when compared to systems in which particles are subject only to hydrodynamic effects.
Experimental results and three-dimensional simulations of instabilities in a rotating lid-driven cylinder
(2012) Kong, Zhao Chad; Houchens, Brent C.
An experimental setup for a rotating lid-driven cylinder problem is designed and constructed in the context of modeling bulk semiconductor crystal growth techniques. Details concerning construction of the experimental setup are included in the interest of reproducibility. Ultrasonic Doppler Velocimetry (UDV) is tested as a viable visualization technique for the lid-driven cylinder and experimental measurements of the flow field are compared to numerical simulations. The aspect ratio of the cylinder and the Reynolds number are the governing parameters for the problem. Experimental and computation results are presented for aspect ratio of 2.5 and Reynolds numbers up to 3000. Accurate UDV measurements of the steady, axisymmetric base flow are demonstrated for both water and a 20% glycerin-water mixture as the working fluid. The expected periodic, axisymmetric instability at Reynolds number of 3000 was unobserved by the UDV. However, related instabilities were observed at lower Reynolds numbers. Associated strengths and weaknesses of UDV for flow measurement are discussed.
Experiments and Modeling of Ternary Alloy Semiconductors
(2012) Tritchler, Stephanie E.; Houchens, Brent C.
Results of the growth of Ga 1-x In x Sb crystals as the pseudobinary Ga 1-x Sb--In x Sb are presented. Special focus is given to the relationship between crystal composition and the cut-off wavelength of transmission for the crystal using Fourier Transform Infrared (FTIR) and Ultra Violet-Visual Wavelength (UV-Vis) spectroscopy. This provides a fast, easy method to determine the composition. A Matlab model to determine the composition of a crystal grown using the Bridgman method is developed. This compares the ideal to the regular solution models for Ga 1-x In x Sb. These two models are used to predict the composition of the grown crystals. The regular solution yields a more accurate pseudo-binary phase diagram and thus a much closer fit to the experimental data.
High resolution numerical study of a liquid bridge Marangoni flow with applied axial magnetic field for low Prandtl number fluids
(2010) Huang, Yue; Houchens, Brent C.
The Full Zone model of the thermocapillary (Marangoni) flow in a liquid bridge with an axial magnetic field, measured by the Hartmann number Ha, is studied using a Chebyshev spectral method for low Prandtl number fluids. By introducing a 2nd order vorticity transport formulation, high resolution Gauss-Lobatto grids can be used to investigate the strong stabilization effects from intermediate magnetic fields, which were impossible with previous formulations. The instability mechanism of the axisymmetric base flow is studied up to Ha=500 for Pr=0.001 and up to Ha=300 for Pr=0.02 using linear stability analyses. Over these parameter spaces, the base flow first transitions to three-dimensional stationary disturbances with different axial symmetries. Solutions from the 2nd order vorticity transport formulation show good agreement with previous studies on weak magnetic fields. This work provides better understanding of the magnetohydrodynamic flow in intermediate field strengths, as well as guidance for optically heated float-zone crystal growth processes.
Instabilities in a Crystal Growth Melt Subjected to Alternating Magnetic Fields
(2013-09-16) Davis, Kenny; Houchens, Brent C.; Akin, John Edward.; Warburton, Tim
In confined bulk crystal growth techniques such as the traveling heater method, base materials in an ampoule are melted and resolidified as a single crystal. During this process, flow control is desired so that the resulting alloy semiconductors are uniform in composition and have minimal defects. Such control allows for tuned lattice parameters and bandgap energy, properties necessary to produce custom materials for specific electro-optical applications. For ternary alloys, bulk crystal growth methods suffer from slow diffusion rates between elements, severely limiting growth rates and reducing uniformity. Exposing the electrically conducting melt to an external alternating magnetic field can accelerate the mixing. A rotating magnetic field (RMF) can be used to stir the melt in the azimuthal direction, which reduces temperature variations and controls the shape at the solidification front. A traveling magnetic field (TMF) imposes large body forces in the radial and axial directions, which helps reduce the settling of denser components and return them to the growth front. In either case, mixing is desired, but turbulence is not. At large magnetic Taylor numbers the flow becomes unstable to first laminar and then turbulent transitions. It is imperative that crystal growers know when these transitions will occur and how the flow physics is affected. Here, the melt driven by electromagnetic forces is analyzed through the use of 3D numerical simulations of the flow field up to and beyond the point of laminar instability. The analysis aims to emulate laboratory conditions for generating electromagnetic forces for both types of alternating magnetic fields and highlights the differences between laboratory forces and the analytical approximations that are often assumed. Comparisons are made between the resulting forces, flow fields, and points of instability as the frequency of the alternating field varies. Critical Taylor numbers and the resulting unstable flow fields are compared to the results from linear stability theory.
Magnetic Control in Crystal Growth from a Melt
(2012-09-05) Huang, Yue; Houchens, Brent C.; Akin, John Edward.; Embree, Mark
Control of bulk melt crystal growth techniques is desirable for producing semiconductors with the highest purity and ternary alloys with tunable electrical properties. Because these molten materials are electrically conducting, external magnetic fields are often employed to regulate the flow in the melt. However, complicated by the coupled flow, thermal, electromagnetic and chemical physics, such magnetic control is typically empirical or even an educated guess. Two magnetic flow control mechanisms: flow damping by steady magnetic fields, and flow stirring by alternating magnetic fields, are investigated numerically. Magnetic damping during optically-heated float-zone crystal growth is modeled using a spectral collocation method. The Marangoni convection at the free melt-gas interface is suppressed by applying a steady magnetic field, measured by the Hartmann number Ha. Using normal mode linear stability analyses, suppression of detrimental flow instabilities is quantitatively determined in a range applicable to experiments (up to Ha = 300 for Pr = 0.02, and up to Ha = 500 for Pr = 0.001). The hydrodynamic flow instability for small Prandtl number P r float-zone is confirmed by energy analyses. Rotating magnetic field stirring during confined crystal growth in an ampoule is also modeled. Decoupled from the flow field at small magnetic Reynolds number, the electromagnetic field is solved in a finite element solver. At low AC frequencies, the force is only in the azimuthal direction but penetrates deep into the melt. In contrast, the magnetic shielding effect is observed at high alternating current (AC) frequencies, where the external magnetic field penetrates only by a skin depth into the electrically conducting media within the short AC cycle. As a result, the electromagnetic body force is primarily confined to the ampoule surface. At these high AC frequencies the magnetic flux lines are drastically distorted within the melt. The body force is fully three-dimensional and is much stronger than at low AC frequencies, but is confined to near the ampoule surface due to the magnetic shielding effect. These models promote fundamental understanding of flow dynamics regulated by electromagnetic body forces. They provide quantitative guidance for crystal growth to minimize trial and error experimentation that is slow and expensive.
Sequentially Optimized Meshfree Approximation as a New Computation Fluid Dynamics Method
(2012-09-05) Wilkinson, Matthew; Meade, Andrew J., Jr.; Akin, John Edward.; Embree, Mark; Houchens, Brent C.
This thesis presents the Sequentially Optimized Meshfree Approximation (SOMA) method, a new and powerful Computational Fluid Dynamics (CFD) solver. While standard computational methods can be faster and cheaper that physical experimentation, both in cost and work time, these methods do have some time and user interaction overhead which SOMA eliminates. As a meshfree method which could use adaptive domain refinement methods, SOMA avoids the need for user generated and/or analyzed grids, volumes, and meshes. Incremental building of a feed-forward artificial neural network through machine learning to solve the flow problem significantly reduces user interaction and reduces computational cost. This is done by avoiding the creation and inversion of possibly dense block diagonal matrices and by focusing computational work on regions where the flow changes and ignoring regions where no changes occur.
Three-Dimensional, Time-Dependent Spectral Element Simulations of a Thermocapillary Liquid Bridge with Magnetic Stabilization
(2011) Davis, Kenneth Edward; Houchens, Brent C.
The spectral element method is used to obtain 3D, time-dependent solutions for a thermocapillary driven liquid bridge with magnetic stabilization, which arises from the float-zone crystal growth process. The methods and implementation of the general, in-house developed fluid flow and heat transfer spectral element solver are discussed and the code is benchmarked. This work compares three-dimensional, time-dependent results to perturbations predicted by linear stability theory for the full-zone problem with Prandtl number of 0.02. Critical points, mode numbers, and azimuthal velocity perturbations are matched for the instabilities. Additionally, the simulations extend the study beyond the initial bifurcation point to find modal competition between two steady modes for the zero magnetic field case. Applying an axial magnetic field damps the perturbations and delays instabilities, providing a quiescent interior region that is conducive to growing defect free, uniform composition crystals. Weak magnetic fields are shown to remove the modal competition that leads to undesirable, time-dependent flow with mode switching.

Browsing by Author "Houchens, Brent C."

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