Browsing by Author "Dr. William Roberts, Committee Member"

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Assessment of Cooling Microelectronics using Piezoelectric Bimorphs
(2004-12-15) Halbur, Simon Gilbert; Dr. Paul I. Ro, Committee Chair; Dr. William Roberts, Committee Member; Dr. Stefan Seelecke, Committee Member
As the heat generation of processors for notebook computers continues to increase, so does the need for smaller, more efficient cooling systems. The cooling capabilities of the piezoelectric bimorph were assessed to determine if it could fulfill this need. This was accomplished by first analyzing the current methods used to cool notebook computers using heat transfer theory. Using this data and the known properties of the cooling system, two techniques were applied to try and predict the temperature profile of a processor. By predicting this profile it would be possible to determine the amount of convective cooling required to dissipate the heat for a specific processor. The cooling capabilities of the piezoelectric bimorph were assessed by looking at the theoretical volumetric flow rates and flow velocities it was able to produce through bulk air flow. It was found that the flow rate increases while the flow velocity decreases as the length of the bimorph increases. This was further supported by experimental testing with a heat source that could be regulated to output a specified amount of power. Testing also showed that there exists an optimal bimorph length and optimal gap between the bimorph and heat source, where maximum cooling is obtained. By comparing the cooling requirements of a processor and the cooling capabilities of a piezoelectric bimorph it was found that the bimorph's capabilities are not sufficient for current high power microelectronics. Instead the bimorph was found to be comparable to a heat sink for an older Pentium® processor, which means the bimorph could be viable for low power devices.
Computational Analysis of Circulation Control Airfoils
(2004-10-26) McGowan, Gregory Zar; Dr. Harvey Charlton, Committee Member; Dr. Ashok Gopalarathnam, Committee Chair; Dr. William Roberts, Committee Member
Current projections for future aircraft concepts call for stringent requirements on high-lift and low cruise-drag. The purpose of this study is to examine the use of circulation control, through trailing edge blowing, to meet both requirements. This study was conducted in two stages: (i) validation of computational fluid dynamic procedures on a general aviation circulation control airfoil and (ii) a study of an adaptive circulation control airfoil for controlling lift coefficients in the low-drag range. In an effort to validate computational fluid dynamics procedures for calculating flows around circulation control airfoils, the commercial flow solver FLUENT was utilized to study the flow around a general aviation circulation control airfoil. The results were compared to experimental and computational fluid dynamics results conducted at the NASA Langley Research Center. This effort was conducted in three stages: (i) a comparison of the results for free-air conditions to those from previously conducted experiments, (ii) a study of wind-tunnel wall effects, and (iii) a study of the stagnation-point behavior. In general, the trends in the results from the current work agreed well with those from experiments, some differences in magnitude were present between computations and experiments. For the cases examined, FLUENT computations showed no noticeable effect on the results due to the presence of wind-tunnel walls. The study also showed that the leading-edge stagnation point moves in a systematic manner with changes to the jet blowing coefficient and angle of attack, indicating that this location can be sensed for use in closed-loop control of such airfoil flows. The focus of the second part of the study was to examine the use of adaptive circulation control on a natural laminar flow airfoil for controlling the lift coefficient of the low-drag range. In this effort, adaptive circulation control was achieved through blowing over a small mechanical flap that can be deflected up or down. Such a blown trailing-edge flap allows for control of the jet direction to be independent of the jet momentum coefficient. This study was performed in two stages. In the first study, a two-dimensional thin-airfoil thin-jet theory and accompanying computer program was developed. With this method, changes to the airfoil ideal lift coefficient were studied for various jet blowing rates and angles showing that the ideal lift coefficient could be adjusted by varying either the blowing rate or the flap angle. In the second stage, a hybrid computational study was conducted. This hybrid method involved the use of the CFL3D Reynolds-averaged Navier-Stokes code in conjunction with an integral boundary layer method. The surface pressure distributions for the airfoil were determined using CFL3D. Using these pressure distributions, the boundary layer transition locations were calculated using the integral boundary layer method. The transition-location data was then used to determine the lift-coefficient range in which extended laminar flow could be achieved for cases with and without blowing. The results of this study confirmed that, in addition to flap angle, blowing across the trailing edge flap can be used to adjust the range of lift coefficients over which extensive laminar flow can be achieved. The blown trailing-edge flap was shown to be more effective at altering the location of the low-drag range than a cruise flap with no blowing. In addition, the blown flap eliminates separation off the flap at high flap angles.
Development of Closed Cell Metallic Foam Using Casting Techniques
(2004-11-29) O'Neill, Adrian Thomas; Dr. Afsaneh Rabiei, Committee Chair; Dr. Jeffrey Eischen, Committee Member; Dr. William Roberts, Committee Member
The research sited in this paper involves the development of a new metal foam composite material using casting techniques. This work included the design of the material and the development of a process to produce the metal foam. The materials used to produce the foam consisted of low carbon steel hollow spheres and an aluminum alloy. The foam is comprised of steel hollow spheres packed into a random dense arrangement, with the interstitial space between spheres infiltrated with a casting aluminum alloy. Using prefabricated hollow spheres assures a uniform pore size and cell wall thickness. Casting a metal into the interstitial space provides a solid media to add structural support to the foam. The goal of this research has been to develop metal foam that demonstrates improvements in product uniformity and mechanical properties over the currently available foams. To accomplish this goal, the study included the identification of the various technologies used to manufacture metal foams, the assessment of the improvements needed to augment the quality of foamed metals, and the design of a new product and processing technique that substantiates these goals. The experimental equipment was designed and procured, while the raw materials were obtained. Then the hollow sphere foam samples were successfully produced. Using these samples a series of characterization studies was done to qualify and quantify the results. These findings were then compared to presently published data to gauge the relative success of the work. The hollow sphere metal foam developed in this study displayed significant improvements in the measures of compressive strength and energy absorption capacity, all the while maintaining the characteristic properties of cellular metals. The improvements were measured against the next best existing technology. The newly developed foam averaged 67 MPa over a region of 10 – 50% strain, with densification beginning at approximately 50% strain. The value for energy absorption is 30 MJ/m3 at 50% strain. This foam also has a strength to density ratio on level with the best reported results to date. The combination of these properties gives opportunity for use in previously unidentified applications, such as an energy absorption media for buildings subject to seismic motion. This foam can also be designed in such applications as automobile crumple zones, as structural members in air and space craft, and in biomedical prosthesis. Several areas for improvement have been identified for this technology. The bonding strength between sphere and matrix needs improvement, and different material choices and processing changes have been identified in this research to achieve these improvements. The packing density of the spheres can be improved, and a new method of vibrating the sphere arrangement prior to molding may increase the packing density. The porosity of the aluminum matrix can be reduced, and the design of the casting mold and processing conditions can be modified to reduce undesirable porosity. Additional testing methods have been identified to further characterize the foam and reveal insights for further improvement. The iterative process of sampling, characterization, and analysis will continue to improve this product to satisfy the objectives of this research program.
Flow Modeling for Micro-filtration through electro-statically charged monolith filters
(2009-11-30) Lad, Ankit Raghunath; Dr. Andrey V. Kuznetsov, Committee Chair; Dr. William Roberts, Committee Member; Dr. Jack Edwards, Committee Member
LAD, ANKIT RAGHUNATH. Flow Modeling for Micro-filtration through electro-statically charged monolith filters. (Under the direction of Dr. Andrey V. Kuznetsov.) This study is a multi-physics problem which aims at modeling fluid flow through electro-statically charged monolith filters with machined micro-channels. The multi-phase fluid (air) considered has suspended micro particles which are the impurities to be filtered out. The resulting particle trajectories due to the effect of the forces exerted on the particle such as the hydrodynamic drag, the electrostatic force of attraction and repulsion and Brownian diffusion are studied. The micro-filtration process is studied under the presence of an electric field developed due to the uniform density charge distributed over the channel surface. The model is validated by comparison with the experimental result. The advantage of using repulsive electric field instead of attractive electric field for filtration is studied. The unit cell filtration system is developed for normal and cross flow and the scope for efficiency improvement is tested. The possibility of Ã¢â‚¬Ëœselective filtrationÃ¢â‚¬â„¢ is examined by using the multiple filter layer model and the role of different hole-orientation pattern is also studied. The experimental setup of the filtration system and the filter material strength for practical applications is discussed.
An Integrated Microoptical Microfluidic Device for Agglutination Detection and Blood Typing
(2007-07-31) Alexander, Stewart Parks IV; Dr. M. K. Ramasubramanian, Committee Chair; Dr. William Roberts, Committee Member; Dr. Kara Peters, Committee Member
Blood type identification is an important requirement for many medical procedures, especially blood transfusions. Currently, medical professionals have several ways for determining a person's blood type; however the potential for human error is a factor in all these ways. No automated process exists that takes this human error out of the equation without great expense. The accuracy of testing methods used on a large scale relies heavily upon the experience of the technician performing the test. Pervious work performed at NC State University in this area made use of the light-scattering properties of particles with a macoscopic sample. The device described in this paper uses a much smaller sample and overall can be miniaturized significantly. The focus is a microfluidic device that is able to detect blood type compatibility. It specifically does this by identifying agglutinated blood cells vs. free non‐agglutinated blood cells. The fluid portion of the apparatus is a polymer based two dimensional microfluidic device. It provides channels for the fluid flow but also holds and very accurately aligns two fiber optic cables that are used for agglutination detection. In short the device has a fluid channel perpendicular to two fiber optic cables. The fluid, a blood/saline mixture, flows in between the two cables. When a red blood cell passes across the beam of light some amount of the light is absorbed by the cell and some it is scattered, the rest continues on to the receiver fiber. When an agglutinated cell passes through the gap between the fibers more of the light is absorbed and scattered than as with the individual cell. This larger reduction in amplitude of light transmitted to the receiver fiber is indicative of red blood cell agglutination and is ultimately how the device determines blood type compatibility. Another way to setup the device makes use of Mie light scattering to detect agglutination. This device is solely a research piece of equipment in its current configuration but has very appealing qualities that would allow it to easily be scaled down into a microelectromechanical system (MEMS) device. From the results obtained one can clearly see that the device is able to detect an agglutinated sample vs. a non agglutinated sample.
A Miniaturized Device for Blood Typing Using a Simplified Spectrophotometric Approach
(2006-08-01) Lambert, Jeremy Brandon; Dr. Kara J. Peters, Committee Member; Dr. M. K. Ramasubramanian, Committee Chair; Dr. William Roberts, Committee Member
A new blood typing technology has been developed by Narayanan et al. using ultraviolet and visible light spectroscopy. Blood groups can be typed using changes in the UV and visible spectra between antibody treated and non-treated samples. These changes can be observed by optical density measurements in the 665 to 1000 nm region. Comparison of the slopes between the optical densities of control samples and antibody treated samples can be used to calculate an agglutination index number (A.I.) that describes whether or not the sample reacts to the antibody treatment. A simplified system using a discrete LED/photodiode system to take the place of the monochromator⁄photodiode array system in the spectrophotometer has been developed by Anthony and Ramasubramanian that has shown promising results reproducing the measurements of the spectrophotometer. This system was used as a starting point for the proposed research. The purpose of this research is to evaluate the scattering⁄absorption effects of red blood cells, designing a miniaturized system, and investigate this approach. The miniaturized system has been able to reproduce similar results to the spectrophotometer and is consistent with the simplified method of Anthony and Ramasubramanian. The miniaturized system also explores the use of fiber optics to improve repeatability of source mounting. A plano-convex lens is used to collimate the source beam incident on the detector and eliminate the need for specific placement of the sample that would be necessary for the converging/diverging beam used previously. This allows the components to be placed closer together and further miniaturize the setup. Packaging of this system into a compact device has been investigated and a device configuration is proposed. This packaged device could be modified further to include fluid handling that would yield a fully automated system. It is concluded that an automated blood typing system or a possible bedside pretransfusion safety device using the spectrophotometric approach is a possibility.
A Model-Based Closure Approach for Turbulent Combustion using the One-Dimensional Turbulence Model
(2007-03-21) Ranganath, Bhargav Bindiganavile; Dr. Tarek Echekki, Committee Chair; Dr. William Roberts, Committee Member; Dr. Kevin Lyons, Committee Member; Dr. Zhilin Li, Committee Member
A new model-based closure approach for turbulent combustion using the One-Dimensional turbulence model (ODT) is developed and validated in context to a turbulent jet diffusion flame. The interaction of turbulence and chemistry provides interesting finite rate chemistry effects including the phenomena of extinction and re-ignition. The ODT model resolves both spatially and temporally all the scales in a turbulent reaction flow problem, thus, combining the accuracy of a DNS like solver with efficiency by reduction in the number of dimensions. The closure approach is based on identifying the mechanisms responsible for the above mentioned effects and parameterizing the ODT results with a minimum set of scalars transported in the coarse grained solvers like the Reynolds-Averaged Navier-Stokes (RANS) or Large Eddy Simulation (LES). Thus, the closure from ODT is based on a "one-way" coupling between the coarse grained solvers and ODT. Two approaches for closure are developed in the present work with respect to a RANS solver; however, they can be easily extended to LES. The first approach relies on ODT to provide the history effects associated with the geometry, which represent the interactions of turbulence and chemistry, by tabulating scalar statistics (first and second moments) on two parameters measuring, the extent of mixing, the radial mean mixture fraction, and the extent of entrainment, the centerline mean mixture fraction. However, based on the above parameterization, the approach is limited to jet diffusion flame geometry. Furthermore, the closure requires a one to one correspondence between the flames simulated in the coarse grained solver and ODT. As a second approach, the results from ODT are parameterized based on general representative scalars; mixture fraction, which specifies the mixedness of the mixture and temperature, which specifies the reactedness of the mixture. The history effects associated with the flow geometry are provided by the RANS solver in the form of probability distribution functions (PDFs). Two classes of turbulent jet diffusion flames; hydrogen⁄air (Flame H3) and piloted methane/air (Sandia flames D and F), are considered for validation of the above ODT-based closure approaches. The piloted methane air flames, owing to higher turbulence, exhibit severe extinction in the near field followed by re-ignition around the flame height. Good comparisons of the conditional statistics for temperature and reactive scalars with the experiments are obtained for both the flames. Good predictions of entrainment as well as mixing for both the flames, as seen in the comparisons of Favre averaged axial and radial profiles, are obtained. Furthermore, the correct trends of extinction and re-ignition are predicted successfully for the piloted methane/air flames. Thus, the results show the capability of ODT to address the closure needs for a turbulent combustion problem both at molecular length scales (conditional profiles) and integral length scales (Favre averaged axial and radial profile) successfully. Refinements in terms correct representation of the PDFs for the second closure approach can be recognized, whereas, a robust "two-way" coupling of RANS and ODT may yield good results.
A Novel Approach for the Direct Simulation of Subgrid-Scale Physics in Fire Simulations
(2010-05-03) Balasubramanian, Sivaramakrishnan; Dr. William Roberts, Committee Member; Dr. Tiegang Fang, Committee Member; Dr. Tarek Echekki, Committee Chair
A Lagrangian framework for computing subgrid-scale combustion physics in Large Eddy Simulations (LES) of fire is formulated and validated. The framework is based on coupling LES formulation, based on the Fire Dynamic Simulator (FDS) with the One-Dimensional Turbulence (ODT) model. The ODT model involves reaction-diffusion and turbulent transport along one-dimensional domains. The one-dimensional domains are attached to the flame brush positions, computed in LES, and are allowed to propagate along its surface. The Lagrangian LES-ODT framework involves various implementations including a) momentum, energy, and species solution along one-dimensional ODT domain, b) Tracking of ODT domains through their anchor points, c) Filtering of ODT solutions on the LES grid, d) Inverse filtering (interpolation) of LES velocity fields in ODT domains, and e) The management of ODT domains at the flow inlets and as they reach the flame tip. Comparison of LES-ODT solutions with FDS solutions shows that the LES-ODT implementation reproduces reasonably well the flame topology and structure.
Numerical Investigation of the Mechanisms of Local Extinction Using Flame Kernel-Vortex interactions.
(2006-11-14) Kolera-Gokula, Hemanth; Dr. Nancy Ma, Committee Member; Dr. William Roberts, Committee Member; Dr. Tarek Echekki, Committee Chair; Dr. Kevin Lyons, Committee Member
The response of premixed flames to unsteady stretch is studied via kernel-vortex interactions. In this configuration a spark ignited kernel interacts with a vortex pair of variable strength. Both detailed and simple chemistry approaches are explored. In the detailed chemistry effort a dilute Hydrogen-air mixture is used. The vortex causes significant distortion of the kernel topography. Two distinct regimes; "Breakthrough" and "Extinction" are observed. A continuous increase in flame area and volumetric reaction rate values are observed throughout interactions in the breakthrough regime. However, corresponding consumption speed values are lower than 1-D laminar flame speed values. Detailed chemistry analysis of downstream interaction at the leading edge is carried out. During intermediate stages of the interaction, the mixture in between the interacting flames shows rich burning conditions. As the interaction proceeds the pool of products expands against the counter velocity gradient imposed by the vortex. The decrease in the temperature causes a steady decrease in the rates of reaction of the chain branching reactions causing. The behavior of various reaction layers is dictated to a large extent by their arrangement across the region of interaction. A simple two-step global reaction mechanism is formulated for lean methane combustion. These simple chemistry computations are carried out in an axis-symmetric configuration in a spherical frame of reference. Four distinct regimes of interaction: 1) the no-effect regime, 2) the wrinkling regime 3) the break-through regime, and the 4) global extinction regime are observed. Interactions in the no-effect regime show only minor deviations from unperturbed kernel values. Vortices in the wrinkling regime impose substantial stretch on the kernel causing major deviations from unperturbed kernel values. A sharp drop in the flame surface area and the integrated reaction rate is observed during breakthrough. The primary mechanism governing global extinction is downstream flame-flame interaction. A turbulent combustion diagram was derived for kernel-vortex interactions. Predominance of the breakthrough regime was observed. The turbulent combustion diagram represents an important contribution of this work.
Numerical Studies of H2 and H2/CO Autoignition in Turbulent Jets
(2010-03-03) Gupta, Kamlesh Govindram; Dr. William Roberts, Committee Member; Dr. Tiegang Fang, Committee Member; Dr. Tarek Echekki, Committee Chair
The present study is carried out in two parts. In the first part, the autoignition of hydrogen in a turbulent jet with preheated air is studied computationally using the stand-alone one-dimensional turbulence (ODT) model. The simulations are based on varying the jet Reynolds number and the mixture pressure. Also, computations are carried out for homogeneous autoignition at different mixture fractions and the same two pressure conditions considered for the jet simulations. The simulations show that autoignition is delayed in the jet configuration relative to the earliest autoignition events in homogeneous mixtures. This delay is primarily due to the presence of scalar dissipation associated with the scalar mixing layer in the jet configuration as well as with the presence of turbulent stirring. Turbulence plays additional roles in the subsequent stages of the autoignition process. Pressure effects also are present during the autoignition process and the subsequent high-temperature combustion stages. These effects may be attributed primarily to the autoignition delay time sensitivity to the mixture conditions and the role of pressure and air preheating on molecular transport properties. The overall trends are such that turbulence increases autoignition delay times and accordingly the ignition length and pressure further contributes to this delay. In the second part of this study, similar autoignition study of mixture of hydrogen and carbon monoxide is conducted. Two different mixture compositions are considered. They correspond to H2:CO:N2 ratios by volume of 15:35:50 and 20:30:50. Each composition is simulated for two oxidizer preheat temperatures and two fuel jet Reynolds numbers at atmospheric pressure. Homogeneous autoignition is carried out for same preheat mixture conditions for comparison with the turbulent jet results. The autoignition delay time recorded for jet cases is lower than the homogeneous autoignition delay time. This is attributed to the differential diffusion of hydrogen, which plays an important and enhancing role of the diffusion of hydrogen into the oxidizer.
Skin Friction versus Fire Propagation
(2006-08-13) Gibson, LaTosha; Dr. William Roberts, Committee Member; Dr. Kevin Lyons, Committee Chair; Dr. Zhilin Li, Committee Member
In examination of skin friction versus fire propagation, two methods of solution were of interest: (1) the viscous solution of the incompressible stagnation point velocity flow and (2) the Amplification Theory. For stagnation point velocity flow, the velocity is assumed to be zero at the stagnation point for the viscous solution. The Amplification Theory, however, deduces that the velocity is characterized by vortexes at the stagnation point. Therefore it was hypothesized that turbulence intensity through the Amplification Theory would render higher values for skin friction. The accounting of flame stretch was believed to have a small effect on the value of skin friction since the stretched laminar burning velocity is a product of the laminar burning velocity, and the pressure and temperature risen by a small power. Because of the direct correlation between the wall heat flux at the stagnation point and shear stress, the associated analytical heat flux equation utilizing the stagnation velocity gradient as a function of turbulent intensity was believed to be in a closer approximation to empirical values than the heat flux associated with the viscous solution for the incompressible stagnation point flow. Overall, the values from the viscous solution of the stagnation point velocity reported lower values than values of the K-epsilon solution involving premixed combustion. However, factoring stretch decreased the skin friction within the stagnation region. The empirical heat flux formula was shown to be in closer proximity to experimental values than the semi-analytical and theoretical heat flux solution.