Browsing by Subject "Computational fluid dynamics"
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Item Adaptive and convergent methods for large eddy simulation of turbulent combustion(2014-08) Heye, Colin Russell; Raman, VenkatIn the recent past, LES methodology has emerged as a viable tool for modeling turbulent combustion. LES computes the large scale mixing process accurately, thereby providing a better starting point for small-scale models that describe the combustion process. Significant effort has been made over past decades to improve accuracy and applicability of the LES approach to a wide range of flows, though the current conventions often lack consistency to the problems at hand. To this end, the two main objectives of this dissertation are to develop a dynamic transport equation-based combustion model for large- eddy simulation (LES) of turbulent spray combustion and to investigate grid- independent LES modeling for scalar mixing. Long-standing combustion modeling approaches have shown to be suc- cessful for a wide range of gas-phase flames, however, the assumptions required to derive these formulations are invalidated in the presence of liquid fuels and non-negligible evaporation rates. In the first part of this work, a novel ap- proach is developed to account for these evaporation effects and the resulting multi-regime combustion process. First, the mathematical formulation is de- rived and the numerical implementation in a low-Mach number computational solver is verified against one-dimensional and lab scale, both non-reacting and reacting spray-laden flows. In order to clarify the modeling requirements in LES for spray combustion applications, results from a suite of fully-resolved direct numerical simulations (DNS) of a spray laden planar jet flame are fil- tered at a range of length scales. LES results are then validated against two sets of experimental jet flames, one having a pilot and allowing for reduced chemistry modeling and the second requiring the use of detail chemistry with in situ tabulation to reduce the computational cost of the direct integration of a chemical mechanism. The conventional LES governing equations are derived from a low-pass filtering of the Navier-Stokes equations. In practice, the filter used to derive the LES governing equations is not formally defined and instead, it is assumed that the discretization of LES equations will implicitly act as a low-pass filter. The second part of this study investigates an alternative derivation of the LES governing equations that requires the formal definition of the filtering operator, known as explicitly filtered LES. It has been shown that decoupling the filter- ing operation from the underlying grid allows for the isolation of subfilter-scale modeling errors from numerical discretization errors. Specific to combustion modeling are the aggregate errors associated with modeling sub-filter distribu- tions of scalars that are transported by numerical impacted turbulent fields. Quantities of interest to commonly-used combustion models, including sub- filter scalar variance and filtered scalar dissipation rate, are investigated for both homogeneous and shear-driven turbulent mixing.Item Advanced analysis of structured packing via computational fluid dynamics simulation(2010-12) Owens, Scott Allen, 1982-; Eldridge, R. Bruce; Bonnecaze, R. T. (Roger T.); Rochelle, Gary; Ganesan, Venkat; Seibert, Frank; Loescher, MitchThis research explored the use of CFD simulations to study single phase flows through structured packing. Flow rates were chosen to approximate those used in the vapor phase of industrial distillation columns. The results were evaluated against experimental results obtained with the same packing model and packed height. Several novel methods were employed to quickly obtain high validity results. A high-fidelity, digital copy of an actual packing element was created in seven hours through CT scanning. The meshing strategy employed adaptive, polyhedral meshing algorithms which resulted in high quality volume meshes with 80 percent less mesh elements than would be required with traditional tetrahedral meshing. Meshing and computation were performed on the TACC clusters. The permitted meshing with up to 57 million volume cells in less than 30 hours while simulations employing a realizable k-[epsilon] model converged in approximately two days using up to 544 processors. Nitrogen simulation predictions were found to be, on average, 7 percent below experimental measurements with water simulations showing considerably more error (~40%). The error is likely attributable a discrepancy between the simulation and experimental geometries. This discrepancy is due to an oversight in sample preparation and not a flaw in the CT scanning process of geometry creation. The volume of data generated in CFD simulation was found to be very valuable for understanding and benchmarking packing performance. Streamlines and contour plots were used to analyze the variation in performance both locally and throughout the packing stack. Significant variation was observed in flow pattern, velocity distribution, and pressure profiles throughout the column. However, the joint regions were found to be most adverse to column energy efficiency.Item Characterization of structured packing via computational fluid dynamics(2014-12) Basden, Michael Allen; Bonnecaze, R. T. (Roger T.); Eldridge, R. BruceCFD simulations were used to study single phase and multiphase flows through structured packing. Simulations utilizing a high fidelity, digital copy of a packing element were validated against experimental results for both single phase and multiphase flows. Single phase simulations were carried out on a variety of periodic packing elements to examine the impact of packing channel geometry on pressure drop. Multiphase simulations on periodic elements were used to examine the effect of hydrodynamic properties and boundary conditions. Single-phase simulations of nitrogen flow through the high fidelity geometry produced via X-ray CT scans showed average deviations less than 15% when compared to experimental measurements. This error was reduced to 7% when a mesh utilizing prism layers to accurately resolve the boundary layer was used. With a validated model for single phase flow, the application of CFD to packing design was investigated on periodic geometries with varied packing parameters (e.g. channel corrugation angle and channel side length). It was found that current industrial packings have channel geometries maximizing pressure drop, indicating some degree of optimization around channel geometry is possible depending on separation needs. Multiphase simulations using the Volume of Fluid model examined the effects of liquid density, viscosity, surface tension, and contact angle on small-scale packing geometries. Contact angle had the most pronounced influence on predicted wetting, and simulations demonstrated that using experimentally determined static contact angles was not an appropriate choice for the simulation contact angle. The predicted influence of surface tension qualitatively matched experimental data for wetted area. Liquid viscosity and density also demonstrated qualitative agreement with semi-empirical models derived from experimental data. Experimental data collected via absorption of CO2 into 0.1 mol/L NaOH were compared to simulation predictions using a geometry generated via X-ray CT scans. Wetted area predictions matched experimental data best when a fully wetting static contact angle (0°) was used, yielding simulated results 3.4% lower than experimental data on average. Irrigated pressure drop and holdup predictions were significantly higher than experimental data.Item Computation of fluid flow with multi-grid and multi-block algorithms(Texas Tech University, 1959-08) Mendu, L.N.Computational Fluid Dynamics (CFD) has wide applications in areas such as aerospace, automobile and materials manufacturing industries. The development of CFD procedures has progressed extremely rapidly during the past two decades. However, the real world processes are usually too large and too complicated to simulate due to the computing and memory limits. The problems that are facing the computational fluid dynamicist can be briefly summarized as discretization and variable storing strategies, convergence acceleration of solution procedure, handling of complex geometries and turbulence modeling. In the present study, an effort is made to develop solution procedures to tackle the above mentioned problems. The evaluation of the pressure field has always been the difficult issue in the primitive variable approach. To eliminate a wavy pressure field, the staggered grid approach was developed by Harlow and Welch (1965), but implementation of the staggered grid for a three-dimensional, curvilinear coordinate system is complicated and tiresome. In the present study, the results and convergence histories with using a solution procedure based on non-staggered grid system are reported and compared with that of staggered grid system. After comparing the flow field and convergence histories, the present non-staggered grid formulation proved as a potential alternative to staggered grid formulation. There has never been any pressure oscillation in this practice.Item Direct numerical simulation (DNS) for incompressible turbulent channel flow at Reτ = 5200(2015-12) Lee, Myoungkyu; Moser, Robert deLancey; Biros, George; Bogard, David G.; Murthy, Jayathi; Oliver, Todd A.Nearly all moving objects on Earth pass through fluids and many of them move at high speed. This makes high Re wall-bounded turbulent flows of great technological impor- tance. To study high Re wall-bounded turbulence, high spatial and temporal resolution is required due to the multi-scale nature of turbulence. Direct numerical simulation (DNS) is a technique for the study of turbulence in which the Navier-Stoke equations, the governing equations of fluid flow, are solved with sufficient resolution to represent all the scales of tur- bulence. Hence, DNS is very expensive and always limited by computational capability. To perform DNS on the most advanced high performance computing systems, extensive code optimization is required. A new turbulence DNS code, PoongBack, was developed for the studies reported here. It shows excellent performance and scalability (∼97%) on upto 786k cores on Mira at Argonne Leadership Computing Facility. We have performed DNS of turbulent channel flow using a Fourier-Galerkin method in the streamwise(x) and spanwise (z) directions and a B-Splines collocation method in the wall-normal (y) direction. The highest Reynolds number based on shear velocity (uτ = √(τw/ρ)), Reτ is approximately 5200. The simulation results exhibit a number of the char- acteristics of high Re wall-bounded turbulent flows. For example, a region where the mean velocity has a logarithmic variation is observed, with von Kármán constant κ = 0.384±0.004. There is also a logarithmic dependence of the variance of the spanwise velocity component, though not the streamwise component. A distinct separation of scales exists between the large outer-layer structures and small inner-layer structures. At intermediate distances from the wall, the one-dimensional spectrum of the streamwise velocity fluctuation in both the streamwise and spanwise directions exhibits 1/k dependence over a short range in wavenum- ber (k). Further, consistent with previous experimental observations, when these spectra are multiplied by k (premultiplied spectra), they have a bimodal structure with local peaks located at wavenumbers on either side of the 1/k range. To study the scale dependence of the dynamics of the Reynolds stress components, we applied a spectral analysis to the terms in the Reynolds stress transport equation (RSTE). It is shown that only the turbulent transport terms show significant Re dependencies. Further- more, the turbulent transport terms can be decomposed into two parts, one that contributes to transport in the wall-normal direction and one that is responsible for transfer between length scales. The results show that the large scale motion in the outer region has direct effects on the flow in the near-wall region through transport of turbulent kinetic energy. Also, a reverse energy cascade from intermediate scales to large scales is observed in the spanwise velocity fluctuations.Item Evaluation of human exposure to indoor airborne pollutants : transport and fate of particulate and gaseous pollutants(2009-05) Rim, Donghyun; Novoselac, AtilaBuilding environmental conditions such as ventilation and contaminant concentrations are important factors that influence occupant health and comfort. The objective of the present work is to investigate how personal exposure to gaseous and particulate pollutants depends on indoor airflow, source characteristics, and occupant activity in commercial and residential environments. The study examines airflow and pollutant transport using experimental measurements in conjunction with computational fluid dynamics (CFD). The results demonstrate that breathing has a measurable influence on the airflow in an occupant breathing zone, but it has very small impacts on the occupant thermal plume. The results also show that breathing can significantly affect inhaled particle concentrations, even though the influence varies with source position and particle size. Also, localized hand motions of a sitting manikin do not significantly disrupt the upward thermal plume. In typical US residences, forced convection driven mixing airflow or buoyancy driven stratified airflow occurs depending on the HVAC fan operation (fan on or fan off, respectively). The measured transition period between mixing flow (fan on) and stratified flow (fan off) is approximately one minute, implying that most airflow in the residence is either dominated by mixing or stratification. A high level of exposure to short-term pollutant sources, such as resuspension of particles from floor surfaces due to human activity, more likely occurs with stratified flow than with highly mixed airflow. This is due to the strong influence of the occupant thermal plume that transports the pollutants into the breathing zone. Furthermore, by transporting air containing ozone across the reactive occupant surface, the occupant thermal plume has a large effect on exposure to ozone reaction products. Due to the reaction of ozone with the skin oils and clothing surfaces, the occupant surface boundary layer becomes depleted of ozone and conversely enriched with ozone reaction products. The parameter ventilation effectiveness quantifies the effectiveness of airflow distribution and can be used for assessment of exposure to gaseous pollutants. Based on the study results, the usefulness of ventilation effectiveness as an indicator of exposure to particulate pollutants depends on the particle size. For small particles (~1 [mu]m), an increase of ventilation effectives caused a decrease in occupant exposure, while for large particles (~7 [mu]m), source location and airflow around the pollutant source are significant factors for the exposure, and the ventilation effectiveness has very little to no effect.Item Experimental comparison of advanced control strategies(Texas Tech University, 1995-08) Joshi, Ninad V.The objective of this research endeavor is to compare experimentally several advanced control strategies on a heat exchanger and fluid flow system. The experimental set-up was established a few years back and consists of a shell-and-tube heat exchanger with several control valves. This heat exchanger uses steam or hot water on its shell side to heat either cold or hot or a mixture of hot and cold water passing through its tube side to a desired temperature. The apparatus also contains many pneumatic control valves for controlling the flow rates of hot or cold water or steam. An experimental comparison of three control strategies (classical PID, internal model control [IMC], and process modelbased control [PMBC]) was done a couple of years earlier. The objective of this study, along with the previous one, was to implement some advanced control strategies, and present a broad-based overall perspective on the advantages and disadvantages of different control strategies. This study picks up where the last study left off, and implemented some more control strategies under similar experimental conditions. The different advanced control strategies ultimately to be implemented were neural network-based control (both inverse and normal model), model predictive control, a combination of model predictive control and neural network-based control, and heuristic-based fuzzy logic control. Thus, as a part of this study, eight different strategies were implemented. Studies on the fuzzy logic strategy were carried out separately by another graduate student.Item High-order accuracy and multigrid acceleration for two-dimensional flow computations(Texas Tech University, 1994-12) Kiris, Ilker MehmetComputational Fluid Dynamics (CID) has come to the point of being a design tool. Hence, as a design tool, it must give physically sound and reasonably accurate results in a reasonable time frame. It is well known that high accuracy can be achieved either by use of high order discretization or by use of finer mesh. The most commonly used convective term discretization technique, first-order upwind, creating artificial dissipation (viscosity) is also a well recognized reality. It is also clear that unlike ID and 2D cases, in 3D simulations grid independence notion does not really exist, especially for real world applications. Hence, CFD users are compelled to get predictions with ever increasing number of grid points (up to about 2 million control volumes, currently) to have better accuracy and representation of flow fields. Unfortunately, accuracy and Central Processing Unit (CPU) time are inversely related. It is also well known that the convergence rate of conventional solution techniques deteriorates very quickly with increasing mesh size. This is due to the fact that, the number of operations for a conventional Single Grid (SG) technique is proportional to the square of the number of control volumes. Hence, CPU time requirements grow non-linearly with increasing grid points for SG solvers. In order to overcome the aforementioned problems, two new approaches, the highorder discretization of convective terms and multigrid, need to be optimized and implemented. The high-order discretization of convective terms seems to promise accurate and stable converged solutions without artificial smearing. Hence, more accurate and physically more sound predictions can be obtained. Whereas, the multigrid algorithms hold the key to order of magnitude CPU time reductions to converged solutions. The quadratic upwind biased polynomial high order schemes, such as QUICK, mixed,UTOPIA, and multigrid algorithm FAS-FMG are tested and optimized via several benchmark cases. Results indicate that the promises of both high-order discretization and multigrid algorithm can be harvested for recirculating flow predictions.Item Hybrid prismatic/tetrahedral grid generation for complex 3-D geometries(1992) Ward, Steven Bryan; Not availableAn algorithm for the generation of hybrid prismatic/tetrahedral grids for complex 3-D geometries is presented. The method marches a triangulated surface grid away from the body to form a semi-unstructured prismatic grid. The outermost layer of this grid is then used in an octree refinement scheme to produce a tetrahedral grid. The tetrahedra are linked directly to the nodes of the outermost prismatic layer. The resulting hybrid grid is suitable for performing Navier-Stokes calculations in the viscous region (prismatic grid) near the body and Euler calculations in the inviscid region (tetrahedral grid) away from the body. The hybrid grid approach provides considerable flexibility in generating meshes around complex 3-D geometries. Other advantages of the developed grid generator are its speed, simplicity, direct control of grid orthogonality and spacing, as well as its generality for treatment of 3-D geometries. An F-16A aircraft was considered in the applications in order to investigate efficiency and to demonstrate robustness of the method in handling relatively complex topologies. Grid generation time for the entire aircraft domain was less than 10 minutes on a Sun workstation running at 2 mflops.Item A method for modeling under-expanded jets(2012-12) Day, Julia Katherine; Schneider, Erich A.; Howell, John RIn nuclear power plants, a pipe break in the cooling line releases a jet that damages other equipment in containment, and is known as a loss of coolant accident (LOCA). This report specifically focuses on boiling water reactor (BWR) applications as a guide for future studies with pressurized water reactors (PWRs). This report presents a methodology for characterizing the jet such that, given a set of upstream conditions, the pressure field and damage potential of the jet can be predicted by an end user with a minimum of computation. The resultant model has many advantages over previous models in that it is easily calculated with knowledge readily available to plant operators and it provides new metrics that allow for a quick and intuitive understanding of the damage potential of the jet.Item Predicting wind driven cross ventilation in buildings with small openings(2012-08) Lo, Liang Chung James; Novoselac, Atila; Siegel, Jeff; Corsi, Richard L.; Ezekoye, Ofodike A.; Banks, DavidDesigning wind driven cross ventilation for a building is challenging due to the dynamic characteristics of wind. While numerous studies have studied various aspects of cross ventilation, few have had an opportunity to examine the topic with a holistic approach utilizing multiple research techniques. Thus, this dissertation combined three different investigation methods: wind tunnel analysis, full scale experiments and computational fluid dynamics (CFD) to examine the physics of wind driven cross ventilation. Following the systematic approaches of the three methods, this study first conducted full scale measurements of wind properties, façade pressures, air flow rates through small window openings, and tracer gas concentrations in a multi-zone test house. Secondly, a scaled model of the test house was studied in a boundary layer wind tunnel (BLWT) for its façade pressures and ventilation rate under various wind incident angles. Finally, a CFD model of the test house was simulated under various constraints to determine the factors which affect indoor air distribution during wind driven cross ventilation events. The full scale experimental results showed a strong correlation between the cross ventilation rate and the wind velocity component normal to the inlet openings. This correlation suggested that the cross ventilation flow rate could be estimated from wind conditions alone. A closer examination of the wind characteristics also revealed that the cyclical pattern of changing wind direction could be impacted by obstructions which are kilometers upwind, suggesting that distant landscapes could have an impact on cross ventilation flows. The combination of CFD and full scale measurements also showed that local heat sources can generate significant buoyancy driven flow and affect indoor mixing during wind-driven cross ventilation scenarios. Experimentally validated parametric CFD analyses demonstrated the effect of interior heat loads in driving internal airflow, and suggest that a small source (35W/m2) can increase the indoor mixing from less than 1 ACH to 8 ACH between indoor spaces. Finally, the wind tunnel and CFD coupled analysis was found to predict the cross ventilation flow which was also validated against the full scaled measurements. The prediction, which may only be applicable to similar building types with small openings, showed significant agreement that such method has potential as an innovative design tool for natural ventilation in buildings.Item Reduced-order Models for Computational Aeroelasticity(2013-11-08) Freno, Brian AndrewThis dissertation presents a proper orthogonal decomposition (POD) method that uses dynamic basis functions. The dynamic functions are of a prescribed form and do not explicitly depend on time but rather on parameters associated with flow unsteadiness. This POD method has been developed for modeling nonlinear flows with deforming meshes but can also be applied to fixed meshes. The method is illustrated for subsonic and transonic flows with fixed and deforming meshes. This method properly captured flow nonlinearities and shock motion for cases in which the classical POD method failed. Additionally, this dissertation presents a novel approach for assessing the number of basis functions used in POD. POD results are compared between subsonic and transonic flows for several cases. It is demonstrated that in order to determine the number of basis functions, it is better to assess the variation of individual energy values, as opposed to the cumulative energy values. Finally, for off-reference flow conditions, interpolation is performed on a tangent space to a Grassmann manifold, and the effect of interpolation order is investigated.Item Serial and parallel dynamic adaptation of general hybrid meshes(2008-08) Kavouklis, Christos; Becker, E. B.; Kallinderis, J.Item Steady-state spherical accretion using smoothed particle hydrodynamics(2011-12) Baumann, Mark Chapple; Matzner, Richard A. (Richard Alfred), 1942-; Dicus, Duane; Klein, Josh; Kopp, Sacha; Marder, MichaelDue to its adaptable nature in a broad range of problem domains, Smoothed Particle Hydrodynamics (SPH) is a popular numerical technique for computing solutions in astrophysics. This dissertation discusses the SPH technique and assesses its capabilities for reproducing steady-state spherically-symmetric accretion flow. The accretion scenario is of great interest for its applicability in a diverse array of astrophysical phenomena and, under certain assumptions, it also provides an accepted analytical solution against which the numerical method can be validated. After deriving the necessary equations from astrophysical fluid dynamics, giving a detailed review of solving the steady-state spherical accretion problem, and developing the SPH methodology, this work suggests solutions to the issues that must be overcome in order to successfully employ the SPH methodology to reproduce steady-state spherical accretion flow. Several techniques for setting initial data are addressed, resolution requirements are illustrated, inner and outer boundary conditions are discussed, and artificial dissipation parameters and methodologies are explored.