Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item CFD INVESTIGATION OF A PULSE JET MIXED VESSEL WITH RANS, LES, AND LBM SIMULATION MODELS(2023) Kim, Jung; Calabrese, Richard V.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pulse Jet Mixed (PJM) vessels are used to process nuclear waste due to their maintenance free operation. In this study we model the turbulent velocity field in water during normal PJM operation to gain insight into vessel operations and to evolve a modeling strategy for process design and operator training. Three transient simulation models, developed using Large Eddy Simulation (LES), unsteady Reynolds-Averaged Navier-Stokes (RANS), and Lattice Boltzmann Method (LBM) techniques, are compared to velocity measurements acquired for 3 test scenarios at 3 locations in a pilot scale vessel at the US DOE National Energy Technology Laboratory (NETL). The LES and RANS simulations are performed in ANSYS Fluent, and the LBM simulations in M-STAR.The LES model well predicts the experimental data provided that the operational pressure profile within the individual pulse tubes is considered. While the RANS model failed to predict the data and exhibited significant differences from LES with respect to turbulence quantities, it is a useful comparison tool that can quickly predict averaged flow parameters. The LBM model’s rigid grid system is deemed unsuitable, as currently configured, for the NETL PJM vessel’s wide range of length scales and curved boundaries, resulting in the longest simulation time and least accurate velocity predictions. Predicted velocity and turbulence metrics are explored to better understand the strengths and failures of the three models. Because the LES model produced the most accurate predictions, it is exploited to generate animations and still images on various 2D planes that depict extremely complex flow patterns throughout the vessel with numerous local jets and mixing layer vortices The study concludes with recommendations for future research to improve the model development and validation strategy.Item ADAPTIVITY IN WALL-MODELED LARGE EDDY SIMULATION(2022) Kahraman, Ali Berk; Larsson, Johan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In turbulence-resolving simulations, smaller eddies account for most of the computational cost. This is especially true for a wall-bounded turbulent flow, where a wall-resolved large eddy simulation might use more than 99% of the computing power to resolve the inner 10% of the boundary layer in realistic flows.The solution is to use an approximate model in the inner 10% of the boundary layer where the turbulence is expected to exhibit universal behavior, a technique generally called wall-modeled large eddy simulation. Wall-modeled large-eddy simulation introduces a modeling interface (or exchange location) separating the wall-modeled layer from the rest of the domain. The current state-of-the-art is to rely on user expertise when choosing where to place this modeling interface, whether this choice is tied to the grid or not. This dissertation presents three post-processing algorithms that determine the exchange location systematically. Two algorithms are physics-based, derived based on known attributes of the turbulence in attached boundary layers. These algorithms are assessed on a range of flows, including flat plate boundary layers, the NASA wall-mounted hump, and different shock/boundary-layer interactions. These algorithms in general agree with what an experienced user would suggest, with thinner wall-modeled layers in nonequilibrium flow regions and thicker wall-modeled layers where the boundary layer is closer to equilibrium, but are completely ignorant to the cost of the simulation they are suggesting. The third algorithm is based on the sensitivity of the wall-model with the predicted wall shear stress and a model of the subsequent computational cost, finding the exchangelocation that minimizes a combination of the two. This algorithm is tested both a priori and a posteriori using an equilibrium wall model for the flow over a wall-mounted hump, a boundary layer in an adverse pressure gradient, and a shock/boundary-layer interaction. This third algorithm also produces exchange locations that mostly agree with what an experienced user would suggest, with thinner layers where the wall-model sensitivity is high and thicker layers where this sensitivity is low. This suggests that the algorithm should be useful in simulations of realistic and highly complex geometries.Item A LARGE EDDY SIMULATION STUDY OF THE EFFECTS OF WIND AND SLOPE ON THE STRUCTURE OF A TURBULENT LINE FIRE(2019) VERMA, SALMAN; Trouve, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to a complex coupling between a large number of physical and chemical processes that happen over a wide range of length and time scales, our current fundamental understanding of wildland fire spread is limited. Wildland fire spread is affected by many parameters, but two of the most important parameters are wind and slope both of which tilt the flame and plume and bring them closer to the unburnt fuel, which, among other things, increases the convective heat transfer and hence the spread rate. The primary objective of this work is to enhance our fundamental understanding of the effects of wind and slope on the structure of a turbulent, buoyant line fire. To meet the aforementioned objective we perform well-resolved Large Eddy Simulations (LES) of a simplified configuration corresponding to a turbulent, buoyant, methane-fueled, stationary, line fire and subjected to wind or slope. Simulations are performed with an LES solver developed by FM Global and called FireFOAM which is based on the OpenFOAM CFD library. For the cases with wind, the transition from the buoyancy-dominated (in which the flame and plume are mostly detached from the downwind surface and have a tilted vertical shape; entrainment is two-sided; downwind surfaces experience convective cooling) to wind-dominated (in which the flame and plume are attached to the downwind surface; entrainment is one-sided; downwind surfaces experience convective heating) regime happens when the Byram’s convection number Nc is ≈1. The flame and plume attachment lengths (defined as the x-wall-distance downwind of the burner within the flame and plume regions, respectively) are found to fluctuate significantly in time. For the cases with slope the transition from the detached regime (equivalent to the buoyancy-dominated regime) to the attached regime (equivalent to the wind-dominated regime) is found to happen between slopes of 16 and 32 degrees. Upslope of the flame zone, the velocity tangent to the surface is found to change from a relatively small negative value (≈ −0.3 m/s) to a relatively large positive value (≈ 2.5 m/s), when the slope is increased from 16 to 32 degrees. The flame attachment length (defined as the tangential-wall-distance upslope of the burner within the flame region) is again found to fluctuate significantly in time. An integral model, capable of describing the effects of cross-wind on the structure of a turbulent, buoyant line fire, is also developed in this work. The model, after some simplifications, suggests that the plume tilt angle is controlled by the Byram’s convection number Nc and the entrainment coefficients α and β. Detailed comparisons are made between the model and LES and show that the model performs well for the cases belonging to the buoyancy-dominated regime (Nc>1) but fails to describe the cases belonging to the wind-dominated regime (Nc<1) because of the absence of a wall attachment sub-model.Item Understanding flame structure in wildfires using Large Eddy Simulations(2014) Verma, Salman; Trouve, Arnaud; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The structure of wildfire flames in the presence of crossflow was analyzed by utilizing suitable non-reacting numerical experiments with low speed flow over a hot isothermal horizontal semi-infinite surface. FireFOAM, a Large Eddy Simulation (LES) based solver developed by FM Global for fire protection engineering applications, was employed for all the calculations. Early-time dynamics of Rayleigh-Taylor Instability (RTI) was first simulated using Direct Numerical Simulations (DNS) so that the solver could be verified against Linear Stability Theory (LST). Then attention was given to late-time dynamics in order understand the different stages (e.g., appearance of secondary instability, generation of larger scales due to interaction between structures) involved in the development of the instability. The onset of thermal vortex instability, in a configuration with low speed flow over a hot isothermal semi-infinite horizontal plate, predicted using DNS was compared with the literature. Spatial evolution of various terms in the streamwise vorticity equation was used to identify the dominant mechanisms responsible for the generation/evolution of vorticity. Streamwise evolution of the instabilities was studied and the effects of the changes in temperature and orientation of the plate on the thermal instabilities were also investigated. Finally, a configuration with low speed flow over a hot isothermal semi-infinite horizontal strip was used to understand the effects of upstream Boundary Layer (BL) height and the length of the strip on both the thin horizontal and larger structures (analogous to Flame Towers (FT) observed in real wildfires and laboratory experiments).Item On The Computation Of Buoyancy Affected Turbulent Wall Flows Using Large Eddy Simulation(2013) Ojofeitimi, Ayodeji; Trouvé, Arnaud; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A high fidelity object-oriented C++ solver was developed in OpenFOAM® for the solution of low Mach number variable density Navier Stokes equations. Employing the Large Eddy Simulation (LES) methodology to compute the turbulent flowfield, the filtered LES equations were subsequently utilized to study buoyancy affected spatially developing boundary layers in natural and mixed convection spatially developing boundary layer flows. For the subgrid scale (SGS) closure, a locally dynamic Smagorinsky SGS model was implemented into OpenFOAM® to enable the backscatter phenomenon intrinsic to transitioning boundary layers. As a precursor to simulating the intricate aero-thermal flowfield of an in-flight aircraft engine pool fire due to a fuel leak, detailed investigations of two canonical problems in the absence of flames were conducted to assess the robustness of the C++ solver and to elucidate the turbulent flow physics; these test cases consisted of a natural convection turbulent boundary layer over an isothermal vertical plate without any forced flow and the mixed convection turbulent boundary layer over an isothermal vertical plate where the effects of a gradually increasing forced flow in the direction opposite to the gravitational vector were assessed. A third canonical case, the mixed convection over an isothermal horizontal plate, was also investigated as an extension of this thesis. For the first two cases, wall-resolved LES computations were compared with experimental data for first and second order turbulent statistics, along with available experimental frequency spectra of temperature and streamwise velocity fluctuations. In an effort to reduce the computational cost, wall-layer modeled LES computations were performed by implementing new wall models into OpenFOAM®. The fidelity of the wall-resolved and wall-layer modeled LES successfully confirmed the ability of the solver in computing high Grashof number transitioning natural and mixed convection spatially developing boundary layers. As it pertains to the third case, while experimental measurements in air of mixed convection over an isothermal horizontal plate is lacking in the literature, the fundamental structure of the boundary layer was qualitatively validated by examining the near-wall vortical flow topology and employing available empirical data. The accuracy of the results acquired for this flow configuration was deemed reliable due to the excellent agreement attained with the prior two test cases. Overall, the level of fidelity illustrated in this thesis has not been previously demonstrated for spatially developing turbulent boundary layers in natural and mixed convection wall flows, especially for LES. Thus, with the establishment of the methodology employed in this work, it can be further utilized as a reliable tool in computing buoyancy affected flame spread problems aboard in-flight aircraft engine fires to shed light upon the complex flow physics inherent to such flows.Item EXPERIMENTAL AND NUMERICAL INVESTIGATION OF TANGENTIALLY-INJECTED SLOT FILM COOLING(2013) Voegele, Andrew; Trouve, Arnaud; Marshall, Andre; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Film cooling is a technique used in gas turbine engines, and blades and rocket nozzles to protect critical surfaces from the hot combustion gases. In film cooling applications, a relatively cool thin fluid is injected along surfaces and subsequently mix with the hot mainstream, thus leading to a reduction of protection at the wall. The breakdown of this film involves complex physics including intense turbulent mixing, heat transfer, conduction, radiation and variable density effects to name a few. In this dissertation, film cooling is both experimentally measured and numerically simulated. The experiments feature non-intrusive Particle Image Velocimetry to provide two-dimensional planes of mean and fluctuating velocity, which are critical in order to characterize and understand the turbulent flow phenomena involved in film cooling. Additionally, through the use of micro-thermocouples, the thermal flow fields and wall temperatures are non-intrusively measured, with very small radiative errors. The film cooling flows are experimentally varied to cover a variety of breakdown regimes for both adiabatic (or idealized walls with no heat loss) and on-adiabatic walls (or walls with a carefully controlled heat loss through them). The subsequent experimental dataset is a unique and comprehensive set of turbulent measurements characterizing and demonstrating the film breakdown and the turbulent flow physics. The experiments are then numerically simulated using an in-house variable density, Large Eddy Simulation (LES) Computational Fluid Dynamics (CFD) code developed as part of this dissertation. In addition to accurately predicting important turbulent kinematic and thermal flow phenomena, the key wall parameters were predicted to within 3% for the adiabatic cases and to within 6% for the non-adiabatic cases, with a few exceptions. Turbulent inflow techniques, crucial for the success of LES of film cooling, are examined. In addition to the turbulent flow physics, radiation and conduction physics at the wall were also simulated with good fidelity. The combined experimental and numerical approach was used to uniquely form a comprehensive study, examining many aspects of film cooling phenomena relevant for engineering applications.Item Numerical Characterization and Modeling of Adiabatic Slot Film Cooling(2011) Voegele, Andrew; Marshall, André W; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Film cooling is a technique used to protect critical surfaces in combustors, thrust chambers, turbines and nozzles from hot, chemically reacting gases. Accurately predicting the film's performance is especially challenging in the vicinity of the wall and the film injection plane due to the complex interactions of two highly turbulent, shearing, boundary layer flows. Properly characterizing the streams at the inlet of a numerical simulation and the choice of turbulence model are crucial to accurately predicting the decay of the film. To address these issues, this study employs a RANS solver that is used to model a film cooled wall. Menter's baseline model, and shear-stress transport model and the Spalart-Allmaras model are employed to determine the effect on film cooling predictions. Several methods for prescribing the inlet planes are explored. These numerical studies are compared with experimental data obtained in a UMD film cooling wind tunnel.