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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Subject: search.filters.subject.Large eddy simulation

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    MULTI-FIDELITY PARAMETRIC SENSITIVITY FOR LARGE EDDY SIMULATION
    (2023) Oberoi, Nikhil; Larsson, Johan Prof.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Designing engineering systems involving fluid flow under uncertainty or for optimality often requires performing many computational fluid dynamics (CFD) calculations. For low-fidelity turbulence modeling simulations such as Reynolds-averaged Navier-Stokes (RANS), such a framework has been established and is in use. However, for high-fidelity turbulence-resolving simulations such as large eddy simulations (LES), the relatively high computational cost of even a single calculation hinders the development of such a framework. The overarching goal of this work is to aid LES in becoming a usable engineering design tool. In this thesis, a computationally affordable approach to estimate parametric sensitivities of engineering relevant quantities of interest in an LES is explored. The method is based on defining a RANS problem that is constrained to reproduce the LES mean flow field. The proposed method is described and assessed for a shock/boundary layer interaction problem, where the shock angle and wall temperature are considered variable or uncertain. In the current work, a proof-of-concept of the proposed method is demonstrated. The method offers qualitative improvements to the sensitivity prediction of certain flow features as compared to standalone RANS simulations, while using a fraction of the LES cost. Different cost functions to infer auxiliary RANS variables are also examined and their influence on the sensitivity estimation is assessed. Overall, the results serve as an important proof-of-concept of the method and suggests the most promising path for future developments.
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    A MULTI-FIDELITY APPROACH TO SENSITIVITY ESTIMATION IN LARGE EDDY SIMULATIONS
    (2022) Arias Ramírez, Walter; Larsson, Johan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    An approach to compute approximate sensitivities in a large eddy simulation (LES) is proposed and assessed.The multi-fidelity sensitivity analysis (MFSA) solves a linearized mean equation, where the mean equation is based on the LES solution. This requires closure modeling which makes the computed sensitivities approximate. The closure modeling is based on inferring the eddy viscosity from the LES data and in predicting the change in turbulence (or the perturbed eddy viscosity) using a simple algebraic model. The method is assessed for the flow over a NACA0012 airfoil at a fixed angle of attack, with the Reynolds number as the varying parameter and the lift, drag, skin friction, and pressure coefficients as the quantities-of-interest. The results show the importance of accurate closure modeling, specifically that treating the eddy viscosity as "frozen" is insufficiently accurate. Also, predictions obtained using the algebraic model for closing the perturbed eddy viscosity are closer to the true sensitivity than results obtained using the fully RANS-based method which is the state-of-the-art and most common method used in industry. The proposed method aims to complement, rather than replace, the current state-of-the-art method in situations in which sensitivities with higher fidelity are required.
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    A FLAMELET APPROACH FOR LARGE EDDY SIMULATIONS OF COUPLED COMBUSTION AND RADIATION IN TURBULENT BUOYANT DIFFUSION FLAMES
    (2021) Xu, Rui; Trouvé, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In traditional computational fluid dynamics (CFD) descriptions of fires, the combustion and radiation models generally rely on a global combustion equation and the assumption of a linear relationship between radiative power and heat release rate. These models may lead to a crude treatment of important phenomena such as flame extinction, formation of soot and toxic species, and the change of radiant emissions in response to evolving fire conditions. The general objective of this Ph.D. study is to evaluate the potential of advanced combustion and radiation models for large eddy simulations (LES) of fires. A flamelet-based modeling framework is proposed that considers established or modified steady and unsteady flamelet formulations. This study is part of an international collaborative project between the University of Maryland and the University of Poitiers (France) aimed at providing a fundamental understanding of coupled combustion-radiation phenomena in fires. It consists of two parts. The objective of the first part is to bring fundamental information on the coupling between combustion and thermal radiation occurring in laminar flames. The study considers a simplified configuration corresponding to one-dimensional counterflow planar laminar diffusion flames subjected to time-evolving moderate-to-slow mixing conditions that are representative of fires. The analysis demonstrates that for conditions far from the extinction limits, the flame belongs to the semi-unsteady regime in which mixing processes occurring in the outer diffusive layers of the flame are unsteady whereas heat release processes occurring in the inner reactive layer remain quasi-steady. The objective of the second part is to develop and validate a fully coupled flow-flame-radiation fire modeling framework. A novel unsteady flamelet model is developed that includes: detailed information on combustion chemistry through a tabulated chemistry approach; a careful description of the combustion-radiation coupling; a description of subgrid-scale turbulence-radiation interactions; and a description of non-grey radiation effects through a Weighted-Sum-of-Grey-Gases (WSGG) model. This new combustion/radiation model is then incorporated into the LES solver FireFOAM (developed by FM Global) and is evaluated by comparisons with experimental data obtained in a turbulent line burner experiment previously studied at the University of Maryland. Results on the global radiant fraction (GRF) obtained in cases with nitrogen dilution suggest that provided that the WSGG radiation model is used, the new modeling framework is capable of simulating changes in the flame radiative emissions with the predicted GRF within 20% of the measured values. Comparisons between the grey and WSGG options in the flamelet model show that, with the WSGG model, the simulated flame is no longer optically thin (the ratio of global absorption divided by global emission is close to 40%). Note that while the flamelet combustion model presented in this study has provided unique insights into the micro physics of fires, it is not a modeling approach that is recommended for engineering-level simulations of fires. First, the flamelet combustion modeling approach assumes the availability of a detailed chemical kinetic mechanism to describe fuel oxidation and this type of mechanism is typically not available for practical fuels in fire problems. Second, the flamelet combustion modeling approach treats the heat release rate implicitly and numerical tests show that the implicit heat release rate is described with limited accuracy (the error in the simulated global heat release rate ranges takes values between a few percent up and 20% in the present work). This limited accuracy on the description of the fire power is viewed as a strong limitation of current tabulated chemistry approaches for engineering-level simulations of fires.
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    ERROR ESTIMATION, GRID SELECTION AND CONVERGENCE VERIFICATION IN LARGE EDDY SIMULATION
    (2019) Toosi, Siavash; Larsson, Johan; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Large eddy simulation (LES) is a modeling approach to simulation of turbulence, in which the large and energy containing eddies are directly resolved, while the smaller scales are modeled. The ``coarse-graining'' length scale (the length scale below which the turbulent eddies are modeled) is an important modeling parameter that is directly tied to the computational grid. As a result, the LES grid controls both the numerical and modeling errors and in most cases (given that the LES model is consistent) becomes the most important factor in determining the accuracy of the solution. The main goal of this dissertation is to enable a systematic approach to grid selection and convergence-verification in LES. Systematic grid selection consists of five essential ingredients: (i) an ``error-indicator'' that identifies the regions of error generation, (ii) some knowledge of the directional structure of error generation (i.e., an anisotropic measure of error generation at each location), (iii) a model that describes the connection between the error generation and the filter/grid resolution (i.e., how it changes with a change in the resolution), (iv) criteria that describe the most ``optimal'' distribution of the error-indicator in space and in direction, and (v) a robust method for convergence-verification. Items (i), (ii), (iv) and (v) are all addressed in this dissertation, while item (iii) has not been a subject of extensive research here (because of its somewhat lower importance compared to the other four). Three error-indicators are introduced that are different in terms of their underlying assumptions, complexity, potential accuracy, and extensibility to more complex flows and more sophisticated formulations of the problem of ``optimal'' grid selection. Two of these error-indicators are inherently anisotropic, while the third one is only a scalar but can be combined with either of the other two to enable anisotropic error-estimation. The ``optimal'' distributions of these error-indicators are discussed in detail, that, combined with a model to connect the error-indicator and the grid/filter resolution, describe our ``optimal'' grid selection criteria. Additionally, a more robust approach for convergence-verification in LES is proposed, and is combined with error-estimation and ``optimal'' grid selection/adaptation to form a systematic algorithm for large eddy simulation. The proposed error-estimation, grid selection, and convergence-verification methods are tested on the turbulent channel flow and the flow over a backward-facing step, with good results in all cases, and grids that are quite close to what is know as ``best practice'' for LES of these flows.