Browsing by Subject "computational fluid dynamics"

Now showing 1 - 5 of 5
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    A Comparison of Two Low-Dimensional Manifold Combustion Models for Nonpremixed Supersonic Combustion
    (2021-03) Mrema, Honest
    High fidelity methods to simulate high speed turbulent combustion are of interest to the advancement of hypersonic air-breathing propulsion systems.These methods need to be cost-effective and efficient in order to be of practical use. To this end, this work seeks to compare and contrast the performance of two combustion modeling approaches that may meet these criteria. The two turbulent combustion modeling approaches are the flamelet progress/variable (FPV) and the evolution-variable manifold (EVM). Both models use tracking variables to reduce the dimensionality of the species transport equations. The main tracking variables being mixture fraction, which tracks the mixing of the fuel and oxidizer, and progress variable, which tracks how far the combustion process has advanced. For high performance at affordable computational cost, the investigated models are utilized with a hybrid large eddy simulation/Reynolds-averaged Navier-Stokes approach along with low dissipation numerical schemes. As a test bed, both approaches are applied to simulate a reacting transverse jet in a supersonic crossflow experiment. we compare OH-PLIF signals obtained from the experiment and simulations. The comparative study revealed that both models exhibit similar burning regions. The flame structures from the EVM simulations resembled the experimental results more compared to the FPV model. For this version of the FPV model, a more sophisticated compressibility correction is needed before a comprehensive comparison can be accomplished. Significant effort was put into improving the compressibility correction for the progress variable source term. This effort yielded a novel scaling approach for the progress variable production rate. This new approach focuses on scaling the reaction progress rates to account for compressibility. Although this type of correction has several benefits, the method struggles with numerical stability. Other efforts to enhance the FPV model have shown that this particular model is sensitive to flamelet manifold boundary conditions and user inputs. This work has revealed that the FPV combustion model, for the application of high speed turbulent combustion, requires further study.
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    Hypersonic Simulations and Analysis of Transition to Turbulence on BoLT-2
    (2022-11) Johnston, Zachary
    The study of laminar to turbulent boundary layer transition has been a flow phenomenonof research for many decades. Recently, there has been interest in understanding how transition occurs for hypersonic boundary layers of increasingly complex geometries. Therefore, the Air Force Research Laboratory/Air Force Office of Scientific Research (AFRL/AFOSR) introduced the Boundary Layer Turbulence (BoLT-2) flight experiment to help in the understanding and prediction of boundary layer transition to turbulence at high-speeds by collecting data in flight. The BoLT-2 research geometry allows for the existence of multiple instabilities to coexist and potentially interact thus leading to transition. This allows for the opportunity to assess current stability analysis tools and numerical methods to help improve prediction of thermal loading under flight conditions. In support of this task, the objective of this dissertation is to quantify transition mechanisms contributing to nonlinear breakdown using a forced DNS approach. Modal analysis techniques are applied to simulation datasets to extract pertinent information associated with dominant instabilities contributing to breakdown. This is meant to help in the understanding of the underlying flow physics contributing to breakdown on BoLT-2. Comparisons are made with experiments conducted in the Mach 6 Quiet Tunnel (M6QT) at Texas A & M University and show excellent agreement. Furthermore, flight conditions are investigated to identify instabilities that are potentially present at flight conditions. This is meant to help with the interpretation of flight data once it becomes available to the research community. The numerical methodology of the DNS approach presented in this dissertation is one that can be used to predict transition and help towards the development of multi-dimensional stability analysis methods for transition prediction.
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    In situ adaptive tabulation of vapor-liquid equilibrium phase change models for the numerical simulation of high-pressure transcritical flows with multiple components
    (2023-11) Zhang, Hongyuan
    Vapor-liquid equilibrium (VLE) models constitute a set of first-principles thermodynamic frameworks tailored to address the complexities of transcritical multiphase flows. They excel at accurately capturing phase transitions occurring under high-pressure conditions, a challenge that remains elusive for alternative modeling approaches. However, VLE-based computational fluid dynamics (CFD) simulation is computationally very expensive for multi-component systems, which severely limits its applications to real-world systems. This limitation severely constrains the practical applicability of VLE in real-world scenarios. In response, this thesis presents the development of a novel ISAT-VLE method based on in situ adaptive tabulation (ISAT). Our primary objective is to significantly enhance the computational efficiency of VLE-based CFD simulations while concurrently reducing memory consumption. To achieve this, we introduce ISAT-VLE solvers, tailored to accommodate both fully conservative (FC) and double flux (DF) schemes. Innovative techniques are proposed to eliminate redundant records within the ISAT-VLE table, further optimizing the method's performance. This thesis encompasses a series of simulations, including high-pressure transcritical temporal mixing layers and shock-droplet interactions, utilizing the ISAT-VLE CFD solvers. Our results demonstrate that the new method achieves a remarkable speed-up factor, ranging from approximately 10 to 60, while ensuring that ISAT errors remain well-controlled within a tight margin of 1%. In our exploration of ISAT-VLE in parallel computing, we encountered a notable performance degradation. To address this challenge, we have developed an innovative ISAT method designed explicitly for parallel computing environments. This novel ISAT approach adopts a hybrid MPI-MPI model to enable shared memory support and employs it to construct a concurrent binary tree, facilitating efficient concurrent read and write operations. This innovation allows us to establish a shared ISAT table within each computing node. Moreover, we have introduced a load balancing algorithm based on shared memory, enabling dynamic workload allocation to minimize imbalance across processors. Additionally, we have implemented a merged shared and local table ISAT strategy to further enhance overall performance. The outcome of these enhancements has been a remarkable transformation in both computational performance and scalability. Specifically, when simulating 3D shock-droplet interactions with 128 processors, our approach has achieved an impressive nearly sixfold increase in performance compared to the original ISAT methodology. Furthermore, the utilization of shared ISAT tables optimizes the usage of table records while concurrently reducing memory consumption. These advancements collectively signify a significant stride forward in the efficiency of parallel computing using ISAT methodologies. Then we investigate supercritical CO2 systems which play a pivotal role in semi-closed sCO2 cycles, holding great promise as the next-generation power cycle. We use the VLE model to investigate the multicomponent effects on the sCO2 systems. VLE-based thermodynamic analyses show that a small amount of combustion-relevant impurities (e.g., H2O, CH4, and O2) can significantly elevate the mixture critical point of the sCO2 systems. As a result, the so-called “supercritical” CO2 systems might be in the subcritical two-phase zone where phase separation occurs. At the relevant conditions in this study (100-300 bar), phase separation only has a small influence on the CO2/H2O/CH4/O2 mixture density, but has a considerable influence on the heat capacity of the mixture. VLE-based CFD simulation of a laminar premixed sCO2 shock tube shows that expansion waves can trigger significant condensation in the systems and the latent heat of the condensation can change the temperature and density fields in the systems. To understand the phase separation during mixing, VLE-based large-eddy simulations (LES) of turbulent jet-in-crossflows in the sCO2 systems are conducted, and the results show that when two subcritical gas or supercritical gas-like streams mix, the mixture can partially condense to subcritical liquid phase. Higher pressure, lower temperature, and higher H2O concentration can enhance the phase separation phenomenon in the systems.
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    Multi-resolution Modeling and Simulation of Marine Hydrokinetic Turbine Arrays at Site Scale
    (2017-04) Chawdhary, Saurabh
    Marine and hydro-kinetic (MHK) energy hold promise to become significant contributor towards sustainable energy generation. Despite the promise, commercialization of MHK energy technologies is still in the development stage. While many simplified models for MHK site resource-assessment exist, more research is needed to enable efficient energy extraction from identified MHK sites. A marine energy company named Verdant Power Inc. was granted first federal license to install up to 30 axial hydrokinetic turbines in the East River in New York City under what came to be known as Roosevelt Island Tidal Energy (RITE) project. Therefore, in this study we investigate issues of relevance to post-site-identification stage for a real-life tidal energy project, the RITE project, using high-fidelity numerical simulations. An effective way to develop arrays of hydrokinetic turbines in river and tidal channels is to arrange them in TriFrame configurations where three turbines are mounted together at the apexes of a triangular frame. The TriFrames serve as the building block for rapidly deploying multi-turbine arrays. The wake structure of a TriFrame of three model turbines is investigated. We employ large-eddy simulation (LES) with the curvilinear immersed boundary method (CURVIB) for fully resolving the turbine geometry details to simulate turbine-turbine wake interactions in the TriFrame configuration. First, the computed results are compared with experiments in terms of mean flow and turbulence characteristics with overall good agreement with bed-flume experiments. The flow-fields are then analyzed to elucidate the mechanisms of turbine interactions and wake evolution in the TriFrame configuration. We found that the wake of the upstream TriFrame turbine exhibits unique characteristics indicating presence of the Venturi effect as the wake encounters the two downstream turbines. We finally compare the wakes of the TriFrame turbines with that of an isolated single turbine wake to further illustrate how the TriFrame configuration affects the wake characteristics and power production in an array of TriFrames. Lastly, we propose a large eddy simulation (LES)-based framework to investigate the site-specific flow dynamics past MHK arrays in a real-life marine environment. To this end, the new generation unstructured Cartesian flow solver, coupled with a sharp interface immersed boundary method for 3D incompressible flows, is used. Optimized data-structures and efficient algorithms were developed to enable faster simulation on high-resolution grids. Multi-resolution simulations on locally refined grids are then employed to model the flow in a section of the East River with detailed river bathymetry and inset turbines at field scale. The results are analyzed in terms of the wake recovery and overall wake dynamics in the array. Comparison with the baseline flow in the East River reveal the effects of tidal array installation.
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    Numerical Simulation Of Instabilities In Three-Dimensional Hypervelocity Boundary Layers
    (2020-03) Knutson, Anthony
    Direct numerical simulation has been used for decades to study the boundary layer transition process. The primary contributions of this dissertation are twofold. First, we identify barriers to performing accurate numerical simulation of instabilities using an existing finite-volume flow solver (US3D) and overcome these barriers by implementing improved numerical methods. In particular, we develop a new type of shock sensor that significantly reduces numerical noise and implement a time-accurate implicit method that significantly reduces numerical dissipation. Second, we perform numerical simulations of two different geometries - the boundary layer transition (BoLT) flight experiment geometry and a cone with a swept fin - to improve our understanding of instabilities in three-dimensional, high-speed boundary layers. We find a vortical mode and traveling crossflow are the dominant instabilities in the BoLT flowfield while a multi-modal instability in the horseshoe vortex leads to transition on the fin-cone geometry.