Browsing by Subject "Direct Simulation Monte Carlo"

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    Consistent modeling of hypersonic nonequilibrium flows using direct simulation Monte Carlo
    (2013-08) Zhang, Chonglin
    Hypersonic flows involve strong thermal and chemical nonequilibrium due to steep gradients in gas properties in the shock layer, wake, and next to vehicle surfaces. Accurate simulation of hypersonic nonequilibrium flows requires consideration of the molecular nature of the gas including internal energy excitation (translational, rotational, and vibrational energy modes) as well as chemical reaction processes such as dissociation. Both continuum and particle simulation methods are available to simulate such complex flow phenomena. Specifically, the direct simulation Monte Carlo (DSMC) method is widely used to model such complex nonequilibrium phenomena within a particle-based numerical method. This thesis describes in detail how the different types of DSMC thermochemical models should be implemented in a rigorous and consistent manner. In the process, new algorithms are developed including a new framework for phenomenological models able to incorporate results from computational chemistry. Using this framework, a new DSMC model for rotational energy exchange is constructed. General algorithms are developed for the various types of methods that inherently satisfy microscopic reversibility, detailed balance, and equipartition of energy in equilibrium. Furthermore, a new framework for developing rovibrational state-to-state DSMC collision models is proposed, and a vibrational state-to-state model is developed along the course. The overall result of this thesis is a rigorous and consistent approach to bridge molecular physics and computational chemistry through stochastic molecular simulation to continuum models for gases in strong thermochemical nonequilibrium.
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    DSMC simulations of near-continuum hypersonic flows: code acceleration techniques and comparisons with state-of-the-art CFD solutions
    (2021-10) Bhide, Paritosh
    The modeling and simulation of hypersonic flow is challenging due to the fact that molecular time-scales, such as for vibrational energy relaxation and chemical reactions, become comparable to the characteristic flow time of the bulk gas moving past the vehicle. This results in gas flow under strong nonequilibrium conditions. In some cases, such as high-altitude flight, wake flow, flow over sharp leading edges, or flow regions involving sharp gradients, even the translational and rotational modes of the gas may be in nonequilibrium. Continuum Computational Fluid Dynamics (CFD) methods, solving the Navier-Stokes equations, have been extended to model chemical and vibrational nonequilibrium. However, such extensions are often not rigorous and continuum closure models are known to become inaccurate under translational and rotational nonequilibrium. Instead, the Direct Simulation Monte Carlo (DSMC) particle method can be used to simulate the Boltzmann equation, which is accurate for conditions ranging from fully continuum, to near-continuum, to free-molecular flow. The problem is that for the simulation of near-continuum flows, where only isolated regions exhibit strong nonequilibrium effects, DSMC becomes very expensive to simulate the entire flow domain. Although CFD solutions are much more efficient, they may be inaccurate. As a result, significant research has investigated the consistency between DSMC and CFD solutions and the development of hybrid CFD-DSMC methods to solve such near-continuum flows. This thesis addresses several key challenges associated with near-continuum hypersonic flow modeling, in the context of enabling accurate and efficient hybrid CFD-DSMC platforms. Specifically, this thesis investigates the phenomena of slip flow near sharp leading edges by comparing CFD solutions with and without slip-models against DSMC simulations that naturally predict velocity-slip and temperature-jump at the surface. While much CFD research and almost all previous hybrid CFD-DSMC research has employed no-slip conditions for CFD, it is concluded that the addition of slip-models leads to significantly improved consistency between CFD and DSMC, possibly enabling hybrid CFD-DSMC interfaces to extend to the wall in future hybrid simulations. This thesis also investigates the consistency of CFD and DSMC solutions for hypersonic flow involving the interaction between an attached boundary layer and a control surface, which induces a separated flow region. Although clear differences in boundary layer structure and separation size are identified (where discrepancy increases with increasing altitude), the predictions by CFD and DSMC for surface properties are remarkably similar, even up to flow conditions often considered to be rarefied. These results provide confidence that robust information transfer between CFD and DSMC regions of a hybrid solver can be obtained and provides a practical understanding of relevant flow conditions that motivate hybrid methodologies. Finally, to address the computational cost inherent in DSMC simulations of near-continuum flow regions, this thesis investigates the accuracy and efficiency of subcell acceleration techniques for the DSMC method. Techniques from existing literature are studied, implemented, and applied to near-continuum flows involving chemical reactions; challenging conditions not tested in previous research. Similar to previous research, it is determined that the level of subcell resolution dictates the accuracy of the solution. However, missing from previous research is the fact that the local number of simulated particles is equally important for solution accuracy. Based on a range of DSMC simulations, “best-practices” recommendations are made for the subcell method. The results of this thesis indicate that a 4x reduction in cell resolution (in each co-ordinate direction) and a 4x reduction in the overall number of simulated particles is able to achieve the same accuracy as baseline (fully-resolved) DSMC simulations. The contributions made by this thesis should enable the development of more efficient and robust hybrid CFD-DSMC simulation tools.