Browsing by Author "Zhang, Chonglin"
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Item Adaptive mesh refinement and cut-cell algorithms for DSMC simulation of hypersonic flows.(2010-05) Zhang, ChonglinAdaptive mesh refinement (AMR) and cut-cell algorithms were developed for a 3-level Cartesian mesh based Direct Simulation Monte Carlo (DSMC) implementation. The simple and efficient AMR algorithm adapts the cell size to the local mean free path of the flow field. Variable time step technique was implemented together with the AMR algorithm to set a time step consistent with the local mean collision time. The control of simulation particles through the use of variable time step was also illustrated. The cut-cell method decouples the flow field Cartesian mesh and the triangulated surface mesh representing any object inside the flow field. Two key aspects of the cut-cell method: cut-cell sorting and volume calculation were discussed in detail. The 3-level embedded Cartesian mesh combined with AMR and variable time step allows increased flexibility for precise control of local mesh size and time step, both vital for accurate and efficient DSMC simulation. Hypersonic flow simulations were conducted to highlight the performance of AMR, variable time step and cut-cell algorithms. Three dimensional simulation of Planetary probe reproduced the experimental heat flux measurement.Item Consistent modeling of hypersonic nonequilibrium flows using direct simulation Monte Carlo(2013-08) Zhang, ChonglinHypersonic 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.