Development and Verification of a Parallel Coupled DSMC-NS Scheme Using A Three-dimensional Unstructured Grid

• Main Authors: Yu-Yung Lian, 連又永
• Other Authors: Jong-Shinn Wu
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/66406964275264217666

博士 === 國立交通大學 === 機械工程系所 === 94 === In this thesis, an efficient and accurate parallel coupled DSMC-NS method using three-dimensional unstructured grid topology is proposed and verified for the simulation of high-speed gas flows involving continuum and rarefied regimes. In addition, breakdown parameters near the solid wall are reinvestigated by a detailed kinetic study and a new criterion of breakdown parameter is proposed. Research in this thesis is divided into two phases, which are described in the following in turn. In the first phase, a parallel coupled DSMC-NS method using three-dimensional unstructured grid topology is proposed and verified. A domain overlapping strategy, taking advantage of unstructured data format, with Dirichlet-Dirichlet type boundary conditions based on two breakdown parameters is used iteratively to determine the choice of solvers in the spatial domain. The selected breakdown parameters for this study include: 1) a local maximum Knudsen number defined as the ratio of the local mean free path and local characteristic length based on property gradient and 2) a thermal non-equilibrium indicator defined as the ratio of the difference between translational and rotational temperatures to the translational temperature. A supersonic flow (M�V=4) over a quasi-2-D 25�a wedge is employed as the first step in verifying the present coupled method. The results of simulation using the coupled method are in excellent agreement with those of the pure DSMC method, which is taken as the benchmark solution. Effects of the size of overlapping regions and the choice of breakdown parameters on the convergence history are discussed. Results show that the proposed iteratively coupled method predicts the results more accurately as compared to the “one-shot” coupled method, which has been often used in practice. Further, two realistic 3-D nitrogen flows are simulated using the developed coupled DSMC-NS scheme. The first one is a flow, which consists of two near-continuum parallel orifice jets underexpanding into a near-vacuum environment. The second one is a flow issuing from a typical RCS (Reaction Control System) thruster. Results are compared with experimental data wherever is available. In the second phase, a detailed kinetic investigation using the DSMC method is used to re-examine previous proposed criteria of continuum breakdown parameters by studying the supersonic flow past a finite-size wedge flow. Choice of this particular flow for kinetic study lies in the fact that it includes a leading edge, a boundary layer, an oblique shock and an expanding fan, which covers most critical flow phenomena for the present hybrid DSMC0-NS method. Velocities of three Cartesian directions at various critical locations in the flow field are sampled and compared with the corresponding local Maxwell-Boltzmann distribution. In addition, degree of thermal non-equilibrium among various degrees of freedoms is computed at these locations. To efficiently indicate the degree of thermal non-equilibrium among various degrees of freedoms, a general indicator of thermal non-equilibrium, with its threshold value 0.03, is used with the consideration of temperature deviations among different temperature modes, including translational temperatures in the three Cartesian directions, rotational temperature and vibrational temperature. From the results, it is found that the degree of the continuum breakdown in the boundary-lay region is overestimated with the previously recommended threshold value of 0.05. Revised criterion near the isothermal solid wall is proposed as 0.8 in the present study.