Development and Verification of a Virtual Mesh Refinement Module in a Parallelized Direct Simulation Monte Carlo Code (PDSC)
| Autor: | Cheng-Chin Su, 蘇正勤 |
|---|---|
| Rok vydání: | 2008 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 96 The objective of this thesis is to develop and verify a virtual mesh refinement module (VMR), based on a new concept, in a parallelized direct simulation Monte Carlo code (PDSC). Cells are used for particle collisions and sampling of macroscopic properties in a DSMC simulation, in which the sizes have to be much smaller than the local mean free path. Unfortunately, it is generally impossible to know the distribution of local mean free path before the simulation. Previously, in our group we have developed several mesh refinement techniques in DSMC, which were based on the concept of h-refinement to unstructured grids. However, particle tracing on the refined unstructured mesh becomes inefficient and mesh quality is generally difficult to maintain. In this thesis, we will utilize the concept of transient adaptive sub-cells (TAS) proposed by Tseng et al. and propose a new type of mesh refinement on unstructured grids for DSMC simulation. This method is a two-level virtual mesh refinement, in which the background mesh is refined based on an initial DSMC simulation. The virtual refined cells are arranged in a way similar to the structured grid, which makes the particle tracing on them very efficient, unlike on unstructured grids. These virtual cells are used for particle collision and sampling. In addition, area of each virtual refined cell is calculated using the Monte Carlo integration method. Approximately 5,000*Nvc particles are required to reach 0.1% error for area calculations of all the virtual refined cells, which takes about 12.5 minutes of computational time for ~300,000 virtual refined cells using 12 processors. Only a virtual refined cell, which includes centroid of the background cell, we output only this data in each background cell. In this way, the original grid data structure is retained and memory cost is comparably low and using dynamic domain decomposition (DDD) to reduce computational time. Finally, two two-dimensional test cases, which are Mach-12 hypersonic flow past a block (argon gas, velocity=1413 m/s, temperature=40 K and Kn=0.05, number density=1.29E21 m-3) and Mach-10 hypersonic flow past a circular cylinder (D=0.3048 m, argon gas, velocity=2634.1 m/s, temperature=200 K and Kn=0.0091, number density=4.274E20 m-3), including quadrilateral, triangular and mixed triangular-quadrilateral mesh have demonstrated in the thesis to show the robustness of this new mesh-refining algorithm. Results of cylinder simulation show that the case using VMR not only can faithfully reproduce the benchmark case, but also can reduce computational time from 15 hours (benchmark) to 3.5 hours (quadrilateral mesh), 4.5 hours (triangular mesh) and 5 hours (mixed quadrilateral-triangular mesh). |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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