Two-dimensional and three-dimensional unstructured simulations and coupling techniques for micro-geometries and rarefied gas flow

The direct simulation of Monte Carlo methods for 2D and 3D using triangular and tetrahedral meshes are developed in this research work for microflow and rarefied gas flow. A set of optimized algorithms are considered and applied to improve the computational time and accuracy when unstructured meshes are used. A couple of the validations for the internal flow and external flow are also accomplished in the 2D and 3D unstructured domains, including Couette flow, pressure-driven pipe flow, and external sphere case. A series of applications with complex geometry are also demonstrated by utilizing different boundary conditions such as riblets with moving wall, pressure-driven channel flow with curvatures (rough channel) and 3D conduit flow (sudden expansion flow). The research work on the spinning rotor gauge (SRG) project is also presented. This project covers the entire Kn range including the continuum and rarefied gas flow simulated under a complicated geometry environment such as a sphere in a pipe. In SRG project, we demonstrate a drag computation and consider the effect of Kn, Re, and Ma. The blockage effect is also studied in the SRG project. With the combination of the developed unstructured DSMC and the spectral element algorithm used in the SRG project, one is capable of handling the gas flow within any Kn range in an arbitrary geometry. The fourth piece in this research work includes the development of various coupling techniques between the continuum NS solver and DSMC. The coupling techniques can be used in the continuum break-down which occurs inside of a domain, or during the phase change, etc. A successful coupling case handled by the spectral element method with slip model and unstructured DSMC is demonstrated. Their coupling and iteration process are also studied in this research work. Specifically, the coupling techniques developed in this work include the flux properties control, the particle capture algorithm, and the artificial moving wall algorithm.