Study of subsonic–supersonic gas flow through micro/nanoscale nozzles using unstructured DSMC solver Authors Masoud Darbandi Center of Excellence in Aerospace Systems, Department of Aerospace Engineering Sharif University of Technology Ehsan Roohi Center of Excellence in Aerospace Systems, Department of Aerospace Engineering Sharif University of Technology Research Paper

First Online: 28 July 2010 Received: 10 March 2010 Accepted: 28 June 2010 DOI :
10.1007/s10404-010-0671-7

Cite this article as: Darbandi, M. & Roohi, E. Microfluid Nanofluid (2011) 10: 321. doi:10.1007/s10404-010-0671-7
Abstract
We use an extended direct simulation Monte Carlo (DSMC) method, applicable to unstructured meshes, to numerically simulate a wide range of rarefaction regimes from subsonic to supersonic flows through micro/nanoscale converging–diverging nozzles. Our unstructured DSMC method considers a uniform distribution of particles, employs proper subcell geometry, and follows an appropriate particle tracking algorithm. Using the unstructured DSMC, we study the effects of back pressure, gas/surface interactions (diffuse/specular reflections), and Knudsen number on the flow field in micro/nanoscale nozzles. If we apply the back pressure at the nozzle outlet, a boundary layer separation occurs before the outlet and a region with reverse flow appears inside the boundary layer. Meanwhile, the core region of inviscid flow experiences multiple shock-expansion waves. In order to accurately simulate the outflow, we extend a buffer zone at the nozzle outlet. We show that a high viscous force creation in the wall boundary layer prevents any supersonic flow formation in the divergent part of the nozzle if the Knudsen number exceeds a moderate magnitude. We also show that the wall boundary layer prevents forming any normal shock in the divergent part. In reality, Mach cores would appear at the nozzle center followed by bow shocks and expansion region. We compare the current DSMC results with the solution of the Navier–Stokes equations subject to the velocity slip and temperature jump boundary conditions. We use OpenFOAM as a compressible flow solver to treat the Navier–Stokes equations.

Keywords
Micro/nanoscale nozzles
Rarefied flow
Subsonic regime
Supersonic regime
DSMC
Unstructured mesh
Navier–Stokes
Slip boundary condition
OpenFOAM

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