Study of rarefied gas flows in backward facing micro-step using Direct Simulation Monte Carlo

A backward facing micro-step is a building block for many microfluidic devices. Due to micron sized characteristic dimensions, the gas flow in such a geometry is rarefied in nature. Such rarefied gas flows are widely solved using the Direct Simulation Monte Carlo (DSMC) technique. Flow separation, circulation and re-attachment are some of the basic characteristics of step flows. The objective of this study is to analyze the effect of rarefaction on the flow properties and the separation of the flow. The range of selected Knudsen number (Kn) covers the slip and transition regime from a value of 0.0311-13.25. The pressure ratios employed are 3 and 5. It is observed that the slip velocity continuously increases while the centre-line velocity first decreases, then remains constant and finally increases with increase in Kn. At the step, separation of the flow is seen for Kn < 0.1325 while no such separation is observed in the range of Kn from 0.198 to 13.25. The corresponding Re for these ranges are 6.43 to 0.67 and 0.392 to 0.012 respectively. The re-attachment length decreases with increase in Kn whereas it increases with increase in Re. A stronger pressure force and a weaker diffusion effect leads to flow separation in the slip regime whereas stronger diffusion and weaker pressure force lead to an absence of flow separation in the transition regime. Finally, this work presents for the first time the existence of the Knudsen minimum for such a backward step geometry

Analytical solution of plane Couette flow in the transition regime and comparison with Direct Simulation Monte Carlo data

This paper provides analytical solution for the steady-state Couette flow problem in the transition flow regime, while capturing the non-linear Knudsen layer near the walls. Slope at the center obtained from Direct Simulation Monte Carlo (DSMC) data and inherent symmetry in the problem have been utilized for obtaining the solution. A detailed study of the solutions obtained from the linearized super-Burnett, augmented Burnett and R13 equations is presented. The analytical results are compared against DSMC data; good agreement between them is shown till Kn = 10. These are among the first set of analytical solutions in the transition regime. The results indicate that the solution tends to become linear as the Knudsen number increases. The results have allowed formulation of a slip relationship, which can potentially yield more accurate slip velocity than Maxwell's slip model in the transition regime. Our analysis suggests that the Knudsen number envelope over which the R13 and higher-order continuum equations can be employed is substantially extendable. (C) 2014 Elsevier Ltd. All rights reserved

Improved theory for shock waves using the OBurnett equations

The main goal of the present study is to thoroughly test the recently derived OBurnett equations for the normal shock wave flow problem for a wide range of Mach number ( $3 \leq Ma \leq 9$ ). A dilute gas system composed of hard-sphere molecules is considered and the numerical results of the OBurnett equations are validated against in-house results from the direct simulation Monte Carlo method. The primary focus is to study the orbital structures in the phase space (velocity–temperature plane) and the variation of hydrodynamic fields across the shock. From the orbital structures, we observe that the heteroclinic trajectory exists for the OBurnett equations for all the Mach numbers considered, unlike the conventional Burnett equations. The thermodynamic consistency of the equations is also established by showing positive entropy generation across the shock. Further, the equations give smooth shock structures at all Mach numbers and significantly improve upon the results of the Navier–Stokes equations. With no tweaking of the equations in any way, the present work makes two important contributions by putting forward an improved theory of shock waves and establishing the validity of the OBurnett equations for solving complex flow problems.</jats:p

Numerical simulation of pitching and plunging motion of flat plate using overset mesh

A numerical simulation of two and three-dimensional pitching and plunging flat plate at Reynolds number of O(104) is presented. This study uses STAR-CCM+ to investigate the physics of flapping wings. The focus of the study is to probe into the effects of kinematics, Reynolds number and three dimensionality with resulting aerodynamic forces and flow structures of the flat plate. A shallow stall and a deep stall motion of a nominally two dimensional flat plate with higher effective angles of attack is considered. Also, in order to examine the three dimensional effects on force coefficients, an aspect ratio 2 flat plate is studied and is compared to its two dimensional counterpart. The results obtained are then validated against the experimental study available in literature. It is observed that due to more aggressive effective angle of attack time history in case of deep stall motion, a stronger LEV and higher lift is achieved as compared to that of shallow stall motion. Also, Reynolds number is seen to have a negligible effect on the aerodynamic structures and forces in the range 10,000 to 60,000. In the investigation of three-dimensionality effects, it is observed that presence of Tip Vortex mitigates the lift produced on 3D flat plate as compared to 2D flat plate. The numerical simulations performed in STAR CCM+ agree well with the experimental results obtained from Particle Image Velocimetry (PIV).by Ritu Gavasane, Preetham Pai and Vijay Kuma