Alireza Mohammadzadeh

A Parallel DSMC Investigation of Monatomic/Diatomic Gas Flows in a Micro/Nano Cavity

In the current study, we performed a DSMC investigation to study the nonequilibrium effects on monatomic and diatomic rarefied flows in micro/nano lid-driven cavity. Our DSMC solver is parallel and benefits from a variable time step algorithm. The results of our simulation showed that growing rarefaction effects increase the maximum temperature of the cavity flow. As the inter-molecular rate of collision diminishes in the nonequilibrium regime, molecules manage to conserve their obtained energy from diffused surfaces; consequently, the flow temperature increases. We also investigate the nonequilibrium effects on the shear stress profile in the micro/nano cavity. Although increasing the Knudsen number decreases flow shear stress, the nondimensional shear stress, which shows the molecular potential for performing collision in the rarefied flow, increases.

Thermal stress vs. thermal transpiration: A competition in thermally driven cavity flows

The velocity dependent Maxwell (VDM) model for the boundary condition of a rarefied gas, recently presented by Struchtrup [“Maxwell boundary condition and velocity dependent accommodation coefficient,” Phys. Fluids 25, 112001 (2013)], provides the opportunity to control the strength of the thermal transpiration force at a wall with temperature gradient. Molecular simulations of a heated cavity with varying parameters show intricate flow patterns for weak, or inverted transpiration force. Microscopic and macroscopic transport equations for rarefied gases are solved to study the flow patterns and identify the main driving forces for the flow. It turns out that the patterns arise from a competition between thermal transpiration force at the boundary and thermal stresses in the bulk.

PREDICTING CONTINUUM BREAKDOWN OF RAREFIED MICRO/NANO FLOWS USING ENTROPY AND ENTROPY GENERATION ANALYSIS

In the current study, the DSMC method is utilized to obtain the entropy, entropy generation and the local gradient length Knudsen number in the rarefied flows. Two particular geometries, cavity and flat plate, are considered to study the departure from equilibrium state in the presence of sudden expansion/contraction, bend in the velocity profile, boundary flow and shock waves. The entire slip regime is considered to investigate small and large nonequilibrium effects on the entropy and entropy productions. Our investigation reveals that the distribution of entropy in the rarefied flow is very similar to the temperature contour. The entropy generation distribution in the micro cavity indicates that the two top corners are the regions around which departure from equilibrium state takes place. The study of entropy generation over the flat plate reveals that the entropy production is maximized along the shock wave. Moreover, increasing the rarefaction effects thickens the nonequilibrium shock wave. We also observed that increasing the nonequilibrium effects reduces the level of entropy generation in the rarefied flow. As the flow density decreases in the nonequilibrium regime, the level of shear stress and heat flux reduces, which subsequently lower the level of entropy generation in the rarefied flows. Furthermore, it was found that although the level of entropy generation in the flow reduces as the Knudsen number increases, the boundaries of the maximum entropy production region extends under large rarefaction effects.

Detailed Investigation of Thermal and Hydrodynamic Flow Behaviour in Micro/Nano Cavity Using DSMC and NSF Equations

We utilized direct simulation Monte Carlo (DSMC) method to investigate the effectiveness of the NSF equations in the slip and transition regimes. Monatomic argon confined in a micro/nano lid-driven cavity is considered in this study. Full NSF equations accompanied by the first and second order velocity slip and temperature jump boundary conditions are used to investigate non-equilibrium phenomena. It is seen that although velocity profiles are predicted quite accurately by means of proper slip boundary conditions, the NSF equations fail to predict correct shear stress distribution and heat flux direction even in the middle slip regime. It is also seen that applying the second order velocity slip boundary condition in the transition regime reduces the accuracy of the continuum approach. Fourier law, which assumes the heat always fluxes from hotter to colder region, loses its validity in the slip regime and beyond.Copyright