Effects of Knudsen number, lid velocity and velocity ratio are investigated on the flow features of single lid driven cavity with an aspect ratio of one and double lid driven cavity of aspect ratio two. Knudsen numbers studied are 0. 01(early slip regime), 0. 1 (slip regime) and 1 (transitional regime). Lid velocities investigated are 100 m/s, 200 m/s and 500 m/s. The velocity ratios explored are 1 and-1. Knudsen number was found to have a huge impact on the flow rigidity. Lid velocity tends to shift the central vortex to the top left of the cavity for a cavity with aspect ratio of one and shifts the upper vortex to the top left of the cavity for a cavity with aspect ratio of two. Lid velocity does not affect the slip to a great extent on the lid. Changing the velocity ratio from 1 to-1 leads to the reversal of the relative vorticity in the top and bottom half of the cavity.

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In this study, the flow field of impinging jets in rarefied condition and high pressure ratios was investigated. Direct Simulation of Monte Carlo (DSMC) method was employed to find the flow field at two different Knudsen numbers (Kn) of 0.1 and 0.01. For each studied Knudsen number the effects of different nozzle-to-plate distances (L) are studied. For all cases a supersonic jet forms downstream of the nozzle. The results show that no shock occurs in the supersonic jet impinging on the plate in contrary to those in the continuum regime. The variations of molecular number density, axial velocity and Mach number on the center line were computed and discussed for two different Knudsen numbers. In general, in these conditions the gas expands from specified upstream condition to a lower background monotonically.

Due to the importance of the slip effect on modeling of microchannel and microcavity, numerical investigations have been introduced in this work for studying the Mixed convection of Al2O3-water nanofluid in a square microcavity. Governing equations are discretized and solved using the Finite Volume Method and SIMPLER algorithm. The Knudsen number is selected between 0.001 and 0.1 to consider slip velocity and the temperature jump boundary conditions in slip flow regime. In this study we investigate the influence of the Knudsen number on the average Nusselt number and heat transfer rate of Al2O3-water nanofluid. Results shows that the average Nusselt number is the function of Richardson number, Knudsen number and volume fraction of nanoparticles. Increasing the Richardson number, makes the forced convection less effective and leads in reduction of the Nusselt number. Hence, increasing the Knudsen number, leads to the temperature gradient reduction and reducing the average Nusselt number. As a result, the average Nusselt number could be enhanced up to 10.93% by using nanoparticles in the base fluid.

The numerical simulation of different types of nose cone shapes involving conical, Bi-conic, parabolic, spherical Blunt cone and tangent ogive in high Knudsen numbers flow and for three different mixtures O2-N2, He-Ar and He-Xe are investigated in this article. A computer code based on the method of Direct Simulation Monte Carlo (DSMC) has been developed to study numerically the supersonic flow around these cone shapes in rarefaction condition. In order to verify the computer code, the flow field in the slip regime is solved and compared with the results of Navier Stokes equation with the slip boundary condition. The results obtained in the supersonic flows regime show that the parabolic and tangent ogive nose shapes have the least mean shear stress distribution and lowest tip temperature.

Thermal creep Knudsen pumps are made up of micro/nanochannels subjected to a temperature gradient. In such a combination, the fluid flows from the cold side to the hot side based on the thermal creep phenomenon. Due to the widespread use of Knudsen pumps in various applications, much research has been done to improve their technology. In this work, the effects of wall surface roughness on the flow field characteristics are investigated using the molecular-based direct simulation Monte Carlo method. The surface roughness is modeled by square bumps on the wall. The roughness parameters, including relative roughness, roughness aspect ratio, and roughness distance, are studied in a wide range, that is, 0 ≤ ε ≤ 10, 0.5 ≤ ξ ≤ 2, and 3 ≤ χ ≤ 10, respectively. It is concluded that the existence of roughness, no matter how small, has a significant effect on the flow conditions; so that the subminiature roughness of ε = 0.5% results in a 24% decrease in the mass flow rate. This emphasizes the importance of considering the role of surface roughness in the calculation of the performance of Knudsen pumps. Furthermore, the results indicate that the mass flow rate is independent of the relative roughness and the roughness distance for ε ≤ 1.25% and χ ≤ 5, respectively.