Abstract
In this work, we have studied the effects of surface roughness on accommodation coefficients using an in-house, direct simulation Monte Carlo solver. The solver has been modified in such a way that enables us to calculate the accommodation coefficients for a rough surface while using as inputs the surface roughness conditions and the accommodation coefficient values of a smooth surface. Supersonic flow over a flat plate is considered to characterize and study the effects of surface roughness on accommodation coefficients. Roughness is created on the flat plate by introducing a number of protrusions (modules) of different shapes and aspect ratios. These parameters (shape, aspect ratio, and number of modules) are varied systematically to analyze the effect of each parameter. It is shown that the effect of surface roughness on accommodation coefficient is distinct and solely geometry dependent, as compared to the flow-related effects such as Knudsen number, Mach number, etc. Changes in the values of accommodation coefficients between smooth and rough surfaces are analyzed in detail, and an effective-smooth surface model is proposed in this work, which is shown to take care of surface roughness effects quite accurately, without the need to model the surface roughness explicitly. Macroscopic properties like coefficient of drag, surface heat flux, and velocity profiles are compared between the rough surface and the effective-smooth surface to show the significance of modeling of roughness in the proposed manner. Through a systematic study, a generic empirical relationship is derived between surface roughness parameters and effective accommodation coefficients, using which, modeling of the surface roughness of a typical engineering surface is addressed.
8 More- Received 25 April 2019
DOI:https://doi.org/10.1103/PhysRevFluids.4.123401
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