Discrete porosity method for roughness modelling in the framework of RANS

Abstract

For various turbomachines, significant difference in calculated efficiency between CFD simulations and experiments is observed due to inappropriate wall-treatment methods for rough surfaces. A discrete porosity method is presented in this paper for roughness modelling in comparison to body-conforming mesh and equivalent sand-grain (ESG) roughness height. A physical dimension of each roughness elements are considered for simulation without creating conformal mesh for the rough surfaces. The roughness elements are defined as porous media with defined porosity factors according to Darcy-Forchheimer equations in OpenFOAM solver. This methodology is compared and validated to body-conforming RANS simulation results, where the roughness elements are designed as spanwise bars. The discrete porosity approach needs much lesser pre-processing effort than body-conforming mesh method, but the flow fields looks similar for both approaches. Moreover, the developed porosity method is applied for a realistic rough surface of the cast iron part which is obtained by confocal microscopy scanning method. The shift in maximum velocity because of one sided rough wall, and the difference in smooth and rough side turbulent Reynolds numbers are thoroughly presented. Additionally, the ESG height Ks+ is calculated, to depict the influence of the roughness on the logarithmic law.