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MODELING OF A WALL OF RANDOM ROUGHNESS FOR CFD

Yutaka Miyake
Department of Mechanical Engineering, Graduate School of Engineering, Osaka University 2-l,Yamadaoka, Suita, Osaka, 565-0871; Department of Mechanical Engineering Fukui University of Technology 3-6-1, Gakuen, Fuki, 910-8505 Japan

Takahiro Fujii
Department of Mechanical Engineering Fukui University of Technology 3-6-1, Gakuen, Fuki, 910-8505 Japan

Qinyin Fan
Software Cradle Nishinakajima, Yodogawa-ku, Osaka, Japan

Abstract

A new idea is proposed in this work to convert an original rough surface of irregularly indented complicated geometry to a simplified model surface. The purpose is to provide a substitutive simple solid boundary surface which allows a practical computer of ordinary memory size and computational speed to conduct LES/DNS of a flow over a rough surface. As tall and comparatively large roughness peaks are expected to affect dominantly the flow over an irregularly indented surface, it is intended here to pick up those peaks adequately and substitute them by equivalent hemispheres. The first task is to coarsen the original surface to remove fast fluctuating component of surface indentation. In place of coarsening by a filtering function as is common in LES, wavelet multi-resolution analysis is introduced. Then, in order to pick up concerned peaks, we specify a threshold height on the coarsened surface and obtain a map of islands of peaks taller than the threshold plane. These peaks are converted to hemispheres of same volume. Among various kinds of canonical roughness elements, hemisphere is least sensitive with respect to interference with surrounding elements and individual hydrodynamic performance is most easily evaluated. Validation of the model is given by comparing wall drag of channel flows having original complex walls and simple model walls, on the basis of RANS simulation. The simulation results confirmed that the model surface of proposed idea represents original wall with reasonable accuracy.