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research-article

Towards a Universal Roughness Correlation

[+] Author and Article Information
Pourya Forooghi

Post-doctoral Researcher, Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Kaiserstraße 10, 76131 Karlsruhe, Germany
forooghi@kit.edu

Alexander Stroh

Post-doctoral Researcher, Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Kaiserstraße 10, 76131 Karlsruhe, Germany
alexander.stroh@kit.edu

Franco Magagnato

Senior Scientist, Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Kaiserstraße 10, 76131 Karlsruhe, Germany
franco.magagnato@kit.edu

Suad Jakirlic

Professor, Institute of Fluid Mechanics and Aerodynamics, Technical University Darmstadt, Alarich-Weiss-Straße 10, 64287 Darmstadt, Germany
jakirlic@sla.tu-darmstadt.de

Bettina Frohnapfel

Professor, Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Kaiserstraße 10, 76131 Karlsruhe, Germany
bettina.frohnapfel@kit.edu

1Corresponding author.

ASME doi:10.1115/1.4037280 History: Received February 01, 2017; Revised July 04, 2017

Abstract

Effects of several surface parameters on equivalent sand roughness ($k_s$) in fully-rough regime is investigated by means of Direct Numerical Simulation of flow in channels with different wall geometries at $Re_\tau\cong 500$. The roughness geometry is generated by randomly distributing roughness elements of random size and prescribed shape on a flat surface. The roughness generation approach allows systematic variation of moments of surface height probability density function, size distribution of roughness peaks and surface slope. A total number of 38 cases are solved. It is understood that a correlation based on surface height skewness and Effective Slope can satisfactorily predict $k_s$ normalised with maximum peak-to-valley roughness height within a major part of the studied parameter space. Such a correlation is developed based on the present data points and a number of complementary data points from the literature. It is also shown that peak size distribution can independently influence the skin friction; at fixed values of rms surface height, skewness, kurtosis and Effective Slope, a surface with uniform size peaks causes higher skin friction compared to one with non-uniform peak sizes. Additionally, it is understood that a roughness generated by regular arrangement of roughness elements may lead to a significantly different skin friction compared to a random arrangement. A staggered and an aligned regular arrangement are examined in this paper and it is observed that the former produces significantly closer results to the corresponding random arrangement.

Copyright (c) 2017 by ASME
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