Journal Article

Turbulent channel flow over heterogeneous roughness at oblique angles

Anderson, W.

2020

Overview

Large-eddy simulation has been used to model turbulent channel flow over a range of surfaces featuring a prominent spatial heterogeneity; the flow streamwise direction is aligned relative to the heterogeneity at a range of angles, defined herein with $\\unicode[STIX]{x1D703}$ . Prior work has established that a sharp roughness heterogeneity orthogonal to the flow streamwise direction ( $\\unicode[STIX]{x1D703}=0$ ) induces formation of an internal boundary layer, which originates at the heterogeneity and thickens in the downflow direction before being homogenized via ambient shear. In contrast, more-recent studies have shown that a sharp roughness heterogeneity parallel to the flow streamwise direction ( $\\unicode[STIX]{x1D703}=\\unicode[STIX]{x03C0}/2$ ) induces streamwise-aligned, Reynolds-averaged secondary cells, where the spacing between adjacent surface heterogeneities regulates the spatial extent of secondary cells. No prior study has addressed intermediate (oblique) cases, $0\\leqslant \\unicode[STIX]{x1D703}\\leqslant \\unicode[STIX]{x03C0}/2$ . Results presented herein show that the momentum penalty exhibits a nonlinear dependence upon obliquity, where internal boundary layer-like flow processes persist over a range of obliquity angles before abruptly vanishing for spanwise roughness heterogeneity ( $\\unicode[STIX]{x1D703}=\\unicode[STIX]{x03C0}/2$ ). This result manifests itself within effective roughness lengths recovered a posteriori : the traditional approach to roughness modelling – predicated upon dependence with surface geometric arguments including height root-mean-square, skewness, frontal- and plan-area index, effective slope. and combinations thereof – is insufficient. A revised model incorporating dependence upon roughness frontal area index and flow-heterogeneity obliquity angle is able to accurately predict effective roughness length a priori .

• Publisher: Cambridge University Press
• Subject: Angles (geometry) / Boundary layers / Cells / Channel flow / Computational fluid dynamics / Computer simulation / Dependence / Direction / Engineering / Equilibrium / Heterogeneity / Large eddy simulation / Momentum / Oceanic eddies / Patchiness / Reynolds number / Roughness / Skewness / Spatial heterogeneity / Turbulent flow / Viscosity