Turbulence Statistics in an Open-Channel Flow over a Rough Bed

This paper presents the results of a large-eddy simulation (LES) of turbulent flow over a channel bed artificially roughened by hemispheres. The Reynolds number of the flow based on the channel depth is 13,680 at a relatively low submergence of 3.42. First- and second-order statistics are compared with corresponding laboratory experiments to validate the LES. The effect of roughness heterogeneity on higher-order statistics is quantified and discussed. The contribution of the dominating turbulent events (i.e., sweeps, ejections) to the Reynolds stress and the anisotropy of turbulence are quantified. Visualizations of the complex three-dimensional turbulence structures reveal the occurrence of a number of different vortex types in the flow. The contribution of turbulence structures to the turbulent kinetic energy and their scaling is assessed through proper orthogonal decomposition.