Investigation of Eddy Structure over Rough Surfaces in a Turbulent Channel Flow

Large-eddy simulation (LES) is employed to study turbulent flow over rough surfaces. Fully-developed flow in two-dimensional channels with one of the walls roughened by sinusoidal waves of varying steepness and rectangular ribs with different spacing is considered to investigate the effect of roughness on the eddy structure. The imposed mean streamwise pressure gradients are adjusted to obtain the same roughness function, i.e., to have the same effect on the mean velocity profile. Examination of the mean and instantaneous flow fields shows that different types of surface roughness produce quite different turbulence transport characteristics. The streamwise, spanwise and normal turbulence intensities and Reynolds stresses are significantly different with different roughness although the mean velocity distributions are essentially the same. Quadrant analysis shows ejection and sweep events are strongly related to the geometry of surface roughness. The implications of these results for modeling of turbulent flow over rough surfaces are discussed.