Flow and skin friction over natural rough beds

Abstract

The first of two sets of data reported here, measured over two-dimensional immobile current ripples, comprises skin-friction time series at four streamwise positions and velocity profiles measured to within a few mm of the bed. The velocity profiles cannot be linked to the local mean skin friction through the law of the wall, so they do not represent an internal boundary layer growing downstream of reattachment. Rather the velocity field is the sum of a spatially averaged rotational profile and a locally inviscid perturbation. Normalized skin-friction spectra are independent of position, but skin-friction probabilty density functions show strong increases in skewness and kurtosis near reattachment. The second set of data comprises measurements of the skin-friction vector field around isolated hemispheres with and without model sedimentary tails, and skinfriction and velocity measurements on arrays of hemispheres with and without tails. The skin-friction field around an isolated hemisphere is not consistent with formation of growth of a sedimentary tail more than about two obstacle heights long. The skin-friction field in a hemisphere array of areal density 0.02 is significantly distorted from the isolated-element case, and the roughness length in arrays of two areal densities is not altered by addition of tails.