(Woods Hole Oceanographic Institution, 1983-06)
Paola, Christopher
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.
