PULSED-WIRE ANEMOMETRY ON ROUGH SURFACES, WITH APPLICATION TO DESERT SAND DUNES

The use of pulsed-wire anemometry to measure surface shear stress in rough wall boundary layers is discussed. It is shown that if the size of the probe is not too dissimilar from the size and spacing of the roughness elements, reasonably accurate measurements of shear velocity are possible, provided the (active) tip of the probe is located near the zero plane displacement height. The technique is illustrated via an experiment on the flow over a 1/200 scale model of a ''barchan'' desert sand dune. Measurements of surface stress using the pulsed-wire probe on a modelled dune are shown to be consistent with those deduced from extrapolated crossed hot wire data and also with the implications of full-scale measurements of sand transport rates. These results, in addition to confirming the viability of pulsed wire anemometry on rough surfaces, have led to the resolution of a long-standing paradox concerning the dynamics of such dunes.