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The response of a turbulent boundary layer to a short length of surface roughness

Extensive measurements were made of the response of a high-Reynolds-number turbulent boundary layer to a short length (about three boundary-layer thicknesses) of surface roughness. The measurements include the mean velocity, all the Reynolds streses as well as all the triple products occurring in the Reynolds-stress transport equations. At the last measurement station, 55 boundary-layer thicknesses from the roughness, the boundary layer still had not relaxed to the universal smooth-wall structure. Comparison of the measurements with a calculation of the flow suggests the major requirement for improving the agreement is a more sophisticated treatment of the turbulent-diffusion process.