Turbulent boundary-layer wall-pressure fluctuations on smooth and rough walls

Turbulent boundary-layer wall-pressure measurements were made with ‘pinhole’ microphones three times smaller (relative to a boundary-layer displacement thickness) than microphones used in earlier work. The improved high-frequency resolution permitted examination of the influence of high-frequency eddies on smooth-wall pressure statistics. It was found that the space-time decay rate is considerably higher than previously reported. Measurements of cross-spectral density made with 5 Hz bandwidth filters disclosed low phase speeds at low frequency and small separation. Measurements were repeated on rough walls and parallels were drawn from knowledge of a smooth-wall boundary-layer structure to propose a structure for a rough-wall boundary layer. The effect of independently varying roughness height and separation on the large and small-scale turbulence structure was deduced from the measurements. It was found that roughness separation affected the very large-scale structure, whereas the roughness height influenced the medium and very small-scale turbulence.