Flow Structure and Pressure Fluctuations of Turbulent Boundary Layers with Sparse Roughness

Measurements of the mean flow and turbulent statistics were made in a 2D flat plate turbulent boundary layer over a fetch of hemispherical roughness elements using Laser Doppler Velocimetry (LDV). These measurements, along with previous LDV measurements of flow over fetches of circular cylinders, were examined to study the effect of sparse roughness on the slope and intercept value of the semi logarithmic portion of the boundary layer. Various methods for estimating the friction velocity were explored. Rough wall surface pressure spectra were measured for the hemispherical roughness case. The dependency of measurement position on the pressure spectra was examined. A technique using two pressure transducers was used successfully to mitigate unwanted noise. Nomenclature Af = projected frontal surface area of a roughness element Ap = planform area of a ‘cell ’ of roughness elements Aw = wetted frontal area of a roughness element B = intercept value of the semi logarithmic region in a boundary layer when scaled on inner variables b = intercept value of the semi logarithmic region in a boundary laye