Fluid Dynamics and Surface Pressure Fluctuations of Turbulent Boundary Layers Over Sparse Roughness

Turbulent boundary layers over rough surfaces are a common, yet often overlooked, prob-lem of practical engineering importance. Development of correlations between boundary layer parameters that can be used in turbulence models and the surface geometry is the only practical option for solving these problems. Experiments have been performed on a two-dimensional zero pressure gradient turbulent boundary layer over sparsely spaced hemi-spherical roughness elements of 2 mm diameter. Laser Doppler velocimetry was used to measure all three components of velocity. The friction velocity was calculated using an inte-gral momentum balance. Comparisons were made with various fitting methods that assume the von Kármán constant is appropriate for rough walls. Results indicate that this is not the case, and that the slope of the semi-logarithmic portion of the mean streamwise profile may be a function of the ratio of inner and outer length scales. Comparisons were also made between various correlations that relate the surface geometry to the behavior of the mean velocity profile. In general, the existing correlations achieved a reasonable agreement with the data within the estimated uncertainties. A detailed study of the local turbulent structure around the roughness elements was per