Effects of irregular roughness on roughness-sublayer turbulence statistics and coherent motions

Near-wall turbulence statistics and structure are compared using DNS data from open-channel flows at Re_τ = 1000, over four different surfaces. The first two surfaces have irregular roughness geometries with height distributions of very similar root-mean-square and skewness, which fall within the fully-rough regime. One is a random sand-grain roughness with distinct peaks in its height power spectrum; the other is a fractal-like representation of the roughness of a turbine blade. The other two surfaces are a regular, cube-roughened (k-type) surface and a smooth wall. It is shown that relatively peaky surface structures contribute much of the pressure drag and are strongly correlated with intermittent, instantaneous generation of wall-normal turbulent fluctuations. Thus, surfaces with a higher density of peaky structures lead to higher friction coefficients and more isotropic Reynolds stresses. Integral length scales reveal a significantly shortened streamwise coherence near sand grains and cubes, but not over the turbine-blade surface. Shape modification of conditional eddies in the roughness sublayers will be discussed.