Spanwise variations in nominally two-dimensional rough-wall boundary layers

Laboratory experiments have been conducted in two separate boundary layer facilities to investigate steady spanwise variations in mean velocity discovered during studies of developing flows over regular arrays of large roughness elements. Regular spanwise variation was found with a steady wavelength, moderated by the growing boundary layer, which was an integer multiple of the repeating unit of roughness. Amplitude variations greater than +/- 5% in the mean were found over the roughness and greater than +/- 10% in turbulence quantities. Due to the dominating nature of this phenomena throughout the layer, care should be taken in undertaking local measurements aimed at identifying flow variations caused by roughness heterogeneity.