Large-scale spanwise periodicity in a turbulent boundary layer induced by highly ordered and directional surface roughness

Abstract

The effect of converging-diverging riblet-type surface roughness (riblets arranged in a 'herringbone' pattern) are investigated experimentally in a zero pressure gradient turbulent boundary layer. For this initial parametric investigation three different parameters of the surface roughness are analysed in detail; the converging-diverging riblet yaw angle α, the streamwise fetch or development length over the rough surface Fx and the viscous-scaled riblet height h+. It is observed that this highly directional surface roughness pattern induces a large-scale spanwise periodicity onto the boundary layer, resulting in a pronounced spanwise modification of the boundary layer thickness. Hot-wire me..