Parameters controlling roughness effects in a separating boundary layer

The effects of wall roughness were examined experimentally for two different rough-wall cases involving flow over a ramp with separation and reattachment. For these cases, the roughness Reynolds number was matched at two different momentum thickness Reynolds numbers. Both flow conditions were fully rough. The effect of increasing the wall roughness was to increase the friction velocity and increase the separation region size. The two rough-wall cases produced different size separation regions and different friction velocity values. This shows that the roughness Reynolds number is not sufficient to characterise the roughness effects. Another parameter such as the ratio of the roughness height to the boundary layer thickness is also necessary. In both cases, the outer layer turbulence was mainly affected by the change in the friction velocity.