Turbulent boundary layer flow through a gap in a wall-mounted roughness element

The development of a turbulent boundary layer flow through a gap in an isolated wall-mounted roughness element has been studied experimentally. Two flow regions were distinguished downstream of the gap; a distortion region followed by a readjustment region. In the distortion region, two counteracting distortion mechanisms were identified, the relative importance of which depended on gap size. Thus flows downstream of large gaps were found to differ significantly from those through small gaps. After distortion the layer readjusts itself and approaches equilibrium conditions of an undisturbed zero pressure gradient layer. The readjustment starts near the wall with the turbulence adjustment preceding the mean flow adjustment. The growth of the internal layer for flow through six different gap sizes can be described by a single function if internal layer height and distance from the gap are non-dimensionalized with the local wall length scale. Well downstream of the gap it is shown that all six flows are similar and are approaching equilibrium conditions in a similar manner.