Scaling the Internal Boundary Layer

The internal boundary layer (IBL) is an important phenomenon in atmospheric flows which is associated with a step change in the surface roughness. This phenomenon also relates to industrial flows, where a wall may exhibit abrupt changes in surface roughness, perhaps related to corrosion. The present study reports new wind tunnel measurements which consider a Smooth–Rough–Smooth (SRS) configuration. The rough surfaces were created using 40-grit sandpaper glued onto the ground plane of a wind tunnel, and mean velocity measurements were collected using a boundary layer Pitot tube. The development of the IBL is clearly evident in the streamwise evolution of the mean velocity profiles. The results indicate that once the flow encounters the step change in roughness, the flow immediately next to the wall is decelerated due to the enhanced skin friction associated with the surface roughness. The roughness effects propagate further into flow as the IBL grows in the streamwise direction. However, when the surface condition changes back to smooth, the flow begins to accelerate, but does not recover to the initially smooth profile. This implies that some regions of the velocity profile preserve a “memory” of the previous surface condition, and therefore are not in equilibrium with the local surface.