When a moving surface slides under a stationary one, the potential exists for a vortical structure to develop close to the junction if the velocity of the moving surface is sufficiently high. The mechanism for the formation of this structure is the convection of the boundary layer material formed on the moving wall over the stationary plate. This results in an unsteady wall jet on the stationary plate that separates and rolls into a vortical structure as shown in the figure. Experiments show that a centrifugal instability develops along the vortex core which results in the eventual turbulent breakdown of the structure.