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 Figure 1. Experiments show that a centrifugal instability develops along the vortex core which results in the eventual turbulent breakdown of the structure. Figure 2 shows a spanwise cross section of the instability developing on the vortex core. This type of structure is generated in a range of industrially significant situations, for example at the head of a piston as it moves through a cylinder.

The work was sponsored by the National Science Foundation.

Reference:

Naitoh, T. & Allen, J. J. Spanwise instability of a junction vortex. 10th International Symposium on flow visualization, August 26-29, 2002, Kyoto, Japan