A VORTEX RING IMPINGING ON A SOLID PLANE SURFACE - VORTEX STRUCTURE AND SURFACE FORCE

A joint numerical and experimental investigation has been performed to study a vortex ring normally impinging on a solid plane surface. Specific emphasis is placed upon the vortex structure and the associated surface force at Reynolds numbers between 500 and 2000. Along the line of Chang [Proc. R. Soc. London Ser. A 437, 517 (1992)], the force was viewed to be contributed by fluid elements with nonvanishing vorticity in the flow. This viewpoint enables the paper to provide a unique opportunity to shed light on the detailed mechanism which causes a small net surface force during the impingement process. It is found that the small net surface force results from the cancellation of a force pushing on the surface, due to the primary vortex ring, and a suction force, due to the induced boundary layer. Nevertheless, both the pushing and suction forces are much larger than that of the net surface force by several orders of magnitude. This fact provides proper directions for flow manipulation regarding forces exerted on the surface. © 1995 American Institute of Physics.