Vortex rings ejected from flush-mounted and elevated pipes

Abstract

This paper describes observations and measurements of the effects of a change in boundary conditions on the vorticity field and circulation of laminar vortex rings ejected from a circular pipe. The change in boundary conditions is from a pipe exit which is flush with a flat wall, to a pipe exit which is elevated above the wall. Measurements of the evolving vorticity field show that the flush-mounted vortex ring has a smaller ring diameter than the elevated vortex ring. This is the result of the vortex rings interaction with the wall, which is equivalent to the influence of an imaginary image vortex ring below the wall (i.e. the method of images). The image induces a radially-inward motion towards the axis of the pipe. Plotting vortex ring circulation as a function of time shows that both vortex rings have the same maximum circulation, but the rate of circulation decay is greater in the flush-mounted vortex ring. This is explained in terms of increased rate of cancellation of opposite-sign vorticity at the vortex ring centre.