Vorticity transport mechanisms governing the development of leading-edge vortices

Abstract

The sequence of events constituting the formation of a leading-edge vortex (LEV) has been investigated for a periodically plunging nominally two-dimensional flat plate and a similarly articulated plate of aspect ratio two. Particle image velocimetry applied in multiple parallel planes and unsteady surface pressure measurements were used to quantify the sources and sinks of vorticity governing the growth of circulation in a control region moving with the plate in each case. In the two-dimensional case, the initial accumulation of (negative) vorticity in the nascent LEV produces a strong surface diffusive flux of vorticity that erodes the connection between the LEV and downstream boundary layer through cross-cancelation, initiating the ‘roll up’ of the LEV. Despite the significant diffusive flux earlier in the vortex development, there is no significant accumulation of secondary vorticity until after the severing occurs. The growth of the secondary vortex reduces the suction near the leading edge, such as to result in a self-limiting mechanism on the diffusive flux. In the finite-aspect-ratio case, a similar development is observed, except that the formation process is regulated or reversed by the spanwise convection of vorticity, which opposes the action of the surface diffusive flux. The physical mechanisms of vortex formation or reversal identified here can provide a basis for the design of passive or active flow control strategies to regulate vortex development.