An Experimental Investigation of the Vortex Whip: Three Unequal Strength Vortices in the Presence of a Wall

Abstract

In this study, a series of three unequal strength vortices was generated with triangular vane-typevortex generators in the boundary layer of a water tunnel. The sense of rotation of the vortices from weakest to strongest circulation was CW (+), CCW (-), CCW (-). The Reynolds number based on vortex generation height was Re_H = 1080. Flow was visualized using dye, and measurements were taken perpendicular to the flow with 2D PIV. The inter-separation distance between vortex generators was varied. The goal of this work is to investigate the vortex dynamics present in this system of three vortices known as the vortex whip. The vortex whip is a novel flow control method for delaying boundary layer separation with implications for aviation safety, energy generation, and pressure recovery in pipes. Results differed from 2D numerical simulations. In half of the cases tested, the vortex whip was found to induce the intermediate strength vortex farther towards the wall than the weakest vortex. This reveals that the vortex whip is a system that induces a counter-rotating pair of vortices towards the wall rather than merely the weakest vortex, as previously thought. Our results indicate that a shorter distance between the vortex generators of the counter-rotating vortices (D1) is more optimal and that there is potentially a unique range of separation distances between the vortex generators for the co-rotating vortices (1.17 < D2/H < 2.4) where the vortex whip is optimal. In addition to study of the vortex whip, vortex generation is compared between a single vortex generator and multiple in close proximity. A circulation boost was found for the counter-rotating pair that increased as the separation distance D1 shortened. This points to a mutual induction during the vortex roll-up being responsible for the boost. A circulation reduction was found for the strongest vortex in a co-rotating pair. This reduction was not directly tied to separation distance D2 but is still thought to be due to interference during roll-up from its partner vortex.