Two-phase wing-tip vortex breakdown

This article presents a new flow feature observed behind a rectangular wing in water: the breakdown of the wing-tip vortex induced by the temporary injection of air into the vortex core downstream of the wing. For certain combinations of Reynolds number and angle of attack, a stationary air bubble is trapped in the core at some distance from the wing and can persist for several minutes after the injection is stopped. For other parameter values, the bubble drifts downstream or upstream until it reaches the wing, or it disintegrates immediately. Measurements of the vortex properties and bubble characteristics are presented and discussed. The breakdown behaviour, which is unrelated to cavitation, is found to depend mainly on the tip vortex circulation and on the axial flow component in the vortex core. Only configurations with a velocity excess, with respect to the free stream, at high angle of attack, allow the formation of a breakdown bubble.