The Bursting of Leading-Edge Vortices Some Observations and Discussion of the Phenomenon

The term bursting refers to the structural change from a strong regular spiral motion to a weaker turbulent motion which can occur at some position along a vortex. Various observations of this phenomenon have been recorded during low-speed tunnel investigations of vortex flows generated from sharp, swept back leading edges. These reveal a sudden deceleration of fluid along the vortex axis and expansion of the vortex around a stagnant core downstream of which the flow is turbulent; it is possible at very low stream velocity to see the axial flow, after deceleration, deflect and perform a regular whirling motion before turbulent breakdown. These features together wit h the effects of bursting on oil-flow patterns and pressures at the wing surface are illustrated and described. Brief reference is made to observations at transonic speeds. The burst is found to be sensitive to several factors, in particular, an increase of pressure gradient along the vortex seems conducive to the occurrence of a burst. It is suggested that a condition essential for the burst to occur is a low total pressure within the vortex core coupled with an adverse pressure gradient along the axis. When bursting occurs in the flow above a plain wing it is likely that the occurrence is related to the adverse pressure gradient associated with the existence of a trailing edge. The experiments described in Appendix I show that a similar type of phenomenon can occur in the flow in a vortex tube.