Experimental Investigation of Vortex Breakdown over a Delta Wing with Consideration of Control by Fluid Injection

Abstract : It is well-known that at high angles of attack, multiple vortex structures form on the wing due to the rolling-up of the viscous shear layers that separate from the upper surface. The highest energy structures result from separation along the so-called primary separation line, which is the leading edge, if it is sharp or highly curved. The intensity of this main vortex increases with the angle of attack until a sudden disorganization occurs. This phenomenon, known as vortex breakdown, is characterized by rapid deceleration of both the axial and tangential mean velocity components inside the vortex. During breakdown, the axial mean velocity component vanishes and then becomes negative on the axis of the vortex, corresponding to appearance in the flow structure of a stagnation point followed by a recirculation bubble. This paper discusses the effects of injecting fluid to control vortex breakdown. It contains data from two different experimental studies. For greater clarity the results are presented separately, since they correspond to tests conducted under different experimental conditions. In the first case, the tests were conducted in a wind tunnel on a delta wing with a 70-degree sweep, placed in a subsonic flow. The focus was to identifying the vortex breakdown location by measuring unsteady surface pressures with the aim of defining a breakdown indicator for a feedback control system. The second experiment, conducted in a water tunnel, concerned a delta wing with a 60-degree sweep. Here, the emphasis was on interpretation of the flow field visualizations and the breakdown phenomenology. The effects of control by fluid injection were measured using a force balance. (13 figures, 12 refs.)