Parametric Investigations of the Leading Edge Vortex on a Delta Wing

A 70 -sweep delta wing was investigated experimentally by means of stereoscopic particle image velocimetry (StereoPIV). The velocity eld was measured at three angles of attack (25 , 30 , 35 ) and at three moderate chord Reynolds numbers (0.78, 1.2 and 1.55 10). The ow eld on delta wings, including vortex breakdown, is complex and highly threedimensional. Many qualitative observations exist (mostly ow visualisations), but only few measurements quantify the ow eld. This study will describe the evolution of crucial vortex parameters along the vortex axis, focusing on the vortex radius and the axial circulation. This study aims at generating a deeper insight into the ow structures of leading edge vorticies. The vortex was investigated upstream and downstream of vortex breakdown. The vortex centre was determined on the basis of ow visualisations as well as on StereoPIV data. Strong deviations between the two measurement techniques arise due to a mismatch between the positions of the maximum vorticity and the minimum azimuthal velocity. Thus, the leading edge vortex does not correspond to a regular Hamel-Oseen vortex and other approaches were used to detect the vortex. In the pre-vortex breakdown region, the radius remains almost constant and Reynolds number independent. In this area, a high concentration of vorticity is observed. Downstream of vortex breakdown, the vortex radius increases drastically and the axial velocity decreases towards zero. The circulation of the vortex tube was quanti ed along the vortex axis and a decrease of the axial circulation of the vortex tube is observed slightly upstream of the vortex breakdown position. Downstream of the vortex breakdown, the highly concentrated vorticity in the vortex tube spreads out whereas the circulation of the ow eld on the wing surface reaches an almost constant level. Such a behaviour is expected with a transformation from axial vorticity to azimuthal vorticity due to the breakdown. In the pre-VB region, a strong linear connection between the axial velocity in the vortex centre and the circulation of the vortex tube is observed.