Transformation of flow structure on a delta wing of moderate sweep angle during pitch-up maneuver

Abstract

The transformation of flow structure on a delta wing of moderate sweep angle is investigated during pitch-up via quantitative imaging in crossflow planes, in relation to qualitative dye visualization. The comparisons are made with the corresponding stationary wing. The effects of different pitch-up maneuver rates and Reynolds numbers on evolution of flow patterns are also addressed. A technique of high-image-density particle image velocimetry is employed for the quantitative analysis. The transformation of the flow structure starts with intertwining of the dual vortex structure which is distinctively seen at a low angle of attack of 50° swept wing, and then continues with the disappearance of one of the vortices of the dual vortex structure, in particular, the vortex located closest to the leading-edge of the wing. At further stages, a single, large-scale, leading-edge vortex, and distinctive form of vortex breakdown is witnessed. This distinctive type of vortex breakdown turns into a more traditional form at later stages of the maneuver. During the final stage of the pitch-up maneuver, the onset of vortex breakdown moves toward the apex of the wing. The general trend of aforementioned flow transformation happens irrespective of the values of pitch rates and Reynolds numbers. When the foregoing patterns are compared with the corresponding patterns on the stationary wing, it is evident that a significant time lag of development of the flow patterns occurs, relative to the motion of the wing.