Near-field behavior of a tip vortex

Abstract

The near-field behavior of a tip vortex trailing behind a low aspect ratio wing, attached to an axisymmetric body, is investigated in this paper. This study was performed in a tow tank and involved the use of particle displacement velocimetry. Evolution of the tip vortex was studied by mapping its instantaneous lateral velocity at several consecutive axial locations. The axial velocity distribution was also measured. Experiments were repeated at various Reynolds numbers and incidence angles. Repeatability was also examined at the same conditions. The results indicate that roll-up is almost complete at the trailing edge and that less than 66% of the root circulation is entrained into the vortex. At Rec = 2.2 x 10s, the results are steady, and the spatially averaged tangential velocity agrees well with the existing models. At Rec<W5 the entire flow stucture, including the overall circulation, fluctuates from one experiment to the next. The tangential velocity profiles seem to be dominated by secondary vortices located primarily outside of the vortex core. Axial variations in the flow structure along the parallel middlebody, during the same run, are limited to the location of these secondary vortices. The vortex is observed to expand in the afterbody region, without a change in its total strength. For all but one case (Rec = 2.2 x 10s and a = 10 deg), there is a substantial axial velocity deficit within the vortex core near the trailing edge of the wing. At Rec <105, the axial velocity deficit increases with incidence angle and decreasing Rec.