AN INVESTIGATION OF TIP VORTICES IN A WATER TUNNEL

This paper deals with the problem of a tip vortex issuing from a finite span hydrofoil. The investigation was conducted in a water instead of a wind tunnel to benefit from the larger Reynolds numbers achieved using smaller test models as well as the possibility to use cavitation within the vortex core as a tracer to investigate its stability and diffusion. In this work the tangential and axial velocities in a tip vortex generated by a finite span hydrofoil of elliptical planform and symmetrical cross section have been measured using LDA at a distance of several chords downstream. The tangential velocity profiles obtained for various operating conditions clearly show the existence of a core region rotating as a rigid body and a nearly potential region outside. Estimates of the circulation in this region were obtained by fitting the Burgers model to the velocity data. Fair agreement between the circulation deduced from the velocity profile and lift measurements was obtained. Some visualisations of the cavitation induced by the tip vortex on a two dimensional hydrofoil at several chord lengths downstream are also presented. It is shown that the velocites induced by the vortex cause a modification of the effective angle of attack along the leading edge of the two dimensional hydrofoil and give rise to a distinct separation region on its surface.The latter problem is important in connection with the operation of helicopter rotor blades.