Generation of a wingtip vortex pair using a pressure-side fin

Abstract

We present results from a combined experimental and numerical investigation of the flow around a rectangular NACA 0012 wing, whose tip geometry is modified by the addition of a perpendicular fin. The purpose of this modification is to generate a system of two closely-spaced concentrated tip vortices, instead of the familiar single trailing vortex. Experiments were carried out in a water tunnel, where velocity measurements were made in the near wake of the wing using Stereoscopic Particle Image Velocimetry, in order to determine the characteristics of the tip vortex pair. Various configurations were tested by changing the position, dimensions and angle of attack of the fin. It was found that the fin could generate a strong and concentrated secondary vortex only when placed on the pressure side of the wing. Visualisations of surface streamlines using oil painting in a wind tunnel, as well as numerical simulations of the flow in the wingtip region, show that separation occurs with the fin on the suction side, even at low angles of attack. Both experiments and simulations were used to determine the influence of fin parameter variations on the tip vortex pair, with respect to its circulation ratio and separation distance. Whereas co-rotating vortex pairs of similar circulation could be generated easily, the fin's ability to produce counter-rotating secondary vortices is rather limited. The current results have relevance in the context of reducing the impact of concentrated tip vortices, which for rotor configurations can lead to strong blade-vortex interactions, causing fatigue and aerodynamic noise.