A Study of Streamwise Vortex Interactions

Abstract

Wind tunnel experiments and numerical simulations using RANS modelling were conducted to investigate the effect of a wall on the evolution of co and counter-rotating vortex pairs. The counter-rotating vortices had a mutually induced velocity that moved the pair away from the wall. Such pairs occur in the wakes of race car front and rear wings, as well as other areas of race cars where vortices are used for flow control purposes. In the experiments, two vortices were generated using two vortex generators, with NACA0012 profiles, attached to a flat plate. Whereas in the simulations the superposition of two Batchelor vortices was used to investigate the evolution of a vortex pair. Despite differences in Reynolds number, there was good correlation between the experiments and simulations. It was found that the presence of a wall increases the at which co-rotating vortices move towards each other and merge. This is due to the lateral and vertical induced velocities due to the image vortices and secondary vorticity respectively. At very low initial heights two initially symmetric vortices will merge like an asymmetric pair. The rate of rotation of the co-rotating pairs increased with decreasing initial height, until the initial height became too small and the rotation was inhibited. The generation of secondary vorticity and the interaction between primary vortices and this secondary vorticity was shown to accelerate the decay of circulation for both co and counter-rotating pairs. For counter-rotating pairs whose mutually induced velocity moved the pair away from the wall, the secondary vorticity was found to have little effect on the trajectory. That is significantly different from counter-rotating pairs whose mutually induced velocity moves the pair towards the wall, which rebound multiple times.