Wing Tip Vortex in the Near Field: An Experimental Study

Abstract

Introduction F LOW close to the wing tip is of importance for both low– and high–aspect ratio wings. Indeed, induced drag could be significantly affected by spanwise lift distribution, because there is an introduction of energy that is concentrated at the wing tip. Furthermore, even small errors in the prediction of flow close to the wing tip can significantly affect the evaluation of the structural loads acting on the wing. Knowledge of the physical behavior of the fully developed tip vortex is well established. However, experimental evidence shows, for the initial roll-up of the wing-tip vortex, complex behavior that is not adequately described by existing empirical models of the near-field flow. In fact, the tip vortex evolves from a complex threedimensional separated flow, and the resultant motion of the trailing vortices is highly unsteady. This implies that the core of the vortex fluctuates in time, and this meandering behavior causes the time-averaged Eulerian point measurement to be an average that is weighted both in time and in space. It is thus evident that the unsteadiness of the flow represents a significant challenge for both numerical and experimental analysis. Current knowledge of tip vortex flows is not adequate, and remains essentially qualitative, especially regarding the details of the mechanism of vorticity transport from the near-surface viscous layers into the trailing concentrated vortex. To achieve a better understanding of the initial phase of the tip vortex development an experimental study was performed, and its main results are described here.