A stereoscopic PIV study on the behavior of near-field wingtip vortex structures

Wingtip vortex flow is of great importance because of its effect on practical problems such as landing separation distances for aircraft, blade/vortex interactions on helicopter blades, and propeller cavitations on ships. Extensive investigations have been conducted to improve the understanding of the tip vortex structure and its dissipation or persistence analytically, numerically, and experimentally. The universal feature of the water/wind tunnel generated wing tip vortex reported in the past is vortex wandering – the slow side-to-side movement of the wing-tip vortex core behind the wing. Thus, a primary result of wandering is that fixed probe measurements of velocity and pressure cannot be trusted at distances more than one chord downstream of the wing. For reliable data, the current study investigates the behavior and structure of the near-field wing-tip vortex generated by a square-tipped, rectangular NACA0012 wing by using the stereoscopic Particle Image Velocimetry (SPIV) technique. SPIV is a spatially resolved, instantaneous, three velocity component non-intrusive measurement technique used to conserve the three key feature of the wing-tip vortex during the measurement – small vortex core dimension, core structure, and strong unsteadiness of the core flow, which wasn’t possible with classical instrumentations. One of the great advantages of SPIV over the classical technique is that the vortex wandering can be removed by tracking the center of the vortex in every SPIV frame. By tracking the center of the vortex, the wandering and turbulence in the vortex can be separated. The results show that after re-centering the velocity field, the T.K.E. and Reynolds stress distributions become lower by more than twice at 4.0c downstream.