| 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. This suggests that the vortex itself is laminar after the rollup and the higher turbulence intensity in the vortex core, reported in past studies, is mainly due to vortex wandering. This SPIV method is applied to investigate the angle of attack effect, downstream effect, and wind tunnel wall effect. Past studies suggest that the vortex rollup is completed about two chord lengths behind the wing trailing edge. The SPIV method confirmed that the vortex rollup is completed at 3.0c downstream for alpha= 5.0° and 4.0c for alpha= 10.0° by observing the re-centered Reynolds stress distributions. As for correcting the velocity profile, Devenport et al. (1996) found an analytical way to predict the wandering free velocity profile using the Reynolds stress at the vortex center. The velocity profile predicted by the Devenport et al. (1996) method is compared with the SPIV re-centered velocity profile. The results show that the two profiles agree with each other very well in the vicinity of core when the vortex wandering is large enough (about 20% of vortex core radius). |
