Formation and early development of wingtip vortices

Wingtip vortices are extremely important phenomena in fluid dynamics for their negative effects in many applications. Despite the many studies on this particular flow, the current understanding is still poor in providing a form base for the design of effective tip geometry modifications and vortex control devices. A rectangular wing with squared and rounded wingtips was tested in order to identify the main mechanisms involved in the formation of the vortex on the wing and in its early development in the wake. The complementarity of a number of experimental techniques adopted, such as surface flow visualizations, wall pressure measurements, smoke visualizations and stereoscopic particle image velocimetry (SPIV), gave a richer insight of the physics and the basic mechanisms of the vortex development. Furthermore, a large number of configurations were tested exploring the effects of several parameters such as wing chord, aspect ratio, wingtip geometry, angle of attack and Reynolds number. The development of the vortex along the wing showed the formation of several secondary vortices which interacted with the primary vortex generating low frequency fluctuations. The structure of the flow at this stage was analysed introducing a compact description through characteristic lines of the vortex system defined from the velocity vector field in the vicinity of the wing surface. The high spatial resolution achieved by the SPIV arrangement allowed a deeper understanding of the vortex structure in the early wake and the turbulence production and dissipation within the vortex core. The relaminarization process of the vortex core promoted by centrifugal motion was observed. The relation between vortex meandering, turbulence, secondary vortices and wake sheet was discussed. A comparison of different methods for the averaging of instantaneous planar vector fields was performed showing the effects and importance of the meandering. An axial acceleration of the flow within the vortex was observed and the formation of different axial flow distributions was discussed. A minimum wake-like flow of 0.62 and a maximum jet-like flow of 1.7 times the freestream velocity were measured and a linear relation between a vortex circulation parameter and the axial velocity peak was found.