Vortex Pairs in the Formation and Evolution of Wingtip Vortex

The three-dimensional flow structures of wingtip vortex of a rectangular, squared-tipped NACA0020 airfoil were investigated at three different angles of attack (AOA). The vortex structures were measured by DPIV, which were also simulated by the improved delayed detached eddy simulation (IDDES) method. From the point of view of three-dimensional vortex structure, DPIV and IDDES results show that there exists a pair of counter-rotating starting and stopping vortices (STV and SPV) near the tips of pressure and suction surfaces respectively. In unsymmetrical cases, SPV from suction surface (SSPV) and STV from pressure surface (PSTV) are co-rotating, and finally merge into one trailing wingtip vortex. In the symmetrical case (alpha = 0 degrees), a symmetrical counter-rotating SPV pair evolves in the wake. The merged wingtip vortex gets away from the suction surface, and also moves towards to the wing root. The merger of SSPV and PSTV strengthens the wingtip vortex, leading to the augmentation of circulation.