Formation of Bifurcated Wakes Behind Finite Span Flapping Wings

A three-dimensional high-order unstructured dynamic-grid spectral-difference Navier Stokes flow solver is used to investigate the wake structures of the finite span flapping wings. The numerical results for the root-fixed plunging wing agree qualitatively with the experimental measurement. After the wing-surface deformation is specified, a polynomial-blending function is used to determine the movement of other grid points. The formation process of the bifurcated wake topology behind the finite span flapping wing is found to be closely related to the interaction between trailing-edge vortices (TEVs) and tip vortices (TVs). The key mechanism is attributed to the fact that the rotational direction of TVs is subject to change during the interaction with TEVs.