Effects of Winglet Dihedral on a Tip Vortex

The effect of winglet dihedral (-87.5<=(delta)<=87.5 deg) on the near-field tip vortex flow structure behind a swept and tapered NACA 0015 wing was investigated at Re = 1.81 × 10^5. The winglet dihedral led to a significantly reduced (increased) peak tangential velocity and vorticity (core radius) compared to a baseline wing. The values of core circulation (core axial velocity) could be below or above the baseline-wing value, depending on the winglet angle, and exhibited a local maximum (minimum) at (delta)=0 deg. The inner region of the tip vortex flow exhibited a self-similar behavior for a wing with and with no winglet. The lift-induced drag was always reduced by the addition of a winglet. The negative dihedral was more effective in reducing the lift-induced drag compared to the corresponding winglet of positive dihedral. A large discrepancy in the lift-induced drag was, however, observed between the wake integral method and the classical lifting-line theory.