Turbulence-Driven Reverse Lift on 2D and 3D Bodies with Deflected Tails*

Deflection of tails is a basic strategy used by many species and man-made structures to induce drift. This process is often performed under flows with a variety of turbulence levels, which may induce distinctive force. Using laboratory experiments and simplified theoretical arguments, we show that the level of turbulence may reverse the direction of the mean lift on two- and three-dimensional structures with relatively short, deflected tails. Planar particle image velocimetry and a high-resolution load cell were used to characterize the near wake region and the instantaneous lift of the body-tail assembly for various geometric configurations, tail angles and turbulence levels at Reynolds number Re=2 X 10^4 based on the body width. The possibility of reverse lift may occur within a critical deflection angle depending on the tail length and turbulence level, which induced changes  in the backward-flow region. This particular phenomenon is explained quantitatively with a simple formulation that accounts for the effects of the body geometry and turbulence. The uncovered phenomenon offers insight for optimum design of vehicles in various flows.