Drag reduction by a flexible afterbody

Abstract

The hydrodynamic mechanism of drag reduction by a flexible afterbody was explored by using the penalty immersed-boundary method. The flexible afterbody is constituted by a trailing closed flexible filament. A volume penalty method was adopted to conserve the volume of a flexible afterbody during deformation. Simulations of a rigid plate without a flexible afterbody and with a rigid afterbody were also performed for comparison. The results of the present simulation show the same tendency as the experimental data, in which the maximum drag reduction was obtained at a particular length of the flexible filament irrespective of the bending rigidity. We analyzed the underlying mechanism of drag reduction in detail by examining wake patterns, pressure distributions, flapping modes, and the average kinetic energy (E¯k) of the flexible afterbody. The contributions of form drag and skin friction drag to the total drag were determined as functions of the length of a flexible afterbody. An appropriately shaped flexible afterbody delays vortex formation and weakens the vorticity and negative pressure, resulting in drag reduction. Active flapping of the afterbody adversely affects drag reduction. Drag is reduced by increasing the Reynolds number (Re). Drag is insensitive to E¯k but sensitive to the flapping amplitude of the flexible afterbody as Re increases.