Computational Investigation of Drag Reduction on a Rotationally Oscillating Cylinder

The reduction of drag on a circular cylinder in two-dimensional incompressible ow, achieved using oscillatory body rotation, is investigated to reveal the physics responsible for the force reduction. Flow is computationally simulated at Reynolds numbers of 100, 300, and 15 000 for a small group of sinusoidal body rotations to gain an understanding of the mechanisms which can control forces in such situations. It is observed that, for certain rotational parameters, drag is reduced over a range of Reynolds numbers from the two-dimensional base ow. At Re = 15 000, an interesting boundary layer instability is observed which could explain the dramatic drag reduction previously noted in an experiment on this ow. Simulations are performed with a high-resolution viscous vortex method which utilizes the particle strength exchange technique for diiusion and multipole expansions for fast summations. Diierent approaches for computing and analyzing forces will be discussed as well as a `hybridization' applied to the scheme to allow particle merging in the far wake.