Abstract
Omnidirectional reductions in drag and fluctuating forces were achieved for a circular cylinder subjected to cross-flow by attaching cylindrical rings along its span at an interval of several diameters. The Reynolds number based on the cylinder diameter, d, ranged from Red = 3000 to 38000 for the experiments. The aspect ratio of the cylinder, L/d, was approximately 20. The addition of the rings reduced the drag force for Red ≥ 20000, even though the projected area increased. This was attributed to the formation of separation bubbles on both sides of the ring, which led to pressure recovery on the rear of the cylinder. The optimum ring configuration for drag reduction was found to be D/d = 1.3, W/d = 1.0, and P/d = 6 at Red ≈ 30000, where D is the ring diameter, W is the spanwise width of the ring, and P is the spanwise pitch of the ring. This configuration reduced the drag force by 15%. In addition, the fluctuating lift, which was estimated from the fluctuating surface pressures, was weakened for all Reynolds numbers due to the suppression of vortex shedding.
