Omnidirectional reductions in drag and fluctuating forces for a circular cylinder by attaching rings

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 diameter of the cylinder, Red, ranged from 3000 to 38 000 for the experiments. The addition of the rings reduced the drag force by 15% for Red⩾20 000, even though the projected area increased. This was attributed to the formation of separation bubbles on both sides of the ring, which lead to pressure recovery on the rear of the ring. 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.