Fluid dynamic effects of grooves on circular cylinder surface

It is shown that a groove on the surface of a circular cylinder affects movement of the separation point backward and reduces draw even at Reynolds numbers of about a few thousand. Several types of circular-arc cross-section grooves are studied using flow visualisations and numerical simulations. Whether these grooves are effective depends strongly on their positions, and the most effective positions are about 80 deg, measured from the foremost point. When they are effective, cavity flows are developed inside the grooves. This effect corresponds to that of dimples on golf balls and will explain unique characteristics of the drag curve.