Mechanism of drag reduction by dimple structures on a sphere

Abstract

The purpose of the present study is to clarify the mechanism of drag reduction on a sphere with arc type dimples. The sphere with dimples has 328 arc type dimples of different depth on their surface uniformly. The present study measures the pressure distributions and the velocity distributions on inside and outside of dimples, and visualizes the flow on a sphere surface by the oil film method. These experiments clarify the following results. The separation bubbles generates at inside of dimple. These separation bubbles transform the laminar boundary layer into the turbulent boundary layer. The critical Reynolds number of a sphere with dimples shifts to lower Reynolds number side and the separation point shifts to further downstream side compared to that of a smooth sphere. Therefore, the drag coefficient of a sphere with dimples becomes smaller than that of a smooth sphere. Then, the critical Reynolds number shifts to lower Reynolds number side as the depth of dimple becomes deeper. However, the separation point shifts to the upstream side and the drag coefficient becomes larger in the super critical Reynolds number.