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 for a sphere with arc type dimples. The sphere has 328 dimples of different depths uniformly distributed on its surface. The present study measured the pressure and velocity distributions inside and between the dimples, and visualized the flow on the sphere surface by an oil film method. The results indicated that separation bubbles were generated inside the dimples and transformed a laminar boundary layer into a turbulent boundary layer. Compared to a smooth sphere, the critical Reynolds number decreased and the separation point shifted further downstream. Therefore, the drag coefficient of a dimpled sphere was smaller than that of a smooth sphere. The magnitude of the decrease in the critical Reynolds number was found to increase with dimple depth. However, the separation point shifted to the upstream side and the drag coefficient became larger in the super-critical Reynolds number region.