Effects of Dimples for Drag and Lift on a Sphere with Rotation

Abstract

The present study clarifies the effects of dimples for drag and lift on a sphere with rotation. The sphere with dimples has 328 arc type dimples on its surface uniformly. The present study measures velocity distribution in wake and pressure distribution on a sphere surface, and visualizes flow around a sphere by the spark tracing method for the case in which Reynolds number Re=0.4×105-1.3×105. For the case of a smooth sphere, separation point on deceleration side shifts downstream remarkably as spin rate ratio (circumferential velocity of a sphere / free stream velocity) increases. The pressure distribution on deceleration side decreases compared to that on acceleration side. The wake becomes small and shifts opposite direction of rotation. Therefore, the drag reduces, and the lift becomes negative. For the case of a sphere with dimples, this phenomenon occurs for the case in which critical Reynolds number Re=0.5×105. The separation point on acceleration side shifts downstream, and the separation point on deceleration side shifts upstream as spin rate ratio increases for the case in which super critical Reynolds number Re=1.3×105. The pressure distribution on acceleration side decreases compared to that on deceleration side. In addition, the pressure distribution on outside dimple decreases compared to that on inside dimple. The wake becomes large and shifts direction of rotation. Therefore, the drag and positive lift increase.