臨界レイノルズ数付近の球に働く流体力の挙動

抄録

Aerodynamic force acting on a smooth sphere in a uniform flow was investigated near the critical Reynolds number. Drag, lift and side forces acting on the sphere and the pressure distibution on its surface were measured in a wind tunnel. The flow around the sphere was also visualized by a smoke wire method. In the range of the critical Reynolds number, steady lift and side forces generate with a sudden drag reduction, because a laminar-turbulent transition occurs partially around the sphere and the flow becomes asymmetric in respect of the flow direction. When the Reynolds number is slightly larger or smaller than the critical Reynolds number, the flow around the sphere becomes unstable. Irregular and unstable lift and side forces appear, although their time-averaged values are zero. Their trajectories with time show random, bistable or arc-like patterns. In the range of the super critical Reynolds number, a pair of streamwise vortices appears in the wake of the sphere. Since the vortex pair rotates clockwise or counterclockwise randomly around the streamwise axis of the sphere, the direction of the resultant force of lift and side forces rotates at random.