Mechanism of drag reduction by a surface trip wire on a sphere

The effect of a surface trip wire on the flow around a sphere is experimentally investigated at subcritical Reynolds numbers of Re = 0.5x10(5)-2.8x10(5) based on the free-stream velocity U(infinity) and sphere diameter d. By varying the streamwise location (20 degrees-70 degrees from the stagnation point) and diameter (0.33x10(-2) < k/d < 1.33x10(-2)) of a trip wire, we measure the drag, surface pressure distribution and boundary layer velocity profiles above the sphere surface, and conduct flow visualization. Depending on the size and streamwise location of the trip wire, three different flow characteristics are observed above the sphere surface. For low Reynolds numbers, the disturbance induced by the trip wire decays downstream and main separation occurs at a streamwise location similar to that of a smooth sphere. As the Reynolds number is increased, laminar separation is delayed farther downstream by the disturbance from the trip wire and the transition to turbulence occurs along the separated shear layer, resulting in the flow reattachment to the sphere surface and thus forming a secondary separation bubble on the sphere surface. Then, the main separation is delayed due to high momentum near the surface and the drag is significantly reduced. When the trip wire produces even larger disturbances through the separation and reattachment right at the trip-wire location for higher Reynolds numbers, the boundary layer flow becomes turbulent soon after the trip-wire location and the main separation is delayed, resulting in drag reduction.