Development of flow structures over dimples

Dye flow visualization conducted for dimples with depth to diameter ratios ranging from 5% to 50% show six different flow stages as the Reynolds number varies from 1000 to 28000. The flow in stage I is fully attached with streamlines that curve towards and then away from the dimple centerline. In stage II, a separated flow region appears in the upstream half of the dimple. In stage III, a pair of counter-rotating vortices connected by a vortex line forms within the dimple. One of these vortices grows and dominates the flow in stage IV. Its rotational direction is fixed and its axis remains tilted from the vertical. In stage V, the axis of this vortex becomes vertical and its rotational direction switches randomly. The flow pattern in stage VI depends on the dimple depth to diameter ratio, though the mean flow is always symmetric about the dimple centerline. This flow development can be achieved either by changing the Reynolds number or dimple depth to diameter ratio. Not all six of these development stages occur for all dimples. Dimples with low depth to diameter ratios show fewer development stages than deeper ones. Sharp and round edged dimples both show similar behavior. (C) 2013 Elsevier Inc. All rights reserved.