A time-resolved DPIV study of the unsteady character of the flow over a surface-mounted prism

Abstract Time resolved digital particle image velocimetry is used to quantify mean and instantaneous flow parameters in the separated flow over a surface mounted prism. Two-dimensional measurements at the mid-plane of the prism of the mean velocity magnitude, reverse coefficient, and vorticity are presented. The instantaneous variations of the vorticity field are also presented, giving quantitative proof of the extent of the unsteady character of the flow. The mean flow results show variations of the edge of the shear layer and the separation bubble as the Reynolds number is increased from 2500 to 16 500. The instantaneous analysis showed a sequence of events that starts with the shear layer becoming unstable and an associated vortex rollup. The interaction of this vortex with the wall forms a reverse flow in the boundary layer on the topside of the prism and generates opposite sign vorticity. A comparison of the mean and instantaneous flow characteristics shows that the flow undergoes significant variations with time such that the concept of a mean flow that consists of a shear layer, a separation bubble and reattachment may be of limited physical significance.