On the spatio-temporal dynamics of turbulent flow separation over a fence with a splitter plate in uniform flow

The spatial and temporal dynamics of separated flow induced by a fence with a splitter plate in uniform flow at moderate Reynolds numbers were investigated using time-resolved particle image velocimetry. The Reynolds numbers, based on the free-stream velocity and the fence height above the splitter plate, were 2700, 5500 and 7500. The mean separated flow is characterised by a large primary clockwise recirculating vortex above the splitter plate and a smaller counter-rotating secondary vortex in the corner of the fence and splitter plate. Guided by different vorticity thickness growth rates, the separated shear layer is partitioned into four regions, whose dynamics are governed by the initial roll-up, shear layer stabilisation, wall-induced large-scale vortex breakdown and post-reattachment recovery. Streamwise momentum transport, entrainment velocity, convective velocity, spectral proper orthogonal decomposition, as well as spatial autocorrelation and whole-field integral time scales are used to examine the dynamics of the shear layer across these four regions. Spectral analysis reveals a distinct frequency migration along the shear layer, wherein the Kelvin–Helmholtz frequency near the fence transitions to the vortex shedding frequency near reattachment through successive vortex pairing events. Analysis of instantaneous reverse flow areas demonstrates that the second half of the primary vortex dominates the global unsteadiness of the separation bubble, whereas the first half exhibits strong coupling between the primary and secondary vortices. Conditional averaging analysis indicates that flapping motion significantly modulates turbulence production, however, the dynamic importance of the mean velocity gradients and Reynolds stresses varies across the four regions.