Unsteadiness in turbulent separated flow over a three-dimensional Gaussian hill

The turbulent separated flow over the three-dimensional Gaussian `Boeing’ speed-bump benchmark geometry exhibits phenomena across a wide range of frequencies. This talk aims to characterize the unsteady dynamics in the separated zone, and to explore the streamwise and spanwise three-dimensional separation topology. High-rate surface-pressure measurements are used to identify the dominant frequencies, while planar two-component particle image velocimetry is employed to elucidate the wake structure along multiple planes.

The flow is dominated by four key frequencies at Re = 226,000 based on the bump height. Two low frequencies are associated with motions of the separation front, while a third, higher frequency results from shear layer vortex shedding observed only along the centreline symmetry plane. Notably, a very low-frequency side-to-side spanwise oscillation of the recirculation zone is observed. The Strouhal number of this oscillation is similar to those seen in other hill-type geometries, but the Bump flow does not exhibit the random switching instability reported for these flows. Proper orthogonal decomposition reveals the anti-symmetric spanwise oscillation interacting with other symmetric modes governing the streamwise length of the recirculation zone. Moreover, the mean field shows that the separated streamsurface does not reattach back onto the wall; instead, a second separation event occurs downstream with centreline upwash.