Bed roughness effect on flow separation beneath partially submerged simulated ice cover in a shallow channel

Abstract

The effect of bed roughness on shear layer separation and coherent structures beneath a partially submerged cover in a shallow channel is evaluated. A planar particle image velocimetry system is used to conduct detailed instantaneous velocity measurements beneath the partially submerged simulated ice cover. The results indicate that roughness influences near-wall turbulence, whiles the separated shear layer dominated the flow dynamics close to the undersurface of the cover. The instantaneous velocity field shows elongated separated shear layer underneath the cover for flow over the smooth bed compared to the rough bed. The bed roughness contributed to a reduction in size of the recirculation bubble at the undersurface of the cover. The instantaneous size of the recirculation bubble shows expansion and contraction of the separated shear layer when compared to the mean bubble size, depicting intense shear layer flapping at the undersurface of the cover, and this is dominant for the smooth bed flow. Close to the leading edge of the cover, the instantaneous spanwise vorticity magnitude shows dominance of small-scale instabilities akin to the Kelvin-Helmholtz type instability at interface of the separated shear layer. The separated shear layer generated large-scale vortices of varying length scale when compared to the bed roughness. Although bed roughness promoted near-wall turbulence with elevated levels of Reynolds stresses compared to the smooth bed, at the undersurface of the cover, the high levels of stresses were due to shear layer separation. A wide range of integral length scales are estimated within the separated shear layer, which contributed significantly to the generation of the Reynolds stresses.