Abstract
The effect of bed roughness on shear layer separation and the structure of turbulence in a shallow channel is evaluated. A planar particle velocimetry system is used to conduct detailed instantaneous velocity measurements beneath the simulated ice cover. The results show that although surface roughness modifies near-wall turbulence, once shear layer separation occurs, it becomes the controlling parameter of turbulence for flow shallow channels. The instantaneous velocity field show elongated separated shear layer underneath the cover for flow over the smooth bed compared to the rough bed. For the current shallow channel, the bed roughness significantly reduced the size of the separation bubble at the undersurface of the cover. The instantaneous size of the separated bubble expands and contracts 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 Q-criterion and swirling strength revealed that separation of the shear layer generated large-scale vortices of varying length scale when compared to the bed roughness. The bed roughness promotes near-wall turbulence with elevated levels of Reynolds stresses compared to the smooth bed. However, at the undersurface of the cover, the high levels of turbulence were controlled by the flow separation. Compared to the bed roughness, a wide range of integral length scales are estimated within the separated shear layer, which contributed significantly to the generation of Reynolds stresses.