Coherent structures at different contraction ratios caused by two spill-through abutments

Detached eddy simulation (DES) at a channel Reynolds number of 45,000 is used to investigate the coherent structures forming around spill-through abutments located at opposite sides of a rectangular channel which contract the flow section. Changes in the coherent structures are investigated for two contraction ratios at two different scour stages. It is observed that at the small contraction ratio the coherent structures forming around the abutments are similar to the ones observed for isolated abutments. However, at the larger contraction ratio there are important changes in these structures. At the beginning of the scour process two counter rotating contraction vortices form close to the bed at the centre of the channel, oriented in the streamwise direction. At this initial scour stage, the upstream part of the main horseshoe vortex is incoherent. On the other hand, at a later stage of scour with the development of the scour hole contraction vortices disappear and the main horseshoe vortex regains its original shape.