Numerical investigation on turbulent Flow and scour leading processes around straight dike in a flat bed channel

The present study investigates the physics of the flow and the role of coherent structures of flow in the scouring process, around a vertical dike, located in a straight channel, at the beginning of scouring process (flat bed). Therefore large eddy simulation (LES) method is applied for a flow with a relatively low Reynolds number (Re=18000). The inflow is thoroughly turbulent and full of realistic turbulent fluctuations. After visualizing the flow, it was found that the intensity of the primary horseshoe vortex (HV) is largest at vertical sections around the tip of the dike. In this region, the core of the horseshoe vortex fluctuated between two modes. In one of them (zero-flow mode) the horseshoe vortex gets closer to the dike and the beneath flow jet gets weaker. In the other one (back-flow mode), the jet flow near the bed, takes the horseshoe vortex farther apart from the dike. As a result, in the distribution of turbulent kinetic energy and pressure fluctuations, two peaks are observed. The largest amounts of bed shear stress are present in the acceleration zone of flow near the tip of the dike, upstream of detached shear layer (DSL). The tail of the horseshoe vortex occasionally interacts with the vortex tubes shed in DSL and the tip of the dike. Furtherrmore some of the vortices shed in DSL may sometimes interact with each other or with the vortices present in the recirculation region behind the dike. This leads to amplification of bed shear stress along their path.