Evolution of coherent structures in the flow around a circular pier with a developing scour hole: A numerical study

Large-eddy simulations (LES) were used to simulate the flow features around a circular pier with evolving scour hole. The bathymetry corresponding to four stages of scour hole evolution, published in the previous literature, was used in the flow domain to recreate the flow at a particular stage of scour hole development. The growth of coherent structures and their interactions in a developing scour hole were investigated. The formation of secondary vortices inside the scour hole and their interactions with the primary horseshoe vortex (HV) made the HV system highly dynamic. High-level pressure fluctuations were observed inside the core of HV. The simulation captured the bimodal aperiodic oscillations in the HV at all stages of scour hole development. The mechanisms responsible for the formation of scour hole behind the pier were discussed. High levels of mean and instantaneous bed shear stress values were observed beneath the horseshoe vortex. These values were found to have decreased with an increase in the scour hole depth. The LES also captured the detachment of vortices from the legs of HV and their convection in the pier downstream.