Coherent structures in the flow around a pier group with scour hole

Local scouring around a pier group is more complex than the one around a single pier. This study used numerical simulations to investigate the influence of coherent structures and their interactions on the development of scour hole around a pier group. The obtained results are validated with previous experimental findings. The simulations revealed that, in addition to the horseshoe vortex (HSV), other secondary vortices are present inside the scour hole. The interactions between them significantly changed the size and coherence of the HSV. The independent vortex interactions at the left and right piers led to differences in scour depth around those piers. The instantaneous velocity histograms revealed bimodal aperiodic oscillations in the HSV. The lateral movement of the detached legs of the HSV was found to be one of the reasons for the lateral expansion of the scour hole. The coherent structures are also extracted using a Lagrangian method called the Lagrangian coherent structures (LCSs). The obtained LCSs extracted the shear layers formed upstream of the pier group. They also revealed the presence of anchor-like structures upstream of the pier group within the scour hole. This study identified new mechanisms contributing to the formation of scour holes around pier groups.