Coherent Structure Variations in the Flow around a Square Pier with an Evolving Scour Hole

Coherent structures play a significant role in the formation and growth of scour holes around bridge piers. The present study used large eddy simulations (LES) to investigate the variations of coherent structures at different stages of scour hole evolution and the role of their interactions on the scour hole development. The study revealed that the interactions of secondary vortices with the primary horseshoe vortex (PHV) significantly changed the size and shape of the PHV and amplified the pressure fluctuations inside the scour hole. The LES captured bimodal aperiodic oscillations in the PHV. The interactions between the legs of the PHV with the separated shear layers (SSLs) were found to be one of the reasons for the lateral expansion of the scour hole. The separation of shear layers very close to each other near the bed surface significantly amplified the bed shear stress behind the pier and was responsible for the growth of the scour hole in the pier downstream. The formation of a U-shaped elongated vortex was observed downstream of the pier, and it was found to be responsible for the transport of sediment behind the pier. The study also reported variations in coherent structures as the pier shape changes from circular to square. The information reported in the study can be used to improve the scour depth prediction methods.