Identification of coherent structures in the flow field around a cylindrical pier using Finite Time Lyapunov Exponents*

Local scouring around a bridge pier is one of the most common reasons for bridge failure. The flow around a bridge pier is three-dimensional. It contains different flow features, such as down flow in front of the pier, vortex formation at the upstream base of the pier, scour hole at the bottom of the pier and vortex shedding in the pier downstream. The interaction among all these features makes the 3D flow around a bridge pier very complex. The coherent structures around the pier play a vital role in forming the scour hole and transporting sediment from scour hole to the pier downstream. Hence, the accurate prediction of these coherent structures and their interactions is necessary to understand the scouring mechanism better. But, the current methods fail to accurately extract the coherent structures from the velocity field.

Lagrangian coherent structures (LCS) are the ridges of Finite Time Lyapunov Exponents (FTLE). LCS is used to understand the overall flow geometry and material transport. These structures separate the regions with different dynamical behaviour in the flow and extract the geometry which is often hidden in the velocity field. Therefore, these structures often provide an excellent tool for understanding the behaviour of time-dependent systems, especially for understanding particle transport. Since the LCS govern the particle transport, it may even influence the sediment moment and hence the formation of the scour hole.

The present study carried out a 3D Reynolds Averaged Navier Stokes (RANS) simulation with a k-ω SST turbulent model to simulate the velocity field around the pier with different stages of scour hole (initial flatbed, intermediate scour hole and equilibrium scour hole). The Lagrangian Coherent Structures (LCS) are extracted from the velocity field as the ridges of the FTLE field. The obtained LCS accurately represented the three-dimensional coherent structures formed around the pier and the interactions among them.