LES of turbulence around a scoured bridge abutment

Abstract

Local scour phenomena around bridge abutments and piers can induce the collapse of hydraulic structures (Cardoso and Bettess, 1999). Understanding of the erosion process is required for the a-priori estimation of the scour-hole geometry. The problem of local erosion is particularly difficult since a complete modeling of the process needs to take into account several phenomena ranging from fluid mechanics to river geomorphology. The turbulence characteristics of the incoming flow field (Sumer, 2007), the dynamics of the coherent structures that forms in junction flows (Simpson, 2001) and the sediments motion which is characterized by large intermittency (Radice et al., 2009), are processes that have to be properly considered. The first step of the present research is aimed at the analysis of the coherent structures dynamics around the bridge abutment and how they change in different scour conditions. In fact many authors found out that the lack in understanding these processes is one of the motivation of the incapability of the models to accurately predict the scour-hole geometry and its maximum depth (Ahmed and Rajaratnam, 2000). The second step is aimed at understanding how the coherent structures and their dynamics can influence the scouring process. It is important to know how the fluctuations and the intermittent character of the vortical structures can be involved in the sediment transport and to single out the most important forces that can destabilize the sediments since a clear view of the incipient motion is still missing. This study focuses on the analysis of the turbulent field around a 45^○ wing-wall bridge abutment at different phases of the scour phenomenon.