Coherent structure dynamics upstream of a long rectangular block at the side of a large aspect ratio channel

Chrisohoides and Sotiropoulos [Phys. Fluids 15, L25 (2003)] recently developed an experimental technique for visualizing and extracting the time scale of coherent Lagrangian structures in turbulent, free-surface flows. They applied their technique to visualize the rich dynamics of the flow in the upstream recirculating region of a long, rectangular, surface-piercing obstacle mounted on the side wall of a shallow open channel at a Reynolds number of 4.2×10⁵ and a Froude number of 0.35. In this paper, detached eddy simulation is carried out for a similar geometrical configuration but using the flat, rigid-lid (zero Froude number) assumption to model the free surface in order to verify and explain for the first time the experimental observations. The calculated flow at the surface in the recirculating zone is found to be dominated by multiple, slowly evolving, large-scale eddies whose structure, dynamics, and Lagrangian time scales are shown to be in good agreement with those reported in the experiment at the free surface. Analysis of the computed solutions shows that the resolved flow in this region is quasiperiodic and that its spatial and temporal richness is due to the complex interaction of the slowly moving recirculating flow and the shear layer that forms at its interface with the fast-moving outer flow. The computed solutions also provide the first comprehensive insight into the highly three-dimensional structure of this flow.