Large-eddy simulation of the flow in an S-duct

Large-eddy simulations of the flow in an S-shaped, two-dimensional duct were conducted using a Lagrangian dynamic eddy viscosity subgrid-scale model and a non-orthogonal grid system. The simulations were performed at Re _b = 13,800 and 30,800, using up to 13.4 million grid nodes. The results show that two factors affect the boundary layer: the concave or convex wall curvature (which, respectively, enhances or dampens the turbulent motions) and the favourable or adverse pressure gradient in the regions of curvature change, which accelerates or decelerates the boundary layer. In the presence of an adverse pressure gradient the boundary layer can separate intermittently, which also enhances the turbulent motions. The strongest separation occurs between the two curves and affects the flow not only in the following curve but also in the recovery region. The flow near the walls displays a logarithmic behaviour, but its slope and intercept vary with curvature and pressure gradient. Taylor–Görtler vortices are observed near the concave surfaces; they are responsible for strong organization of the streamwise and wall-normal velocity fluctuations, and contribute significantly to the Reynolds stresses. The turbulent kinetic energy budgets show that the production and dissipation are similarly affected by the wall curvature as the turbulent motions: they are increased by the concave wall and decreased by the convex. The mean flow advection transports turbulent kinetic energy from the region with concave curvature to the region with convex curvature.