Large Eddy Simulations of convergent–divergent channel flows at moderate Reynolds numbers

•TKE budgets are similar at separation/re-attachment points at different Reynolds numbers.•At separation points, TKE budget is essentially composed of 〈u′u′〉 normal stress.•At re-attachment point, TKE budget is mainly composed by 〈u′u′〉 and 〈w′w′〉 stresses.•Streak bursting is associated with the presence of adverse pressure gradient regions.•Flow separation enhances the effects of the streak bursting mechanism.

The paper is focused on the study of fully turbulent channel flows, using Large Eddy Simulations (LES), in order to address the effects of adverse pressure gradient regions. Analyses of the effects of streak instabilities, which have been shown to be relevant in such regions, are extended to moderate Reynolds numbers. The work considers two different channel geometries in order to further separate influences from wall curvature, flow separation and adverse pressure gradients. Turbulent kinetic energy and Reynolds stress budgets are investigated at separation and re-attachment points. The numerical approach used in the present work is based on the incompressible Navier–Stokes equations, which are solved by a pseudo-spectral methodology for structured grids. Wall-resolved LES calculations are performed using the WALE subgrid scale model. The study shows that the streak instability mechanism persists at higher Reynolds numbers with and without wall curvature in the adverse pressure gradient regions. Moreover, the observed effects are also present regardless of the existence of flow separation regions. Finally, the study of turbulent kinetic energy budgets indicates that, independently of the flow condition, there are well-defined patterns for such turbulent properties at separation and re-attachment points.