The three-dimensional structures in turbulent Couette-Poiseuille flows on the verge of separation

This work investigates flow structures in Couette-Poiseuille (C-P) flows with the adverse pressure gradient (APG) adjusted to create zero mean skin friction on the stationary wall. The vortical structures are identified as regions with a positive value of the discriminant of the velocity gradient tensor (Chong et al., 1990; Soria et al., 1994). The data used in current study are from direct numerical simulation (DNS). The enstrophy density and the dissipation of kinetic energy carried by flow structures are instigated by examining the second invariants of the symmetric strain rate tensor S_{i j} and the skew-symmetric rate of rotation tensor W_{i j} (Chong et al. (1998)). Some characterizations of the topological flow structures in the near-wall region are similar to a viscous sub-layer in channel flows (Blackburn et al., 1996).