Modification of hairpin auto-generation process in turbulent channel flows due to polymer stress

To study the influence of dynamical interactions between turbulent vortical structures and polymer stress on turbulent friction drag reduction, a series of simulations were performed for channel flow at Re#X003C4; =395. The initial eddy extracted by the conditional averages for the Q2 event from fully turbulent Newtonian flow is self-consistently evolved in the presence of polymer stresses by utilizing the FENE-P model (finitely extensible nonlinear elastic-Peterlin). The initial polymer conformation fields are given by the solutions of FENE- P model equations for the Newtonian mean shear. For a relatively low Weissenberg number, defined as the ratio of fluid relaxation time to the time scale of viscous diffusion, (We = 50) the generation of new vortices is inhibited by polymer-induced counter torques, which results in fewer vortices in the buffer layer. However, the head of primary hairpin is unaffected by the polymer stress. For larger values of We(#X022DB;100), the hairpin head becomes weaker and vortex auto-generation and Reynolds stress growth are almost entirely suppressed.