Hairpin vortex dynamics and polymer-induced turbulent drag reduction

It has been known for over six decades that the dissolution of minute amounts of high molecular weight polymers in wall-bounded turbulent flows results in a dramatic reduction in turbulent skin friction by up to 70%. While direct numerical simulation of turbulent flow of dilute polymer solutions predicts drag reduction (DR), dynamical interactions between the coherent structures present in turbulent flows and polymer conformation field that could lead to DR are not clearly understood. This is investigated by self-consistent dynamical simulations that track the evolution of hairpin vortices, i.e., counter-rotating pairs of quasi-streamwise vortices whose nonlinear auto-generation, growth, decay and break up are centrally important to turbulence (Reynolds) stress production. It is shown that auto-generation of new vortices is suppressed by the polymer stresses, thereby decreasing the coherent as well as incoherent Reynolds stress.