The role of vortex stretching in drag reduction of polymer-laden turbulent flow

An addition of polymers can significantly reduce drag in wall-bounded turbulent flows, such as pipes or channels. This phenomenon is accompanied by a noticeable modification of the mean-velocity profile. Starting from the premise that polymers reduce vortex stretching, we derive a theoretical prediction for the mean-velocity profile. After assessing this prediction by numerical experiments of turbulence with reduced vortex stretching, we show that the theory successfully describes experimental measurements of drag reduction in pipe flow.