The intriguing mechanism of drag reduction by dilute polymer solutions

The mechanism of drag reduction by dilute polymer solutions is investigated using results from DNS in turbulent channel flow. Drag reductions of up to 56\%, corresponding to Virk's MDR, are achieved through a most intriguing, energetically insignificant, yet dynamically significant, mechanism. The cornerstone of this mechanism is a redirection of a small fraction, of no more than 5\% on a volume-averaged basis, of the turbulence kinetic energy (TKE) into the elastic energy of the polymer at select turbulent scales. This redirection of energy leads to a decrease in the fluctuating strain-rate at the affected scales, which, in turn, results in a drop in the pressure-strain correlation at these and neighboring scales. The drop in the pressure-strain correlation inhibits the transfer of TKE from the streamwise to cross-stream directions, resulting in a highly anisotropic state at the affected scales. This anisotropy inhibits the normal cascade of TKE to smaller scales. Thus the minute extraction of energy by the polymer initiates a ``self-amplifying'' sequence of events in which turbulence loses its three-dimensionality and the turbulence energy cascade is inhibited. The magnitude of drag reduction is determined by the range of affected scales, which is a function of $We_\tau$ and polymer concentration. For maximum drag reduction, all large scales throughout the channel need to experience the minimal initial extraction of TKE by the polymer.