Exceeding the asymptotic limit of polymer drag reduction

Abstract

The drag of turbulent flows can be drastically decreased by addition of small amounts of high molecular weight polymers. While drag reduction initially increases with polymer concentration, it eventually saturates to what is known as the maximum drag reduction (MDR) asymptote; this asymptote is generally attributed to the dynamics being reduced to a marginal yet persistent state of subdued turbulent motion. Contrary to this accepted view we will show in the following that for an appropriate choice of parameters polymers can reduce the drag beyond the suggested asymptotic limit, eliminating turbulence and giving way to laminar flow. However at higher polymer concentrations the laminar state becomes unstable, resulting in a fluctuating flow with the characteristic drag of the MDR asymptote. The asymptotic state is hence dynamically disconnected from ordinary turbulence.Comment: 6 pages, 6 figure