Elasto-inertial turbulence and the nature of the maximum drag reduction asymptote

Addition of long chain polymers in small quantities has proved to be highly effective in reducing friction losses in turbulent flows. The drag reduction measured is known to have an upper limit, characterized by the maximum drag reduction (MDR) asymptote. The MDR state is often identified with a marginal state of inertial turbulence, which cannot be surpassed by further addition of polymers. We visualize the flow in pipe in the high drag reduction limit, and show that flow structures characterizing this state are indicative of a mechanism distinct from that of classical turbulence. We show that in the MDR limit the flow is dominated by azimuthally extended vortex sheets. The same vortex sheets are identified in low Reynolds number flows (Re<1500) in elasto-inertial turbulence (EIT). In addition we demonstrate that the energy spectra associated MDR at high Reynolds numbers match those of EIT at low Re, and are distinct from the spectra of inertial turbulence. Based on these observations we argue that flows in the asymptotic drag reduction limit are dominated by EIT and that the familiar MDR friction factor scaling is the characteristic frictional drag associated with elasto-inertial turbulence.