Elasto-inertial turbulence in straight pipes at low Reynolds numbers

As demonstrated recently, polymers delay transition and can for selected parameters even fully relaminarize turbulent flow. At high polymer concentration however a different fluctuating state emerges, “elasto-inertial turbulence” (EIT), and it is only in this limit that the flow assumes the characteristic friction factor values corresponding to the "maximum drag reduction asymptote". While Newtonian turbulence at onset is spatio-temporally intermittent and requires finite amplitude perturbations, EIT appears globally and sets in at a critical polymer concentration (corresponding to a critical Weissenberg number) regardless of the flow being subject to external perturbations or not. We demonstrate that EIT even occurs in pipes at Reynolds numbers of the order 1 provided that the Weissenberg number is sufficiently large. We additionally discuss possible connections to the case of “purely elastic turbulence”, found at even lower Re in flows with curved streamlines; this has recently been reported also for channel flow however requiring a complicated triggering mechanism. In addition to the nature of the transition scenario we also investigate structures and spectra of EIT and compare them to those found in purely elastic turbulence.