Could hairpin structures in the polymer stretch field be the link between elastic turbulence and elasto-inertial turbulence?*

For polymer flows, Elasto-inertial turbulence (EIT) and elastic turbulence (ET) are both turbulent states in wall-bounded flows and are driven by polymer dynamics. So far, it has been assumed that ET needs curved streamlines but exists in inertialess flows, whereas EIT may happens in parallel wall-bounded flows for Re>1 to super-critical Reynolds numbers. Using specific initial boundary conditions, 2D simulations of polymer additives in a channel flow based on the FENE-P model reveal peculiar structures in the polymer stretch field. At higher Reynolds numbers, the structures look like hairpin. The pressure signature of the hairpin structure is a bow shock-like structure at the center of the channel. The hairpin shape is a consequence of the flow's inertia. The structure has a very long time scale. At lower Re, the polymer stretch is still structured into thin sheets, where polymers are highly stretched. These sheets, like the hairpin structure, occupy the core of the flow.