The minimal flow unit and origin of two-dimensional elasto-inertial turbulence

The research on elasto-inertial turbulence (EIT), a new type of turbulent flow, has reachedthe stage of identifying the minimal flow unit (MFU). On this issue, direct numericalsimulationsof FENE-P fluid flow in two-dimensional channels with variable sizes areconducted in this study. We demonstrate with the increase of channel lengththat thesimulated flow experiences several different flow patterns, and there existsan MFU for EITto be self-sustained. At Weissenberg number (Wi) higher than the one required to exciteEIT, when the channel length is relatively small, a steadyarrowheadregime (SAR)flowstructure and a laminar-like friction coefficient is achieved. However, as the channel lengthincreases, the flow can fully develop into EIT characterized with high flow drag. Close tothe size of the MFU, the simulated flow behavesintermittently between the SAR state withlow drag and EIT state with high drag. The flow falling back to 'laminar flow' is causedby the insufficient channel size below the MFU. Furthermore, we give the relationshipbetween the value ofthe MFU and the effectiveWi, and explain its physical reasons.Moreover, the intermittent flow regime obtained based on the MFU gives us an opportunityto look into the origin and exciting process of EIT. Through capturing the onset process ofEIT, we observed that EIT originates from the sheet-like extension structure located nearthe wall,whichismayberelatedto the wall mode rather than thecentremode.The fractureand regeneration of this sheet-like structure is the key mechanism for the self-sustainingof EIT.