(573i) Critical-Layer Mechanisms in Elastoinertial Turbulence

We describe direct numerical simulations (DNS) of channel flow turbulence in a FENE-P fluid. At Reynolds numbers very close to transition, the flow first relaminarizes upon increasing Weissenberg number (Wi) or polymer concentration, but then becomes turbulent again, displaying features of elastoinertial turbulence (EIT). The present work reports computations and analysis that elucidate the structures and mechanisms responsible for sustaining EIT and their ties to predictions from linear theory i.e. linear stability (LSA) and resolvent analyses. In both analyses, the concept of critical-layers, i.e., wall-normal positions where the fluid velocity equals the wavespeed of the linear mode, is important. We show that EIT at low Re has highly localized polymer stress fluctuations that strongly resemble the viscoelastic extension of so-called Tollmien-Schlichting (TS) modes. We also identify the fundamental period (minimal flow unit (MFU)) where EIT can self-sustain and further characterize these fundamental structures.