(342g) Dynamics and Flow Structures of the Laminar-Turbulent Edge State for Understanding Polymer Drag Reduction

Recent advances in viscoelastic turbulence research indicate that answers to some most prominent questions in this field are likely held in the laminar-turbulent transition region. Questions such as the maximum drag reduction (MDR) -- the universal upper limit for polymer drag reduction, transition to turbulence and onset of drag reduction (DR) have remain unsolved after decades of efforts. Although the exact nature of MDR is still unclear, it becomes increasingly apparent that MDR is probably dominated by a class of weak or marginal turbulent states, characterized by significantly lower levels of turbulent activity, drastically reduced friction drag, and distinctly different flow features than Newtonian turbulence. The current study focuses on the edge state, a pivotal (both literally and figuratively) point in the dynamics of laminar-turbulent transition. Edge states are computed for various parameters (Reynolds number, Weissenberg number, and box size) and their flow statistics, structure and dynamics are analyzed. These states show a strongly shear-dominated flow pattern that is distinct from Newtonian turbulence and closely resembles MDR. Polymers are not significantly extended at moderate Weissenberg numbers. As a result, flow statistics do not notably depend on viscoelasticity -- the defining characteristic of MDR. Dynamics of edge states are much less chaotic than regular turbulence and are normally dominated by a few key states. This systematic investigation into the edge state paves way for further exploration of the near-transition regime, which holds the key to solving the mystery of MDR as well as interpreting the conflicting observations of the laminar-turbulent transition in polymer solutions.