(577d) Polymer Effects on the Development and Bursting of Turbulent Vortices: Implication on High-Extent Drag Reduction

Two major problems in viscoelastic turbulence, the effects of polymers on the laminar-turbulent transition dynamics and the origin of the maximum drag reduction asymptote, are both better understood in the regime near the margin of turbulence. Direct numerical simulation trajectories initiated from the edge state are used to follow its unstable manifold into the turbulent basin. In Newtonian flow, the growth of turbulence starts with the intensification of velocity streaks and a sharp rise in the Reynolds shear stress. It is followed by a quick breakdown into high-intensity small-scale fluctuations before entering the core of turbulence. This breakdown process not only occurs during the transition but also shows up in steady-state turbulence as intermittent bursting events. Adding drag-reducing polymers does not affect the initial growth of turbulence but stabilizes the primary streak-vortex structure. As a result, the breakdown stage is circumvented. Throughout the process, polymers act in reaction to the growing turbulence and do not drive the instability. Before the MDR regime, there is also a transition between low-extent and high-extend drag reduction (LDR vs. HDR) where qualitative differences are observed in flow statistics. Its existence indicates that there are at least two primary mechanisms for drag reduction. Our research shows that the second one, which does not occur until the LDR-HDR transition, results from the suppression of bursting and the corresponding shift of vortex-sustaining mechanism. Understanding this transition is important for achieving high-extend drag reduction in flow control practices.