Maximum Dissipation Reduction in Bulk Polymeric Turbulence

We present an experimental study of the flow regime and vortex structure in bulk polymeric turbulence through three-dimensional velocity measurements with unprecedented high spatial resolution. We demonstrate that, as the polymer concentration increases, the effect of polymers on the fluid energy dissipation rate can be categorized into two distinct regimes: the dissipation reduction regime and the maximum dissipation reduction (MDR) regime, and in the latter the effect of the polymers saturates. We reveal that the MDR regime occurs when the scale range affected by the polymer additives extends up to the integral length scale, rather than the generally accepted Lumley scale. This finding enables us to map out the phase diagram of the flow regimes of polymeric turbulence on the Reynolds number and Weissenberg number. Furthermore, we demonstrate that in the MDR regime, sheetlike vortex and strain structures dominate, which is traced to the fact that polymer stress stabilizes shear layers and reduces perturbation to shear instability. These results suggest that the MDR regime is a universal flow state in bulk polymeric turbulence and is the counterpart of the maximum drag reduction regime in wall-bounded flows. Our findings emphasize the significance of sheetlike vortex structures and offer new insights into predicting the critical conditions for the emergence of various flow regimes in viscoelastic fluids.