(186y) Drag Reduction of Viscoelastic Turbulent Channel Flows In Minimal Flow Units

Near-wall turbulent flow motions are characterized by recurrent coherent flow patterns. With the addition of small amount of polymer molecules, these coherent structures are modified and the friction drag of the turbulent flow can be significantly reduced. In this study, we conduct direct numerical simulations (DNS) in the minimal flow unit (MFU) of viscoelastic channel flows in which turbulence is able to sustain. These MFUs well capture the key components of the self-sustaining process in turbulent flows. Specifically, we focus on moderate values of Re which are not far above the transitional Re. Our earlier study of the traveling-wave solutions in viscoelastic turbulent flows predicts that all key phenomena in drag reduction, from its onset to the maximum drag reduction (MDR), can be observed in a limited parameter regime near the laminar-turbulence transition. With this MFU approach in the near-transition regime, we study the dynamics of the viscoelastic turbulent flows, especially the effects of polymers on the recurrent coherent structures, based on which the mechanism of drag reduction can be investigated. Special attention will be given to MDR, where an upper limit of drag reduction is reached.