(321j) Effects of Drag-Reducing Polymers on the Transition and Growth of Turbulent Coherent Structures

Drag-reducing polymers have profound impact on the dynamics of turbulence, not only in the statistically-converged stage but also in its transient development process. Our interest in this area is motivated by two prominent unsolved problems in viscoelastic turbulence. The first is the so-called maximum drag reduction (MDR), an asymptotic upper limit of polymer-induced drag reduction whose mechanism has eluded decades of investigation. Recent progress in the area increasingly points to the laminar-turbulent transition regime as the key to understanding MDR. The second is the transition to turbulence in viscoelastic fluids itself, where complex and sometimes contradictory observations have been made, suggesting the possibility of multiple transition mechanisms. The pivotal solution object controlling the laminar-turbulent transition, the so-called "edge state", is numerically computed. Direct numerical simulation is then used to track the transient development of these marginally turbulent states into fully developed turbulence. The transition process of Newtonian fluids is characterized by a strong breakdown event where turbulent structures sharply intensify across the channel before their eventual decay into the typical coherent structures in the turbulent basin. Polymer additives are found to effectively suppress these high-intensity vortices, which leads to a different transition pathway where a strong breakdown is bypassed.