(257b) Asymptotic Drag Limits in Turbulent Taylor-Couette Flow of Dilute Polymeric Solutions

It is well known that adding a minute amount of long-chain flexible polymer leads to dramatic drag reduction (DR) in turbulent channel and pipe flows. Moreover, our earlier studies show the existence of an asymptotic maximum drag enhancement flow state in a wide-gap turbulent viscoelastic Taylor–Couette flow (TCF). This intriguing asymptotic flow state arises due to creation and eventual saturation of small-scale elastic and inertio-elastic Görtler vortices in the inner- and outer-wall regions, respectively. Since these vortical structures arise due to competing effects of polymer-induced stresses that either suppress or promote turbulent vortices, one should be able to use elastic forces to realize the maximum drag reduction (MDR) state in viscoelastic TCF. To that end, we performed extensive direct numerical simulations over a wide range of Reynolds numbers (1500~8000) in a low-curvature small-gap TCF at Wi of O(100) with the FENE-P model with maximum chain extensibility of 50-200. As expected, we demonstrate the existence of the MDR asymptote in a flow composed of elastically modified small-scale turbulent vortices and large-scale Taylor vortices. The MDR state realized is distinct from those in channel and pipe flows due to its moderate to high DR, i.e., <60%. A careful examination of the flow structures reveals that this difference is due to the persistence of large-scale vortices that transport angular momentum efficiently and stretch polymers dramatically. Similar to other drag-limiting states, both inertial and elastic forces are important in sustaining turbulent flows, especially for larger polymer chain lengths. Hence, the MDR asymptote reported here belongs to the elasto-inertial turbulence regime. Overall, this study provides concrete evidence for our earlier postulation that polymer-induced asymptotic behaviors are an inherent property of viscoelastic wall-bounded turbulent flows.