UNS: Origins of maximum drag reduction in viscoelastic turbulence

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The proposed research is focused on the investigation of drag reduction that occurs in turbulent flows and on the reason for its occurrence. While this is a very important fundamental problem in turbulent flows, it also has very significant application areas in energy costs for the transportation of fluids across long distances. An important technical problem in transporting fluids at high rates or when a solid body travels through a fluid (e.g., an air or naval vessel) is to reduce the fluid drag. In the case of internal flows (i.e., inside conduits), turbulent drag can be dramatically reduced by low levels of polymer or surfactant additives. While this effect is well-known, there is still debate about how it happens. A feature of turbulent drag reduction is the existence of a so-called "maximum drag reduction" (MDR) asymptote. For a given flow geometry at a given pressure drop, there is an asymptotic flow rate that can be achieved through addition of polymers. Changing the concentration, molecular weight or even the chemical structure of the additives has no effect on this asymptotic value -- it is universal. Prior results from the PI's group have showed that viscoelasticity serves to suppress active turbulence and gives rise to a less active, intermittent turbulence state which is termed 'hibernating'. The main hypothesis of the proposal is that this hibernating turbulence state is the one that is responsible for the occurrence of the MDR asymptote. The proposed research will explore turbulent flow of viscoelastic fluids in minimal channel turbulence at high Weissenberg number (the dimensionless Weissenberg number is the ratio of the fluid relaxation time scale to the characteristic time scale of the flow), and in so doing possibly also gain insight regarding near-wall turbulence production in Newtonian flows. The PI would collaborate with a group at the University of Liverpool to perform experiments and he will also investigate the validity of the minimal turbulence cell to large domains via large scale direct numerical simulations.