Drag Reduction in Homogeneous Turbulence by Scale-Dependent Effective Viscosity

The phenomenon of drag reduction by polymer additives had been studied in simulations on the basis of non-Newtonian fluid mechanical models that take into account the field of polymer extension (conformation tensor) and its interaction with the velocity field. Drag reduction was found in both homogeneous and wall bounded turbulent flows. In the latter case it was shown recently that the notion of scale-dependent effective viscosity allows quantitative predictions of the characteristics of drag reduction in close correspondence with experiments. In this paper we demonstrate that also drag reduction in homogeneous turbulence is usefully discussed in terms of a scale-dependent viscosity. In other words, the essence of the phenomena under study can be recaptured by an "equivalent" equation of motion for the velocity field alone, with a judiciously chosen scale-dependent effective viscosity that succinctly summarizes the important aspects of the interaction between the polymer conformation tensor field and the velocity field. We will also clarify here the differences between drag reduction in homogeneous and wall bounded flows.