Lubricating layer in drag-reducing solutions of rigid polymers*

Existing theories on the mechanism for polymer drag-reduction (DR), imply that the phenomenon typically relies on the solution being viscoelastic. While most solutions of flexible polymers exhibit pronounced viscoelasticity, rigid polymer solutions do not. Solutions of rigid polymers have been shown to demonstrate larger viscosities and a noticeable shear-thinning trend, well approximated by the Generalized Newtonian models. The aim of the present investigation was to explore how the spatial gradient of mean viscosity in the wall-normal direction affects DR. Experiments in a turbulent channel flow were performed using a solution of xanthan gum with varying Reynolds number (Re). Quantities of DR grew from 20% to 32% with increasing Re. Planar particle image velocimetry and steady shear viscosity measurements were used to characterize the velocity and shear rheology of the fluid. Normalized mean velocity profiles overlapped in the inner layer for the different Re scenarios, despite unique DR values. We observed different mean viscosity profiles across the channel for each DR case. All profiles demonstrated a thin low-viscosity layer in the vicinity of the wall. This "lubricating layer" is proposed to play a major role in drag-reduction using rigid polymers.