Near-wall lubricating layer in drag-reduced flows of rigid polymers

Abstract

The current theories on the mechanism for polymer drag-reduction (DR) are generally applicable for long-chain flexible polymers that form viscoelastic solutions. Rigid polymer solutions that generate DR seemingly lack prevalent viscoelastic characteristics. They do, however, demonstrate larger viscosities and a noticeable shear-thinning trend, well approximated by generalized Newtonian models. The following experimental investigation scrutinized the flow statistics of an aqueous xanthan gum solution in a turbulent channel flow, with friction Reynolds numbers ($Re_{\tau}$) between 160 and 680. The amount of DR varied insignificantly between 28% and 33%. The velocity field was measured using planar particle image velocimetry and the steady shear rheology was measured using a torsional rheometer. The results were used to characterize the flow statistics of the polymer drag-reduced flows at different $Re_{\tau}$ and with negligible changes in DR; a parametric study only previously considered by numerical simulations. Changes to the mean velocity and Reynolds stress profiles with increasing $Re_{\tau}$ were similar to the modifications observed in Newtonian turbulence. Specifically, the inner-normalized mean velocity profiles overlapped for different $Re_{\tau}$ and the Reynolds stresses monotonically grew in magnitude with increasing $Re_{\tau}$. Profiles of mean viscosity with respect to the wall-normal position demonstrated a thin layer that consists of a low-viscosity fluid in the immediate vicinity of the wall. Fluid outside of this thin layer had a significantly larger viscosity. We surmise that the demarcation in the shear viscosity between the inner "lubricating" layer and the outer layer cultivates fluid slippage in the buffer layer and an upward shift in the logarithmic layer; a hypothesis akin to DR using wall lubrication and superhydrophobic surfaces.