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

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 higher viscosities and a noticeable shear-thinning trend, approximated by generalized Newtonian models. The following experimental investigation scrutinizes the flow statistics of an aqueous xanthan gum solution in a turbulent channel flow, with friction Reynolds numbers Re-tau between 170 and 700. The amount of DR varies insignificantly between 27% and 33%. The velocity field is measured using planar particle image velocimetry and the steady shear rheology is measured using a torsional rheometer. The results are 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 are similar to the modifications observed in Newtonian turbulence. Specifically, the inner-normalized mean velocity profiles overlap for different Re-tau and the Reynolds stresses monotonically grow in magnitude with increasing Re-tau. Profiles of mean viscosity with respect to the wall-normal position demonstrate a thin layer that consists of a low-viscosity fluid in the immediate vicinity of the wall. Fluid outside this thin layer has a significantly higher viscosity. We surmise that the demarcation in the mean 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.