PIV Measurements of the Structure of Drag-Reduced Turbulence and the Mechanisms of Polymer Drag Reduction

Abstract-
The addition of parts-per-million quantities of long-chain flexible polymers to flowing liquids can have profound effects on flow properties that appear incommensurate with polymer concentration. The most notable effect is a large reduction in turbulent drag of wall bounded flows - polymer concentrations as low as 1 part-per-million have yielded drag reductions as high as 80% when compared to that of the solvent alone. This unprecedented reduction in turbulent drag has attracted considerable attention from both fundamental and practical viewpoints since its discovery over fifty years ago. In spite of this long history, the exact mechanism responsible for polymer drag reduction is not well understood, probably for good reason, since it involves the interaction between two fields, namely, fluid turbulence and dilute polymer dynamics, neither of which are fully understood by themselves.

In an effort to more fully understand the mechanism of polymer drag reduction, a collaboration between experiments and direct numerical simulations of polymer drag reduced flow and simulations of polymer chains in turbulent flow has been undertaken at Stanford University. This talk describes the experimental part of this collaboration. In the present work, the near-wall structure of a turbulent boundary layer, with and without polymer addition, is measured using particle image velocimetry (PIV). Measurements are performed in the plane parallel to the wall (xz), for thirty planes across the thickness of the boundary layer, and also the side-plane (xy). For increasing drag reduction, we find a significant modification of the near-wall structure of turbulence with a coarsening of the low-speed velocity streaks and a reduction in the number and strength of the near-wall vortices. Flow visualization, using planar laser induced fluorescence (PLIF) of the injected polymer, shows local accumulation of polymer concentration near the low-speed velocity streaks. A mechanism of polymer drag reduction as determined from the PIV measurements will be discussed.

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