Laser Velocimeter Measurement of Reynolds Stress and Turbulence in Dilute Polymer Solutions

Abstract

Measurements of Reynolds stress and axial and transverse turbulence intensities have been made in drag-reducing turbulent pipe flow of a dilute solution of high molecular weight polymer and compared to measurements made with pure water. A newly developed laser velocimeter capable of measuring these turbulence parameters has been utilized and is described in detail. Axial turbulence intensities measured in polymer solution are consistent with previous polymer results and viscous sublayer thickening is observed. New results include demonstration that the turbulent shearing stress is reduced in the turbulent core by an amount proportional to the observed decrease in pressure gradient at the wall, and extrapolates to a wall value in agreement with calculated local wall shear. Near the wall polymer solution Reynolds stress is reduced below that measured for water consistent with observed velocity profiles. Polymer radial turbulence intensities are comparable with those for water in the turbulent core, but exhibit similar dramatic suppression near the wall. These and other recent results strongly suggest that dilute polymer solution drag reduction is primarily a wall phenomenon. Polymers appear to have little or no effect on turbulent flow away from a solid boundary where turbulent velocities scale with u_τ, the shear velocity based on the observed wall shear.