Investigation of near-wall turbulence in relation to polymer rheology

Abstract

An experimental investigation was carried out to characterize the rheology of polyacrylamide solutions and its effect on the structure of a turbulent channel flow. The shear viscosity of 10 and 20 ppm solutions had similar magnitudes as that of water with a Newtonian behavior, while the 90 and 160 ppm solutions had a shear-thinning behavior. The elasticity and relaxation time of the solutions monotonously increased with an increase in polymer concentration. Pressure drop measurement at a Reynold number of 20 000 showed 25, 43, 51, and 57% drag reduction for 10, 20, 90, and 160 ppm solutions, respectively. Time-resolved planar particle image velocimetry was used to characterize the turbulent structure. The polymers were more effective in reducing the strain-rate in the buffer layer due to the larger strain rate and stretching of the polymers. This was consistent with larger values of the Weissenberg number in the buffer layer compared with the log layer. The distributions of the Weissenberg number showed two distinct distributions at the low and high drag reduction regimes. The addition of the polymers to the turbulent flow was observed to balance the local strain rate and rotation. This effect was observed in the inner layer for all polymer concentrations, while it was observed in the logarithmic layer only for the 90 and 160 ppm solutions. The power spectral density of turbulence kinetic energy in the buffer layer showed that the high frequency content was damped for the 10 and 20 ppm solutions, while a wider frequency range was attenuated at a higher polymer concentration.