Turbulent velocity field in heterogeneously drag reduced pipe flow

Abstract

Two-dimensional Laser Doppler Velocimetry and pressure drop measurements were carried out in a heterogeneously drag reduced turbulent pipe flow of 20 000 < Re < 30 000. A polymer solution of 0.75 wt.% PAMH was injected in the centerline of a 30 mm × 30 mm quadrangular duct. A detailed statistical analysis consisting of mean velocity, second and higher moments, discrete Fourier analysis, and correlation of the recorded velocity signals is presented. The results are consistent with the assumption that the heterogeneous drag reduction system is due to the entangled polymer molecules present in the flow and not due to a large scale interaction of the polymer thread with the outer flow field. Following the assumption that the achieved drag reduction is due to dissolved polymer, the differences in the onset of the effect and in the mean velocity profile is explained by an agglomerate model. That means that, the active drag reducing elements are entanglements of multiple polymer molecules capable of affecting the entire pipe diameter even at low drag reduction levels. It is also shown that the velocity signals for drag reduced flow are affected mainly in the buffer layer whereas the injected polymer thread practically never approaches the wall closer than γ + = 100.