VELOCITY FLUCTUATIONS IN NON-HOMOGENEOUS DRAG REDUCTION

Abstract

Non-homogeneous drag reduction is obtained by injecting a polymer solution thread into the center of a turbulent pipe flow. Under the proper conditions the magnitude of this non-homogeneous drag reduction is much larger than that for the same polymer molecules uniformly distributed in the flowing solvent. In this work the axial velocities and velocity fluctuations near the wall were examined in such a non-homogeneous drag reduction and also in the flow of a homogeneous (pre-mixed) solution of poly(ethylene oxide). The drag reduction and the Reynolds number were the same for both fluids. Significant differences were found in the velocity profile and in the statistical analysis of the velocity fluctuations. The skewness profile for the non-homogeneous case remained positive to a much greater distance from the wall than the skewness profile for the premix solution. In the injection case the velocity fluctuation spectra had the same sharp peak at 5−10 Hz for y + between 11 and 50. This peak corresponded to the bursting frequency measured from the short time autocorrelation. In the pre-mix case a broad increase in velocity fluctuation spectra was seen at y + = 11 which was attributed to polymer molecule rotations in the laminar shear flow near the wall. This band was absent in the non-homogeneous drag reduction, suggesting that no polymers were present at y + = 11. The observations imply that non-homogeneous drag reduction involves a modification of the large eddy structure.