A Mechanism of Polymer Induced Drag Reduction in Turbulent Pipe Flow

Polymer induced drag reduction in turbulent pipe flow was investigated using a non-intrusive laser based diagnostic technique, namely Particle Image Velocimetry (PIV). The drag reduction was measured in a pressure-driven flow facility, in a horizontal pipe of inner diameter 25.3 mm at Reynolds numbers ranging from 35 000 to 210 000. Three highmolecular-weight polymers (polyethylene oxide 2×10 6 – 8×10 6 Da) at concentrations in the range of 5 – 250 wppm were used. The results, obtained from the PIV measurements, show that the drag reduction scales with the magnitude of the normalized streamwise and spanwise rms velocity fluctuations in the flow. This scaling seems to universal, and is independent of the Reynolds number and in some cases also independent of the distance from the wall where the velocity fluctuations are considered. Furthermore, the instantaneous PIV observations indicate that as the level of drag reduction increases, the flow in the pipe is separated into a low-momentum flow region near the pipe wall and a high-momentum flow region in the turbulent core. Based on these findings a new mechanism of polymeric drag reduction is proposed in this paper. INTRODUCTION High molecular weight polymers or surfactants dissolved at very low concentrations in a solvent lead to a significant reduction of mechanical drag in turbulent flow when compared to the equivalent flow of the pure solvent. This phenomenon, known as drag reduction (DR) and reported for the first time by Toms [1] and Mysels [2], is of great industrial relevance, e.g., in the oil-and-gas industry (pipeline systems or hydraulic fracturing), agriculture (field irrigation) and civil engineering (firefighting, plane refueling). The phenomenon of drag reduction has been extensively studied in the past, however, previous studies were limited to the gross-flow characterization, i.e., the effect of drag reducers on the friction factor at a given Reynolds number Re [3,4]. Although highly valuable, such studies were unable to provide a detailed insight into the mechanistic turbulence-polymer interactions at the microscopic level. Recently, the utilization of advanced laser-based flow diagnostics techniques, e.g., Laser Doppler Velocimetry and Particle Tracking Velocimetry, for the characterization of drag reducing solutions has provided both quantitative and qualitative results. Liberatore et al. [5] observed that the frequency and the intensity of large scale ejections decreased in the presence of polymer additives. Warholic et al. [6] identified turbulent structures close to the wall which are typical for Newtonian solvents. These structures were characteristic by the ejection of low momentum fluid to the outer velocity-defect region. Such structures were identified as locations of large Reynolds stresses. These structures, however, were absent at high measured levels of drag reduction.