Turbulent drag reduction by polymer additives in parallel-shear flows

In this study, we experimentally investigate the turbulent drag-reduction (DR)mechanism in flow through ducts of circular, rectangular and square cross-sectionsusing two grades of polyacrylamide in aqueous solution having different molecularweights and various semidilute concentrations. Specifically, we explore the relationshipbetween drag reduction and fluid elasticity, purposely exploiting the mechanicaldegradation of polymer molecules to vary their rheological properties. We also obtaintime-resolved velocity data for various DR levels using particle image velocimetryand laser Doppler velocimetry. Elasticity is quantified via relaxation times determinedfrom uniaxial extensional flow using a capillary breakup apparatus. A plot of DRagainst Weissenberg number (Wi) is found to approximately collapse the data, withthe onset of DR occurring atWi≈0.5 and the maximum drag-reduction asymptotebeing approached forWi&5. Thus quantitative predictions of DR in a range of shearflows can be made from a single measurable material property of a polymer solution,at least for this particular flexible linear polyme