Aspects of Mechanisms in Type B Drag Reduction

Abstract

Drag reduction by macromolecular additives is briefly reviewed and shown to comprize two extreme forms of gross flow behaviour. Types A and B, that are respectively associated with random-coiling and extended additive conformations. The mechanism of drag reduction is discussed within the framework of an “Additive-Burst Matrix”, devised such that its rows represent macromolecular states while its columns connote turbulent burst events. Experiments are presented wherein Type A and Type B behaviour were both induced by polyelectrolyte additives of the identical backbone structure and molecular weight. Results for Type B drag reduction revealed that, at fixed Re∞f, flow enhancement increased almost linearly with additive concentration; also, a typical gross flow trajectory commenced on the maximum drag reduction asymptote and then exhibited “retro-onset” into the polymeric regime of less than maximum drag reduction. Comparisons between selected Type A and B trajectories also delineated “additive equivalence” at “iso-slip” points, where different polymer solutions achieved the same flow enhancement. Interpretation of these experimental results suggests that Type B drag reduction is likely more fundamental to the mechanism of additive-turbulence interaction than the more widely studied Type A.