Drag Reduction and Molecular Structure

Abstract

GADD¹ has discussed the mechanism of the reduction of turbulent drag in liquids in terms of a relaxation time following mechanical distortion. A mechanism based solely on extension and relaxation of flexible elements seems to be inapplicable to drag reducing systems consisting of suspensions of substantially inelastic particles such as asbestos fibres². It is therefore worth considering the alternative hypothesis that a polymer molecule or a particle acts by bridging micro-vortices³. The critical parameters would then be not simply molecular flexibility of the drag reducing agent but the maximum dimension of the shape assumed by the molecules or particles in the conditions of shear stress concerned, and the relationship of this dimension to the size of the vortices to be damped, as proposed by Ellis².