Drag Reduction in External Flows of Additive Solutions

Abstract

A series of measurements of the turbulent drag on a flat plate using homogeneous solutions of high-molecular-weight additives, and a series of visual studies concerning diffusion and entrainment of jets with additive solutions flowing into a turbulent stream of pure Water have been conducted. The experiments include tests with additive solutions of various concentrations and different channel velocities. The drag reduction obtained in these experiments is generally lower than that for pipe flows without taking into account differences in experimental Reynolds numbers; however, the maximum reduction occurs at about the same concentration (50–100 ppmw) in both cases. A comparison between the ejection study (studies with additive solutions ejected into a pure water boundary layer) data and the present results indicates very poor mixing between the ejected fluid and its surroundings. Photographs of submerged jets confirm that additives suppress turbulent diffusion. The flat plate results also suggest that for efficient drag reduction, the solution ejected into the boundary layer should be dilute and that the rate of ejection should be comparable to the discharge within the inner boundary layer (the wall controlled region). These findings suggest that smaller amounts of additive are needed for ejection than are usually estimated, and therefore brighten the hope for the practical application of additives to achieve drag reduction in external flaw cases, such as on a ship hull.