The remarkable ability of low concentrations of certain additives to reduce the frictional resistant in turbulent flow to as little as one-quarter that of the pure solvent is known as drag reduction. It is well known that polymers are the most effective drag reducers. It is not unusual to see up to 80 percent drag reduction with only a few parts per million of added polymer.
Despite the extensive research in the area of drag reduction over the past four decades, there is no universally accepted model that explains the mechanism by which macromolecules act to bring about frictional reduction. A comprehensive mechanism would have to address the role of the polymer structure, composition and microstructure as well as polymer-solvent interactions in the drag reduction phenomena. A comprehensive mechanism will also have to explain the differences observed for flexible versus rigid rod polymers. Even though a comprehensive mechanism does not exist, several theories have been presented. One school of thought, believes that there are multiple mechanisms must exist to bring about the drag reduction effect. (1) Isolated polymer molecules extend in elongational flow fields present in turbulence, thereby increasing the thickness of the elastic sublayer. (2) Polymer aggregates may exist to form large hydrodynamic domains which could suppress small scale turbulence by resisting rapid changes in alignment. (3) In heterogeneous drag reduction, e.g. injection of a concentrated polymer solution in the center of a pipe, long threads of the polymer solution interact with the larger turbulent disturbances or eddies in the center of the pipe. As one may discern, there is varying views upon the mechanism of drag reduction, but through the work of our group and others an understanding of the principles dictating drag reduction efficiency.
Our research group has undertaken extensive analyses of polymers of widely differing structures and compositions. These polymers include hydrophobically modified polyacrylamide polymers, anionic and cationic polyelectrolytes, and polyampholytes. It was discovered that all copolymers were found to conform to a universal curve for drag reduction, when normalized for hydrodynamic volume fraction of polymer in solution. this method of plotting allows the facile comparison of drag reduction efficiencies of polymers of widely differing structures, compositions and molecular weights. The most efficient drag reducers are the polymers which yield the greatest values of (%DR/[h]C) at a particular volume fraction. The drag reduction efficiency seems to be related to the efficiency with which the polymer hydrodynamic coil interacts with and disrupts the eddies and microvortices present in turbulent flow.
Current work within our group is continuing in this area by utilizing a rotating disk rheometer to determine drag reduction efficiencies of novel water soluble polymers.