Drag reduction in turbulent flow by polymer additives

A brief survey is given of the drag reduction phenomenon, emphasizing the aspects which must be explained by any theory: onset, the existence of intrinsic drag reduction, the Newtonian plug, saturation with increasing concentration, and the maximum drag reduction asymptote. In addition, the polymer properties observed to be favorable are noted. Experimental and theoretical arguments are cited, indicating that sublayer stability arguments are not relevant. Recent evidence is given, linking the onset phenom-enon to a molecular time scale. The behavior of isolated molecules in flow fields is briefly surveyed, indicating the possibility of large increases in viscosity in relatively rotation free strain rate fields; indirect experimental evidence for such behavior is cited, and it is shown how, by reducing the intensity of the smallest eddies, this can explain the various aspects noted. The maximum drag reduction asymptote is discussed, in connection with recent measurements of turbulent fluctuations in drag reducing flows, and it is shown how both of these may be related to changes in the large eddy structure caused by the polymer induced changes in the small eddies.