Abstract
Drag reduction by polymers in turbulent wall-bounded flows
exhibits universal and non-universal aspects. The universal
maximal mean-velocity profile was explained in a recent theory.
The saturation of this profile and the crossover back to the
Newtonian plug are non-universal, depending on Reynolds number
e, concentration of polymer cp and the degree of
polymerization Np. We explain the mechanism of
saturation stemming from the finiteness of extensibility of the
polymers, predict its dependence on cp and N in the
limit of small cp and large e, and present the
excellent comparison of our predictions to experiments on drag
reduction by DNA.