Drag reduction by polymers in turbulent wall-bounded flows exhibits universaland non-universal aspects. The universal maximal mean velocity profile wasexplained in a recent theory. The saturation of this profile and the crossoverback to the Newtonian plug are non-universal, depending on Reynolds number Re,concentration of polymer $c_p$ and the degree of polymerization $N_p$. Weexplain the mechanism of saturation stemming from the finiteness ofextensibility of the polymers, predict its dependence on $c_p$ and $N$ in thelimit of small $c_p$ and large Re, and present the excellent comparison of ourpredictions to experiments on drag reduction by DNA.
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