  Drag reduction by polymers in wall turbulence is bounded from above by a
universal maximal drag reduction (MDR) velocity profile that is a log-law,
estimated experimentally by Virk as $V^+(y^+)\approx 11.7 \log y^+ -17$. Here
$V^+(y)$ and $y^+$ are the mean streamwise velocity and the distance from the
wall in "wall" units. In this Letter we propose that this MDR profile is an
edge solution of the Navier-Stokes equations (with an effective viscosity
profile) beyond which no turbulent solutions exist. This insight rationalizes
the universality of the MDR and provides a maximum principle which allows an
ab-initio calculation of the parameters in this law without any viscoelastic
experimental input.
