  We investigate the phenomenon of drag reduction in a viscoelastic fluid model
of dilute polymer solutions. By means of direct numerical simulations of the
three-dimensional turbulent Kolmogorov flow we show that drag reduction takes
place above a critical Reynolds number $Re_c$. An explicit expression for the
dependence of $Re_c$ on polymer elasticity and diffusivity is derived. The
values of the drag coefficient obtained for different fluid parameters collapse
onto a universal curve when plotted as a function of the rescaled Reynolds
number $Re/Re_c$. The analysis of the momentum budget allows to gain some
insight on the physics of drag reduction, and suggests the existence of a
maximum drag reduction asymptote.
