  We present a numerical study aimed at comparing two approaches to the
evaluation of relative permeability curves from 3D binary images of porous
media. One approach hinges on the numerical solution of Stokes equations, while
the other is based on the Johnson-Koplik-Dashen (JKD) universal scaling theory
of viscous frequency-dependent flow [D.~L. Johnson, J.~Koplik, and R.~Dashen,
\emph{Theory of dynamic permeability and tortuosity in fluid--saturated porous
media}, Journal of Fluid Mechanics \textbf{176} (1987), 379--402.] and the
method of maximal inscribed spheres.
  JKD steady-flow simulations only require the solution of a boundary-value
problem for the Laplace equation, which is computationally less intensive than
the solution of Stokes equations.
  A series of numerical calculations performed on 3D pore-space images of
natural rock demonstrate that JKD-based estimates are in good agreement with
the corresponding Stokes-flow numerical simulations.
