  We report on generic relations between fractional flow and pressure in steady
two-phase flow in porous media. The main result is a differential equation for
fractional flow as a function of phase saturation. We infer this result from
two underlying observations of steady flow simulations in two and three
dimensions using biperiodic boundary conditions. The resulting equation is
solved generally, and the result is tested against simulations and experimental
relative permeability results found in the literature.
