Effective Rheology of Immiscible Two-Phase Flow in Porous Media
  We demonstrate through numerical simulations and a mean field calculation that immiscible two-phase flow in a porous medium behaves effectively as a Bingham viscoplastic fluid. This leads to a generalized Darcy equation where the volumetric flow rate depends quadratically on an excess pressure difference in the range of flow rates where the capillary forces compete with the viscous forces. At higher rates, the flow is Newtonian. 
Santanu Sinha and Alex Hansen
physics.flu-dyn