# Relative permeability

The relative permeability of a phase is a function of its effective saturation: (1) In this equation is the residual saturation for phase . If then the relative permeability is zero, while if then the relative permeability is unity. Otherwise, the relative permeability is given by the expressions below.

## Constant

`PorousFlowRelativePermeabilityConst`

The relative permeability of the phase is constant (2) This is not recommended because there is nothing to discourage phase disappearance, which manifests itself in poor convergence. Usually as is a much better choice. However, it is included as it is useful for testing purposes.

## Corey

`PorousFlowRelativePermeabilityCorey`

The relative permeability of the phase is given by Corey (1954) (3) where is a user-defined quantity. Originally, Corey (1954) used for the wetting phase, but the PorousFlow module allows an arbitrary exponent to be used.

## van Genuchten

`PorousFlowRelativePermeabilityVG`

The relative permeability of the phase is given by Genuchten (1980) (4) This has the numerical disadvantage that its derivative as tends to infinity. This means that simulations where the saturation oscillates around do not converge well.

Therefore, a *cut* version of the van Genuchten expression is also offered, which is almost definitely indistinguishable experimentally from the original expression: (5) Here the cubic is chosen so that its value and derivative match the van Genuchten expression at , and so that it is unity at .

## Brooks-Corey

`PorousFlowRelativePermeabilityBC`

The Brooks and Corey (1966) relative permeability model is an extension of the previous Corey (1954) formulation where the relative permeability of the wetting phase is given by (6) and the relative permeability of the non-wetting phase is (7) where is a user-defined exponent. When , this formulation reduces to the original Corey (1954) form.

## Broadbridge-White

`PorousFlowRelativePermeabilityBW`

The relative permeability of a phase given by Broadbridge and White (1988) is (8)

## FLAC

`PorousFlowRelativePermeabilityFLAC`

A form of relative permeability used in FLAC, where (9) This has the distinct advantage over the Corey formulation that its derivative is continuous at .

## References

- P. Broadbridge and I. White.
Constant rate rainfall infiltration: a versatile nonlinear model, 1. analytical solution.
*Water Resources Research*, 24:145–154, 1988.[BibTeX] - R. H. Brooks and A. T. Corey.
Properties of porous media affecting fluid flow.
*J. Irrig. Drain. Div.*, 92:61–88, 1966.[BibTeX] - A. T. Corey.
The interrelation between gas and oil relative permeabilities.
*Prod. Month.*, 19:38–41, 1954.[BibTeX] - M. Th. van Genuchten.
A closed for equation for predicting the hydraulic conductivity of unsaturated soils.
*Soil Sci. Soc.*, 44:892–898, 1980.[BibTeX]