The coupled flow of two, different, fluids in a partly saturated rigid media is now considered. Unsaturated soils provide a common example, where the two fluids are water and air. Often, the air phase is considered to be at constant pressure, which is generally a relevant approximation as air pressure doesn’t highly affect the water flow. In such an arrangement only one d. o.f. per node is sufficient, and the classical diffusion equations (see Eq. 6.10) are relevant, with parameters depending on the suction or saturation level.
Mixing between different fluids is sometimes possible. Then two or more d. o.f. per node are to be considered. The permeability and storage equations of each phase depend on the suction or saturation level, and so the problem may be highly nonlinear. However, it is not difficult to numerically develop coupling, as the formulation is similar for each phase.
11.3.2.2 Diffusion and Transport Coupling
Heat and single fluid flow in a rigid porous media are considered here. The fluid specific weight and viscosity are dependant on the temperature or salt concentration, and the heat or salt transport by advection-diffusion process is dependant on the fluid flow. Then a diffusion process and an advection-diffusion process have to be solved simultaneously. In this case there are 2 d. o.f. per node: fluid pore pressure and salt concentration or temperature.
