FLOW-3D can simulate both saturated and unsaturated flows within porous media. Saturated porous media flows are for situations where there is a sharp (or nearly sharp) interface between the saturated zone and the unsaturated zone, with a specific capillary pressure present at the interface. Such situations occur in groundwater flows. Unsaturated porous media flows are for situations where there is a gradual transition from the saturated zone to the unsaturated zone. Here, there is no set capillary pressure; the capillary pressure is a function of the current saturation level, and the history of the saturation within the porous material.
In both cases, different porosity, permeability and wettability (capillary pressure or capillary pressure vs. saturation) can be specified independently for each component, and permeability can be either isotropic (the same in all directions) or anisotropic (permeability depends on flow direction).
The animation below shows the case of a water drop permeating into a porous material, like paper. In this case, the porous material is modeled as unsaturated, and thus the wettability varies with the local saturation. Therefore, areas that are already wetted exhibit a stronger capillary pressure, while regions at the fringe of the drop have a lower capillary pressure. This is accomplished with separate filling and draining curves. Therefore, the drop does not spread out uniformly through the material. Such behavior can be seen by wringing out a paper towel: it is far easier to get it wet than it is to squeeze out all of the water.
Simulation of a drop absorbed into a porous medium