This result falsely implies that permeability is changing along the length of the preform. Since permeability is a property of the preform, it cannot change. The reason for the apparent change is that the flow encountered one level of resistance when it initially went past the fiber bundles. Later, as the flow begins to impregnate the tows, the resistance felt by the flow decreased. Here is a schematic of what was happening to cause the curved permeability profile.
Injection takes place from the left side of the diagram.
Region 1 contains fiber bundles which are not impregnated at all. Region 2 contains fiber bundles which are partially impregnated, and Region 3 contains bundles which are completely impregnated. A two-step permeability model can account for the varying degrees of resin infusion in the tows. Where the fibers are completely impregnated, in Region 3, the permeability is higher than where the fibers are not impregnated, in Region 1. The flow in the unsaturated region is considered to be like flow around solid cylinders.