Diffuse flow means that the viscous, or dynamic, forces are more dominant than the gravity forces, so that vertical variation in saturations may be neglected. Typical characteristics for this type of flow condition are low vertical permeability, combined with a high horizontal pressure gradient.
The converse to diffuse flow condition is the more common condition of segregated flow. Segregated flow is where the gravity forces are more dominant than the the viscous, or dynamic, forces, so that vertical variations in saturations are significant. Typical characteristics for this type of flow condition are high vertical permeability, combined with a low horizontal pressure gradient.
Both of the above flow conditions are implemented within this routine, and are discussed in more detail in the following sections
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