Inserting Darcy’s law (Equation 4-1) into the continuity equation produces the generalized governing equation

Insert the definition of fluid compressibility χf = (1/ρ)(∂ρ/∂p) to the right-hand side and rearrange to arrive at

Using this relation, the generalized governing equation (Equation 4-4) takes the following form:

In this equation, S is the storage coefficient (SI unit: 1/Pa), which can be interpreted as the weighted compressibility of the porous material and the fluid. The storage S can be an expression involving results from a solid-deformation equation or an expression involving temperatures and concentrations from other analyses. The Darcy’s Law interface implements Equation 4-5 using the Storage Model node, which explicitly includes an option to define S as the linearized storage (SI unit: 1/Pa) using the compressibility of fluids and porous matrix.