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8.5 Cautionary Note for Steady-State Biphasic and Multiphasic Analyses
Biphasic, biphasic-solute and multiphasic analyses are generally transient analyses, involving mass transport (solvent and solutes) through a porous permeable domain. The default setting for these analyses is the transient mode (see Section 3.2↑). When a steady-state analysis is requested (<analysis type="steady-state"/>), the code simplifies the governing equations by setting time derivatives of the solid displacement and solute concentrations to zero. It is important to understand that this simplification is only applicable to analyses where interface conditions between sub-domains, and boundary conditions with the external environment, allow mass exchanges for the solvent and all solutes. If a domain or sub-domain is completely sealed such that it prevents solvent or solute exchanges, a steady-state analysis should not be used, as it would not capture these sealed conditions and would lead to erroneous results. For those types of problems, always use a transient analysis to obtain the correct solution. In those cases, a steady-state response may be obtained efficiently by temporarily increasing the transport properties (hydraulic permeability and solute diffusivities) by orders of magnitude via load curves, and/or by increasing the size of time steps.