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8.6.2 Prescribed Boundary Conditions
In most analyses, it may be assumed that the ambient fluid pressure and electric potential in the external environment are zero, thus and , where the subscripted asterisk is used to denote environmental conditions. Since the external environment does not include a solid matrix, the fixed charge density there is zero. For example, for a triphasic analysis, . It follows that the effective fluid pressure in the external environment is and the effective concentrations are . Therefore, in multiphasic analyses, whenever the external environment contains solutes with concentrations , the user must remember to prescribe non-zero boundary conditions for the effective solute concentrations and the effective fluid pressure.
Letting also implies that prescribed mixture normal tractions (Section 18.104.22.168↑) represent only the traction above ambient conditions. Note that users are not obligated to assume that . However, if a non-zero value is assumed for the ambient pressure, then users must remember to incorporate this non-zero value whenever prescribing mixture normal tractions. Similarly, users are not required to assume that ; when a non-zero value is assumed for the electric potential of the external environment, the prescribed boundary conditions for the effective concentrations should be evaluated using the corresponding partition coefficient, .