Porous Interface
The Porous Interface condition is designed to properly account for a sudden jump in porosity, εp, across a boundary between different domains. It describes possible discontinuity of stress across the interface, and also reveals modified behavior of tangential velocity due to boundary layer on the upstream side of the interface. Choose among the following options under the Continuity conditions:
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 Velocity and stress — stress continuity is invoked;
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 Velocity and porosity-corrected stress (default)— the sum of stress and a contribution due to the inertial term being constant across the interface;
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 Velocity and porosity-corrected stress with loss — the sum of the stress and a contribution due to the inertial term is lost in the downstream direction.
All three options give continuity of the velocity across the interface in constant-density flows, while in case of a jump in density:
1
give continuity of the mass flux (product of density and normal velocity) and tangential velocity;
2
account for the density-jump correction in the stress condition.
When Velocity and porosity-corrected stress with loss is chosen, a Pressure loss model needs to be specified. The options are:
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 User-defined loss coefficients (default). Enter values for Expansion loss coefficient and Contraction loss coefficient;
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 Borda–Carnot. Specify Vena contracta model.
Vena contracta model allows two options:
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 User defined (default) — enter value of Vena contracta coefficient;
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 Weisbach — enter values of Lower limit and Exponent.
Notice that the notions ‘expansion’ and ‘contraction’ used in this section correspond not to geometrical changes, but to the variations of the microscopic structure across the interface in the flow direction — increasing or decreasing porosity, εp, respectively.