The semiPermeableBaffleMassFraction is a boundary condition that provides a mass-fraction boundary condition for a semi-permeable baffle.

This condition models a baffle which is permeable to a some species and impermeable to others. It must be used in conjunction with the corresponding velocity condition, semiPermeableBaffleVelocity.

The mass flux of a species is calculated as a coefficient multiplied by the difference in mass fraction across the baffle.

\[\phi_{Yi} = c A (Y_i - Y_{i,n})\]


Property Description
\(\phi_{Yi}\) Flux of the permeable species [kg/s]
\(c\) Transfer coefficient [kg/m^2/s]
\(A\) Patch face area [m^2]
\(Y_i\) Mass fraction on the patch [-]
\(Y_{i,n}\) Mass fraction on the neighbour patch [-]

A species that the baffle is permeable to will, therefore, have a coefficient greater than zero, whilst a species that does not transfer will have a coefficient equal to zero.

This condition calculates the species flux. The fluxes are summed up by the velocity condition to generate the net mass transfer across the baffle. This mass-fraction condition then generates a corrective diffusive flux to ensure that the correct amounts of the permeable species are transferred.


The condition requires entries in both the boundary and field files.

Boundary file

    type            patch;

Field file

    // Mandatory entries
    type            semiPermeableBaffleMassFraction;

    // Optional entries
    c               <scalar>;
    phi             <word>;

    // Inherited entries


Property Description Type Required Default
type Type name: semiPermeableBaffleMassFraction word yes -
c Transfer coefficient scalar no 0
phi Name of flux field word no phi

The inherited entries are elaborated in:

  • mappedPatchBase.H
  • mixedFvPatchFields.H

Further information


  • N/A

Source code:



  • Introduced in version v2206