[cstuart]

I have a custom boundary condition for a mass diffusion solver of mine. The boundary condition calculates the gradient based on an empirical formula for the outflowing mass flux. I want my end user to be able to view in Paraview the changing outflow mass flux as well as the total quantity of mass escaped (i.e. integral of mass flux with time).

To achieve this, I want to have a separate volScalarField in my solver which holds only this integrated quantity in its boundary field and nothing else. (So in fact I should probably be using a surface field but I'm trying to keep things simple for me for the moment.)

My problem is accessing that storage field from within the boundary condition of a different field. So, say I have mass concentration C_T with my custom BC, I need to get a writeable reference to storage field 'store'. The closest I have worked out how to get to this is:

    const fvPatchField<scalar>& storageField =
        patch().lookupPatchField<volScalarField, scalar>("store");

This gives me the right section of the boundary field of the storage volScalarField, but lookupPatchField() returns a const reference, so I cannot store anything in it!
Is there a way of getting a non-const reference?