[Gareth_CN]

Hello,

I want to calculate the flux 'phip' based on the interpolation velocity obtained by boundary `timeVaryingMappedFixedValue`.

Here is my Code:

 
const Field<scalar>& phip =
        this->patch().boundaryMesh().mesh().Sf() & fieldMapper_.map();

`fieldMapper_.map()` is the mapped patch field.

I get the error message as belows when I compile the new BC

error: invalid initialization of reference of type ‘const Foam::Field<double>&’ from expression of type ‘Foam::tmp<Foam::Field<Foam::Vector<double> > >’
     const Field<scalar>& phip =
                          ^~~~

error: no match for ‘operator&’ (operand types are ‘const Foam::Vector<double>’ and ‘const double’)
 PRODUCT_OPERATOR(innerProduct, &, dot)
/home/gareth/OpenFOAM/OpenFOAM-8/src/OpenFOAM/lnInclude/FieldM.H:272:31: note: in definition of macro ‘TFOR_ALL_F_OP_F_OP_F’
                               OP2 List_ELEM(f3, f3P, i);                       \
                               ^~~
/home/gareth/OpenFOAM/OpenFOAM-8/src/OpenFOAM/lnInclude/FieldFunctions.C:775:1: note: in expansion of macro ‘PRODUCT_OPERATOR’
 PRODUCT_OPERATOR(innerProduct, &, dot)
 ^~~~~~~~~~~~~~~~

I think this error is caused by the different types of objects on both sides of '='. But I am not cleared about this(I am newbie to OpenFOAM)

Hope someone could give me some advice！

Best wishes！

Gareth

[Gareth_CN]

Hello，

I got the outputs of `this->patch().boundaryMesh().mesh().Sf()`, and found that this was not what I thought. It returns face area vectors of all faces, including all patch faces and internal faces.

How could I only get the face area vectors of the current boundary patch?

Best regards,

Gareth