[Opaque]

You should be able to compute the mass flow on the boundaries of the domain interface. What do I mean?

An ANSYS CFX domain interface joins two sides, say Side 1 and Side 2. Those sides are represented by boundaries. Usually named as <interface name> Side 1, and <interface name> Side 2

You should be able to monitor

massFlow()@<interface name> Side 1

as well as

massFlow()@<interface name> Side 2

but you cannot

massFlow()@<interface name>

For a conservative domain interface, massFlow()@<interface name> is identically 0 by definition. What comes through one side (say >0) has to leave through the other side (<0); therefore, the total sum is 0. It may be non-zero due to discretization details on non-conformal meshes.

Hope the above helps,

[Opaque]

Originally Posted by Ali Di

Thank you for your comment. But, if you mean

MyAVExpressionX=Density*x

and

CofMx = volumeInt( MyAVExpressionX)@Domain / volumeInt(Density)@Domain

it didnt work, and I faced to the same error.

I should have been more explicit. I did not say add

MyAVExpressionX=Density*x

I say create an Additional Variable, see Insert menu for details. Once created,
- activate the variable in the domain of interest
- select algebraic equation
- In the additional variable value widget, add you expression:

Additional Variable Value = Density*x

or

Additional Variable Value = MyAVExpressionX

- Press Apply,
- Add your monitor expression

It will work.