Goal Plot indicates negative or unrealistically low mass flow rates

Hey hi,

I am experimenting with the internal flow feature to simulate a flow inside a grooved duct. I am setting the inlet total pressure and the required outled mass flow rate. Even though that the flow velocity across the cross section of the part appears to indicate the right mass flow rate, upon creating a goal plot the global mass flow rate is either unrealistically small or even at some cases becomes negative unlike the velocity field which appears to be correct!

Any ideas?

Cheers