negative global mass flow rate

Gimli

Hi,

I am modeling a free surface very thin flow. It's a thin film of water shear-driven by an air flow in a channel, so I use the VOF multiphase model.

My problem is I obtain a negative global mass flow rate when I check it in the flux report panel: there is more fluid flowing out than flowing in.

Apparently it's linked to the water phase since the mass flow rate is close to 0 when I only check the air phase. I thought it could be a numeric diffusion problem, but isn't mass supposed to disappear (positive global mass flow rate) in this case?

If anyone knows how to interpret this, I'd appreciate an explanation.

Thanks