Thrust calculation

Tony

I simulated the operation of a small engine with an inlet, a combustor, and an outlet. The engine is sitting on the ground so the thrust is static. I monitored the average velocity-u and mass flow rate at the exit. At the inlet, velocity and mass flow rate is known constant. Thrust is calculated from momentum force only: (mdot* mass flow rate) @ outlet ¨C (mdot*mass flow rate) @ inlet. If the exit velocity is sometimes negative (opening boundary), is this still right? (negative exit velocity*negative exit mass flow rate=positive thrust, which is not phyisic)

Is there a more accurate way to calculate thrust in this case? Thanks.