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.

sorry, the thrust is (mdot*velocity) @ outlet -(mdot*velocity)@ inlet. where mdot=density*area*velocity

[jinks]

hey, I am simulating an oscillating/flapping fin in FLUENT. I plotted the cd vs the flow time and if the time averaged Cd comes out negative then it amount to a net THRUST rite? I need to know if this is the best way of calculating thrust. Also i should tell you, that for this oscillating fin, the average drag is coming out positive, contradictory to our hypothesis. Some one please help me with this.
Thank you.

[ghorrocks]

Try the fluent forum.

[juliom]

In the report of the CFFX solver, you see the forces and moments at walls, try to look at them.
You can define a Result vector from this report and you will have the real force!!
Good luck!