Thrust calculation
 

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.

Re: Thrust calculation
 

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

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.

Try the fluent forum.

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!