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Boundary Conditions for Podded Engine

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Old   October 22, 2013, 18:24
Default Boundary Conditions for Podded Engine
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Hello all, first post here. I am in the process of modeling a podded turbine engine that is in flight. The boundary conditions are giving me trouble and confusion. The podded engine is flying at 35000 ft, Mach 0.75, zero angle of attack. There is an inlet face to the engine (throat) and an outlet face (nozzle). The inner details of the engine are not being modeled. The mass flow rate, pressure and temperature (total and static), etc, at the throat and nozzle are all known from hand calculations. We are trying to match the hand calcs to what CFX is producing.

The engine is modeled at a 2D section 3 mesh cells wide. The section width is set such that the area of the throat and nozzle match the hand calculations and are appropriate for the known mass flow rate. The bounding "box" is a half-ellipse that extends to about 20C in front, 40C in back, and 15C above and below the engine.

The domain is set to ideal gas w/ reference pressure of 3.45psi (atmospheric pressure at 35000 ft)
The domain inlet to the half-ellipse is set as a velocity inlet with a U velocity of ~730 ft/s, no V or W velocity
The domain outlet is set to pressure outlet with zero relative pressure
The engine inlet (throat) is set to a mass flow outlet
The engine exit (nozzle) is set to a subsonic mass flow inlet equaling the mass flow of the throat

I am attempting to solve on total energy with k-omega SST.

The main issue is that the conditions at the outlet are not correct so it is not producing the gross thrust we expect. The issue appears to be that the total pressure at the nozzle is not right. The nozzle should be nearly choked at Mach 0.96, but it is showing about Mach 0.8.

Since the engine is technically adding energy to the domain, do I need to rethink my boundary conditions? Would a supersonic or mixed mass flow inlet work for the engine nozzle?

Thank you for your time!
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Old   October 28, 2013, 13:08
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So I ended up putting a momentum source at the nozzle face and inputted the pressure, temperature, and normal velocity for this source. This seems to work much better and the gross thrust within 20 lbf of the hand calculations. However it does not seem to be matching the velocity I'm inputting. It's about 10 ft/s too low. Is there something else I'm missing?

Thanks again.
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Old   October 28, 2013, 17:43
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This sounds like a FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

What have you done to show that your simulation numerics is accurate? Whether your simulation setup is accurately reproducing reality is another question, but you need accurate numerics before you even start.
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