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Simulating aircraft at High Altitude

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Old   July 4, 2012, 11:55
Default Simulating aircraft at High Altitude
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Hello everyone, this is my first post and is concerning a simulation on an airplane meshed in GAMBIT. I would really appreciate some feedback about my values and help me with my mistakes.

My parameters are these:

- 10.000 m altitude
- Pressure (10k m) = 26437 Pa
- Density (10k m) = 0.4161 kg/m3
- Velocity (velocity-inlet): variable
- Temperature (10k m)= 223.15 K
- Viscosity (10k m)= 1.3067e-05

I do not want to explain my mesh otherwise would take ages, just say it is formed by a large cube involving a cylinder and this involving the aircraft. The boundary conditions are velocity-inlet, interior (for the cylinder), wall (aircraft and cube's faces) and pressure-outlet.

Solver parameters:
- Node-gauss based
- Density-Based
- k-omega SST
- PRESTO
- Second-Order Upwind each parameter

I set Pressure to 0 Pa and when asked, Gauge Pressure to 26437, is it correct?
In Materials section, both density and viscosity are constant values.

Reference Values:
- Area: here is where I have more questions, so far I have been using the projected area (1,0,0) in X-axis, which is the frontal view of the aircraft, with 0.01m of error.
- Length: I have been using the longitudinal length of the aircraft, along the y-axis, but I'm not sure about this.

I think these are the main parameters of the simulation.
I would appreciate any help or feedback from anybody, just need to be sure that I am doing the right procedure, or at least close to it.


Many thanks in advance
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Old   July 4, 2012, 12:16
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Why do you use a "wall" boundary condition on the faces of the cube?
A slip-wall (symmetry) would be better in my opinion.

Since your aircraft might travel at a high speed, effects of compressibility may be important. So you should not use constant values for your fluid properties.
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Old   July 4, 2012, 13:01
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When you say symmetry, you mean the four faces of the tunnel (cube), don't you? I will try this solution, thanks a lot!

On the other hand, the aircraft is not meant to fly at high velocities, just up to 200 Km/h or so. However, flying below 200 Km/h I struggle to balance the Weight with the net Lift force the aircraft generates.
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Old   July 5, 2012, 07:16
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In addition, when fluent starts iterating, it says Reversed flow in 1000 cells in pressure-outlet, and as iteration goes on it decreases until the warning does not appear anymore.

Plus, if I try Ideal-gas, hence Energy equation = On, Fluent shows another warning saying this settings are not appropriate for Velocity-inlet boundary conditions. Why is that happening?

Thanks in advance
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Old   July 5, 2012, 08:39
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For the warnings indicating reversed flow at outlet cells:

Initialize the velocity field with the inlet velocity. Note that this will only speed up convergence. If the warnings disappear after a few iterations, the simulation is still valid.

For the ideal gas calculations, try using a mass-flow boundary condition instead of the velocity inlet.
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