# Simulating aircraft at High Altitude

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 July 4, 2012, 10:55 Simulating aircraft at High Altitude #1 New Member   Join Date: Jun 2012 Posts: 3 Rep Power: 7 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

 July 4, 2012, 11:16 #2 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,531 Rep Power: 25 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.

 July 4, 2012, 12:01 #3 New Member   Join Date: Jun 2012 Posts: 3 Rep Power: 7 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.

 July 5, 2012, 06:16 #4 New Member   Join Date: Jun 2012 Posts: 3 Rep Power: 7 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

 July 5, 2012, 07:39 #5 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,531 Rep Power: 25 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.

 Tags aircraft, density, projected areas, reference values, viscosity models

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