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Old   November 7, 2013, 16:29
Default Convergence Issues
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I've been having some issues getting my model to converge. I am trying to model a podded turbine engine in flight at 35000 ft Mach 0.75, zero angle of attack. The podded engine is essentially a NACA symmetrical airfoil that has been cut in half and spread apart, with a mass flow outlet at the engine face, and a momentum source inlet at the engine nozzle. The momentum source has a higher total temperature, total pressure, and velocity than ambient because the engine is doing work on the flow. The insides of the engine are not being modeled however.

The engine is being modeled as a 2D section, 3 cells wide, about 100k cells in total. Min orthogonal quality is about 0.22 on some inflation layer cells at the leading edge. First layer thickness is about 0.00005 ft to achieve a y+ around 5. Any thinner and the orthogonal quality was very poor. The width of the 2D section has been set to create the necessary area to match the mass flow rate of the engine inlet and nozzle and Mach number from hand calculations.

The model appears to converge well, but will suddenly become unstable and fail. I have tried simplifying the model down to inviscid (by using free slip walls), laminar, upwind advection, etc. The timescale has also been varied from 1e-5s to 1s; the smaller timescale will simply delay the solution becomes unstable. The solution always seems to diverge at about the same number of solutions (~40 for auto timescale factor 1.0). I've attached some screenshots with an auto timescale factor of 0.1.

Could anyone provide insight on where I should begin to look for issues? Thank you very much for your time.
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Old   November 7, 2013, 17:21
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Glenn Horrocks
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This type of effect is usually caused by some weirdness in the flow which convects with the flow and when it hits the exit of the domain goes bezerk. So I would produce a series of results files from before the convergence problem and after it and see if you can spot the weirdness.

And using a laminar flow model does not simplify the simulation, it makes it harder. You need the dissipation of a turbulence model.
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