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Old   April 8, 2015, 06:59
Default Numerical problem?
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Hi everybody,

I need your help with an issue. I have been dealing with it for a long time and I donít understand what exactly happening.

I am working in the development of a compressible CFD code and we are now trying to validate it with a test case from NASA ( of the NACA0012 airfoil. The mesh is the ďunstructured family IIĒ with 2x1793x513 nodes ( The case I am running is the same at the first webpage, with angle of attack of 0ļ. Riemann invariants are applied to boundary conditions in our code.

As can be seen in picture, it seems like numerical oscillations are appearing in the outflow boundary and this is perturbing the convergence upstream. I donít know if it could be due to the aspect ratio of cells near the outflow boundary. The code works successfully with a coarser grid of Naca0012, meshed by ourselves (without grid for boundary layer).

Do anyone know what could be happening and what could be a solution of the problem?

Thank you guys!
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Old   April 9, 2015, 16:12
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just curious, what turbulence model are you using? and how are the turbulence BC at the outlet set up?
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Old   April 20, 2015, 03:39
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Hi nvarma,

thanks for answering. I am using the Spalart-Allmaras model with the usual B.C. (nu_tilda=3*nu) at farfield. I have done some changes in the code and I have solved partially the problem, but it is still doing weird things at outflow boundary. Now I have compiled it with real variables in double precision, and changed the two outflow surfaces (by defect in NASA grid) to one (because there were two repited nodes and they were generating problems). Now it seems like the convergence is advancing because the solution starts to be as it should, but suddenly the outflow pressure decay and then gives negative pressure. I don't know what to do...

thanks for help!
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