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Wing in compressible flow convergence problem

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Old   February 25, 2017, 21:02
Default Wing in compressible flow convergence problem
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Ferruccio Rossi
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Hi,

I am simulating compressible flow over a 737 wing approximately 20 m long (see pictures) at cruise conditions:
Mach = 0.745
Angle of attack = 2.5 deg
Absolute pressure = 24998 Pa
Temperature = 220 k

My solution only converges to 3e-1. It doesn't even get to my target broad convergence value, which I set to 1e-3.

I am using a mesh which I believe is fine (see pictures).
Total number of elements = 6,203,332
Total number of nodes = 2,597,576
Wing face elements size = 5e-2 m
Inflation layers = 36 layers, 5e-4 m first layer thickness, 1.05 growth rate

So why my simulation is not converging? Do I need a finer mesh or finer inflation layers? Or something else could be causing the problem? I read the documentation on CDF online regarding convergence problem, but I didn't find anything that could help.
Moreover, when I look at my residuals plot, the values are stable although not converged to my target (1e-3). Are the results still reliable if they are stable but not converged? I am very confused. Anything helps.

Thank you all!
Attached Images
File Type: png wing.PNG (29.9 KB, 9 views)
File Type: png wing_2.PNG (38.4 KB, 9 views)
File Type: png mesh.PNG (68.4 KB, 9 views)
File Type: png mesh_2.PNG (126.1 KB, 9 views)
File Type: png mesh_3.PNG (47.1 KB, 8 views)
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Old   February 26, 2017, 16:37
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Glenn Horrocks
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Despite what you say, this FAQ describes exactly what is going on and how to fix it: https://www.cfd-online.com/Wiki/Ansy...gence_criteria

The bits especially relevant to you are:
1) The element volume change from your inflation mesh to the bulk mesh is too much. You want your top layer int he inflation mesh to be approximately the same volume as the nearby volume mesh.
2) Your inflation mesh is almost certainly too fine but your bulk mesh is almost certainly too coarse. You need to balance it up a bit there.
3) Your bulk mesh is so coarse that it is causing noticeable faceting on the wing leading edge. You need to resolve the curvature smoothly.

So in short your problem is mesh quality, mesh quality, mesh quality.
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convergence, drag, lift, residuals, wing

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