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simpleFoam always diverges

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Old   January 25, 2012, 07:54
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Hi RugbyGandalf,

I made an investigation on NACA 0012 airfoil in low and high mach number flows. One of the things that I found to be very important is the windtunnel investigation itself. Check if the results have been corrected somehow for wall interference and all these things. I found it very helpful to get the pressure distribution right first and the look for drag.
To me your domain looks much too small. I got good results with at least 5 to 10 chord lengths distance between the airfoil and any boundary at 0 angle of attack. For 15 angle of attack I needed 22 chord lengths.
Personally I would use the slip boundary condition on the cylinder for velocity, but the zeroGradient condition instead. With slip you disable flow through the walls, which seems reasonable at first glance. However you don't really know (or you need a detailed report) if the wind tunnel is slotted of the test section is even open. Therefore you impose a constraint, which is not present in the wind tunnel.
My simulation showed a drag coefficient, which over estimated drag by 40%. I suppose better is possible. So don't give up.

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