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NACA 0012 Convergence trouble

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Old   May 28, 2009, 14:06
Question NACA 0012 Convergence trouble
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Carl
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Hi,

I'm running some 3ddp simulations around the naca aerofoil in the wind tunnel they used originally. for low angles of attack with the Spalart Allmaras model and a fine hex mesh (~4million cells) I get excellent lift prediction, drag is ok and convergence is good after about 3000 iterations.

However, as the angle of attack increases to 14-16 degrees, the convergence is not great - the residuals never level off, there are apparent low frequency errors and the drag monitor shows variation without any real convergence.

Any ideas? I have been working on this for months! Is it just that RANS is just no good when separation occurs? I've checked wall y+ values and my boundary conditions are fine. Any advice on how to tame the residuals would be very much appreciated - relaxing the solution has no effect. Also, k-epsilon models are even worse!

Carlos.
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Old   May 29, 2009, 07:07
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Dmitry
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Hi!
It seems to be unsteady flow at such angles of attack.
S-A model was developped specially for external flows, such flow around airfoil and it should be better than k-e for this flow.
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Old   June 1, 2009, 11:11
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Dimitrios Mylonas
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Mecarlg, could you be more specific on the Fluent settings you are using for your simulation?

Dimo
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Old   June 1, 2009, 11:40
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Carl
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Dimo,

I'm using the Spalart Allmaras model with the QUICK scheme for everything but pressure which is 2nd Order. Velocity inlet boundary condition with magnitude of 74m/s, turbulence intensity = 0.25%, turbulent length scale = 0.07*chord. Pressure outlet is 20 chord lengths behind the aerofoil with atmospheric pressure, symmetry plane used on one side of the domain with a wall on the other to simulate the actual wind tunnel used in the NACA tests. Default relaxation factors. 3.8 Million cells used.

I have played around with turbulence models, different turbulence intensities and relaxation factors but to no avail. Ideally the SA model would be used but I cannot carry out an optimization study if the drag varies by 20% throughout the solution history (this happens beyond 10000 iterations so its not down to convergence error).

Surely people have use the SA model for simple external aero like this and had more success than me! Any help would be much appreciated.

Best regards,

Carlos.
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Old   August 27, 2009, 11:45
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Hi Mecarlg,

I've done the same problem on NACA 0012, but within Star-CCM+. In this software, has a tutorial about airfoil problem. But now I want to change the fuid is H2O, so the velocity must be lower. Ex: 1~10 m/s in my simulation. I tried to use every type turbulent model (k-ep; k-w and SA) but there is no good result. The lift coef and the drag coef are incorrect. So if you have any experience on this issue please help me!
Thanks alot!
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Old   September 3, 2009, 00:10
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Sean Delfel
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Hi mecarlg,

I believe dmitry is right -- the problem is most likely that there is vortex shedding making your problem unsteady. You therefore won't get good convergence using the steady solver regardless of the inputs/grid/turbulence model. This is especially most likely the case since you were getting good results at lower angles-of-attack.

Cheers,
-sean
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Old   September 3, 2009, 19:31
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Ivan
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you have not posted any reply to this thread for 3 months, have you solved the problem and how? I support the other replies and you might need unsteady simulation. What is the Re number?


Quote:
Originally Posted by mecarlg View Post
Dimo,

I'm using the Spalart Allmaras model with the QUICK scheme for everything but pressure which is 2nd Order. Velocity inlet boundary condition with magnitude of 74m/s, turbulence intensity = 0.25%, turbulent length scale = 0.07*chord. Pressure outlet is 20 chord lengths behind the aerofoil with atmospheric pressure, symmetry plane used on one side of the domain with a wall on the other to simulate the actual wind tunnel used in the NACA tests. Default relaxation factors. 3.8 Million cells used.

I have played around with turbulence models, different turbulence intensities and relaxation factors but to no avail. Ideally the SA model would be used but I cannot carry out an optimization study if the drag varies by 20% throughout the solution history (this happens beyond 10000 iterations so its not down to convergence error).

Surely people have use the SA model for simple external aero like this and had more success than me! Any help would be much appreciated.

Best regards,

Carlos.
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