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Numerical Schemes for Vortex Capturing

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Old   February 7, 2012, 00:01
Default Numerical Schemes for Vortex Capturing
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Kalyan
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Hi all,

I am trying to model the wing tip vortices from a round tip wing ( NACA 0012) using OpenFOAM. I tried both the Spalart Allmaras and KW-SST models and realized that the results are no where near the experimental results.

When I tweaked the fvSchemes file, I got results which are at least comparable to the experiment but they are still quite a bit away. I understand that the diffusion of the vortex depends majorly on the diverging schemes. I am using Gauss QUICK and when I use MUSCL, the solution blows up.

Can anyone suggest what schemes I can employ to get lower diffusion?

Kalyan
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Old   February 7, 2012, 02:48
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Vincent RIVOLA
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You might also want to look at your mesh in this region since it will play an important role.
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Old   February 7, 2012, 10:19
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vinz,

Thanks for the reply. I have a pretty good mesh and I am confident to an extent that it is not a mesh problem

Kalyan
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Old   February 7, 2012, 10:38
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Out of curiosity, do you have some pictures of the mesh in this region?
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Old   February 7, 2012, 11:45
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Vinz,

Here are some pics of the mesh.

The first mesh is around the wing with a cylinder over the round tip.

The second mesh is in the wake behind the wing, with the fine mesh in the region of the vortex.

Please excuse the weirdness of the cells, since paraview doesn't render well. The mesh was made in snappyHex

wing.png

wake.png
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Old   February 7, 2012, 12:01
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Ok, thanks for images. A small tip for paraview, you can use "extract cell by region" instead of "Slice". It will keep the cells instead of cutting them.

When you say "the results are no where near the experimental results."
What are the parameters that your are looking at? forces? coefficients? flow topology? vortex shedding?
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Old   February 7, 2012, 12:08
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Vinz,

Thanks for the tip.

The main parameters which I was comparing were the Normalized axial velocity through the vortex core and the cross flow velocity through the vortex core.

Immediately behind the wing, the norm. axial velocity is around 1.77*U, I get around 1.74*U which is very reasonable but at around x/c=0.6 downstream of the wing, the experimental results have 1.69*U and I have around 0.2-0.3 *U which is a clear indicator of over diffusion either due to the turbulence modeling or the numerical method.

I used both the SA and the KW-SST model with Gauss linear and Gauss QUICK conditions. Gauss QUICK gives slightly better results compared to the Gauss linear but not drastically different.

One thing to note, the residuals have flat lined in just around 600 iterations are about 10e-2 which is not really a good convergence criterion considering the basic geometry involved,and they dont even change. I am running a steady state simulation by the way.

Do you think I need to run a transient ?
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