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Convergence versus meshing granularity

Posted Sep 12, 2014, 5:47 p.m. EDT Computational Fluid Dynamics (CFD), Mesh, Studies & Solvers Version 4.4 2 Replies

John Stasko
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I have noticed interesting behaviour in my models.

If the model will solve using a coarser mesh, it may not solve well using a finer mesh.

And although use of the boundary layer tool can reduce the wall lift-off, it does not necessarily reduce convergence time, and often can lead to convergence instabilities. Sometimes I can only add 2 boundary layers.

Also, if I use an initial field from a coarsely meshed solution, the convergence behaviour for the finer mesh will only begin to act erratically earlier.

I have noticed these effects using the k-epsilon and k-omega turbulence models.
2 Replies Last Post Sep 24, 2014, 3:24 a.m. EDT
Victor Koppejan
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Posted: 10 years ago Sep 19, 2014, 3:44 p.m. EDT
Could you upload one of your models? I've had a similar problem which turned out to be due to improper usage of the turbulence length scale in the inlet.
Could you upload one of your models? I've had a similar problem which turned out to be due to improper usage of the turbulence length scale in the inlet.
Mikael Noerregaard Nielsen
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Posted: 10 years ago Sep 24, 2014, 3:24 a.m. EDT
"I have noticed these effects using the k-epsilon and k-omega turbulence models."

The closure problem of the Navier-Stokes equations are tricky, and introducing 2-equation turbulence models does provide a closed set of PDEs however, they have their strength and weaknesses. None of the mentioned models solve for the entire flow field. Furthermore, k-omega often performs better for steep gradients but it is not as robust as the k-epsilon. Often you can use k-epsilon for an initial guess before using k-omega. If you are doing wind-turbine simulations, I believe the Spalart-Allmaras is specifically designed for this but I can't remember precisely.

Turbulence Modelling for CFD by David.C.Wilcox provides a lot of insights and also describes where the constants related to each model comes from.

Best Regards
"I have noticed these effects using the k-epsilon and k-omega turbulence models." The closure problem of the Navier-Stokes equations are tricky, and introducing 2-equation turbulence models does provide a closed set of PDEs however, they have their strength and weaknesses. None of the mentioned models solve for the entire flow field. Furthermore, k-omega often performs better for steep gradients but it is not as robust as the k-epsilon. Often you can use k-epsilon for an initial guess before using k-omega. If you are doing wind-turbine simulations, I believe the Spalart-Allmaras is specifically designed for this but I can't remember precisely. Turbulence Modelling for CFD by David.C.Wilcox provides a lot of insights and also describes where the constants related to each model comes from. Best Regards

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