Menu

Discussion Forum

New Discussion
Filtern

Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Convergence versus meshing granularity

Posted 12.09.2014, 17:47 GMT-4 Computational Fluid Dynamics (CFD), Mesh, Studies & Solvers Version 4.4 2 Replies

John Stasko
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

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 24.09.2014, 03:24 GMT-4
Victor Koppejan
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 19.09.2014, 15:44 GMT-4
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 24.09.2014, 03:24 GMT-4
"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

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content