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CFD, wall lift-off and the Boundary Layer attribute

Posted 25.08.2014, 16:45 GMT-4 Computational Fluid Dynamics (CFD), Mesh Version 4.4 2 Replies

John Stasko
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This question comes from my poor understanding of the "boundary layer" tool in the mesher.

1. It appears that these "boundary layers" are one technique to ensure that "wall lift-off" is of the proper order. IS THIS CORRECT? I have worked through this fabulous model www.comsol.eu/model/download/176325/models.cfd.pipe_elbow.pdf and have discovered that they are using boundary layers to get good solution.

2. When I mesh without the boundary layer, I get a certain "appearance" of the mesh, and the minimum element quality is 0.14, which is considered "good." Then I add the boundary layer. This increases the number of elements from about 2.3M to 2.9M, but the minimum element quality now drops to 3E-8 and the mesh does not change in appearance. My interpretation of this is that the boundary layer is a mathematical construct outside the problem at hand, so it does not appear in the mesh, and so this new, lower and "bad" minimum element quality is not a concern. IS THIS CORRECT?

3. I have heard that k-epsilon is degerate near the walls and therefore requires wall functions, whereas the k-omega model does not. Does this mean that wall lift-off is not considered in the k-omega model, and consequently the work with boundary layers is obviated?

Thank you for your kind help!
2 Replies Last Post 19.09.2014, 15:54 GMT-4
John Stasko
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Posted: 1 decade ago 25.08.2014, 16:53 GMT-4
Actually, that model uses both k-omega and boundary layers, so that answers the last question: wall lift-off and boundary layers are important considerations for the k-omega model as well.
Actually, that model uses both k-omega and boundary layers, so that answers the last question: wall lift-off and boundary layers are important considerations for the k-omega model as well.
Victor Koppejan
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Posted: 1 decade ago 19.09.2014, 15:54 GMT-4
The difference for the kappa-epsilon and kappa-omega model (as well as other turbulence mode) is quite nicely covered in the theory sectios of the CFD manual.

In a nutshell, k-e uses a wall function based on self-similarity to cover the viscous effects at the wall. k-om uses a specific turbulent energy dissipation (k-e uses non-specific) and integrates up until the wall.
The difference for the kappa-epsilon and kappa-omega model (as well as other turbulence mode) is quite nicely covered in the theory sectios of the CFD manual. In a nutshell, k-e uses a wall function based on self-similarity to cover the viscous effects at the wall. k-om uses a specific turbulent energy dissipation (k-e uses non-specific) and integrates up until the wall.

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