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Wall lift-off and wall lift-off in viscous units

Posted Oct 31, 2011, 6:09 a.m. EDT Fluid & Heat, Computational Fluid Dynamics (CFD), Results & Visualization Version 4.2a 2 Replies

Carla Cotas
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Hello,

I'm simulating a pipe flow and I have some doubts about the wall lift-off and the wall lift-off in viscous units. I'm sorry for asking this, but I don't find good information about this two variables and it's very difficult for me interpret the information obtained in the 2D plot (that are attached). The case A and the case B have mesh differents. The mesh in case A is very coarse.

Someone can help me to interpret these results or give me some references where I can find information related to these variables. Thank you.

Carla
2 Replies Last Post Apr 20, 2012, 11:02 a.m. EDT
Amir Fadel
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Posted: 1 decade ago Oct 31, 2011, 7:24 a.m. EDT
Hi,

The first step would be to have an idea of what comsol calls wall lift-off distance, there is some explanation in the CFD module user's guide, pages 162 and following.

It would be a good idea to understand also the concept of boundary layer in fluid mechanics, laminar and turbulent; some good books have been written by Schetz and White.

Finally, you may have a look at the literature quoted in the comsol user's guide. Where it seems there is a conceptual error in the choice of the wall lift-off distance in viscous units, but that is a different (and longer) story as Kipling may say.

Cheers
Hi, The first step would be to have an idea of what comsol calls wall lift-off distance, there is some explanation in the CFD module user's guide, pages 162 and following. It would be a good idea to understand also the concept of boundary layer in fluid mechanics, laminar and turbulent; some good books have been written by Schetz and White. Finally, you may have a look at the literature quoted in the comsol user's guide. Where it seems there is a conceptual error in the choice of the wall lift-off distance in viscous units, but that is a different (and longer) story as Kipling may say. Cheers
Sin Kim
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Posted: 1 decade ago Apr 20, 2012, 11:02 a.m. EDT
According to k-e turbulence model the first grid point must be located such that the wall lift off in viscous units (actually a non dimensional distance measured from the wall to the wall adjacent grid point) is 11.06 (according to comsol 4.2 manuals) however according to fvm codes it can be within 30 to 100 as well. You can get a value of 11.06 by further refining the mesh near the wall as you have shown in fig B for wall lift off in viscous units. Remember k-e turbulence models are highly grid dependent and you must make sure that the wall lift off in viscous units complies to their demand.

Regards
According to k-e turbulence model the first grid point must be located such that the wall lift off in viscous units (actually a non dimensional distance measured from the wall to the wall adjacent grid point) is 11.06 (according to comsol 4.2 manuals) however according to fvm codes it can be within 30 to 100 as well. You can get a value of 11.06 by further refining the mesh near the wall as you have shown in fig B for wall lift off in viscous units. Remember k-e turbulence models are highly grid dependent and you must make sure that the wall lift off in viscous units complies to their demand. Regards

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