Wall Function on entrance region

dasang · January 31, 2018, 04:08

Hi,
I'm simulating a pipe flow but entrance region is expeced to dominate the tube.
Is it suitalbe to use the wall function method to simulate the situation?

Thank you all!

sbaffini · February 5, 2018, 04:39

It actually depends from several factors, among which Re number, relative grid refinement and your intended inlet profile are the most important.

Let us make an example. You say entrance region dominates. Thus I assume your inlet profile is flat. You also want to use wall functions, thus I assume your grid is coarse with respect to what you expect to happen, and also Re is high then.

In this case, using a Wall Function on a high Re case with a flat profile actually means assuming that your inlet profile is not flat, but that at the walls it conforms to the wall function at that given Re number. In the limit case, you might end up with an almost fully developed region already at the very entrance of your domain.

I' m not saying this is wrong, just maybe not what you would expect.

In general, when dealing with models that describe things at a smaller scale with respect to your grids (wall functions, wall roughness, SGS models in LES, etc.) you need the mechanism described by that model to NOT be relevant in your simulation. Otherwise your grid (and the resulting simulation) are clearly wrong.

If you think that describing the development region in our domain is important, you should avoid describing it with Wall functions. Obviously, if your code cannot switch off the wall functions (e.g., Fluent) and you enter the domain with a perfectly flat profile, the very initial development zone will always be affected, no matter what you do with your grid.

January 31, 2018, 04:08	Wall Function on entrance region	#1
dasang New Member Join Date: Jul 2017 Posts: 1 Rep Power: 0	Hi, I'm simulating a pipe flow but entrance region is expeced to dominate the tube. Is it suitalbe to use the wall function method to simulate the situation? Thank you all!

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February 5, 2018, 04:39		#2
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,151 Blog Entries: 29 Rep Power: 39	It actually depends from several factors, among which Re number, relative grid refinement and your intended inlet profile are the most important. Let us make an example. You say entrance region dominates. Thus I assume your inlet profile is flat. You also want to use wall functions, thus I assume your grid is coarse with respect to what you expect to happen, and also Re is high then. In this case, using a Wall Function on a high Re case with a flat profile actually means assuming that your inlet profile is not flat, but that at the walls it conforms to the wall function at that given Re number. In the limit case, you might end up with an almost fully developed region already at the very entrance of your domain. I' m not saying this is wrong, just maybe not what you would expect. In general, when dealing with models that describe things at a smaller scale with respect to your grids (wall functions, wall roughness, SGS models in LES, etc.) you need the mechanism described by that model to NOT be relevant in your simulation. Otherwise your grid (and the resulting simulation) are clearly wrong. If you think that describing the development region in our domain is important, you should avoid describing it with Wall functions. Obviously, if your code cannot switch off the wall functions (e.g., Fluent) and you enter the domain with a perfectly flat profile, the very initial development zone will always be affected, no matter what you do with your grid.