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-   -   Is boundary layer "a must" in turbulence flow? (http://www.cfd-online.com/Forums/fluent/64803-boundary-layer-must-turbulence-flow.html)

Pingaroo May 25, 2009 07:06

Is boundary layer "a must" in turbulence flow?
 
I wonder in modelling a turbulence flow, is constructing a boundary layer before meshing very necessary? Can we still run a turbulence model problem without it?

And if the model is very complex, how are we suppose to calculate for the height of the first row in boundary layer??? :rolleyes:

Byron May 25, 2009 08:50

Quote:

Originally Posted by Pingaroo (Post 217078)
I wonder in modelling a turbulence flow, is constructing a boundary layer before meshing very necessary? Can we still run a turbulence model problem without it?

And if the model is very complex, how are we suppose to calculate for the height of the first row in boundary layer??? :rolleyes:


Hi ,Pingaroo

I would like to talk with you about this interesting issue.Yes, Ii is necessary to decide which turbulence model shoul be choosed to match our object.As all known, the Navier-Stokes equations is not closed themselves.Therefore, the turbulence model is indispensable.

Whether the model is quite complex or simple, the height of the first row is same to calculate before meshing grids.Here, I just ensure this problem.If you wonder knowing how to calculate the ds, I am also glad to discuss it with you .

Byron.25th,May.

Ralf Schmidt May 25, 2009 09:02

Hi!

If you focus your simulation in heat transfer or drag, it is VERY important to have a properly BL treatment. For that, there are basically two types of turbulence modells (RANS):
1 near wall treatment and
2 wall function.

For 1. you need to resolve the BL with your mesh down to a value of yplus < 5 (best is yplus about 1). And there should be a number of cells within the BL (something like min. 5, but for good results i would suggest 15-25)

For 2. the BL is not resolved and there is a standard log function assumed. Your yplus could be between 30 and 300 (best is about 30).

For calculation of the first row in your BL fitting to the desired yplus see: http://www.cfd-online.com/Wiki/Dimen..._wall_distance

You don't have to use the BL option in Gambit to resolve the BL with a mesh. You might use any other cell type, as long it is fine enough.

BUT, the BL option is a very nice tool and it will result in general in a very good quality mesh.

Another hint: if your yplus in the simulation is out of the desired range, you can adapt the yplus in the adapt menu (->adapt->yplus, or adapt-> region)

Best wishes!
Ralf

Pingaroo May 25, 2009 12:09

Hi Byron and Ralf, Thank you for your replies.

So basically boundary layer is a must when we need to focus on drag or heat transfer.... and is it always a good idea to check whether the turbulence model is in the wall yplus range?

I know that we can calculate the height of the first row by using flat plates skin friction but it is for a very simple model. So, Byron would you please discuss more about complex model?


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