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-   -   Mesh independence study and Wall Y+ (http://www.cfd-online.com/Forums/main/12219-mesh-independence-study-wall-y.html)

sam September 14, 2006 00:18

Mesh independence study and Wall Y+
 
Dea all

I would to know about the issues related to mesh indepedence and Y+ .

1. What is relationship between Mesh indepdence and y+

2. First we set the y plus or mesh independence .

3. what should be the strategy for meshing while studying mesh independence

4. What should be the strategy for meshing while studying y+

Thanks in advance


Mani September 14, 2006 10:22

Re: Mesh independence study and Wall Y+
 
That's a good question. I am assuming you are referring to turbulent flow. The y+ requirement comes from your turbulent model. Depending on the model, the first cell off the wall has to be within a certain y+ range. Whether the model uses wall functions or integrates to the wall will be an important factor. Anyway, as you perform your "mesh independence" study you should always meet this requirement. In other words, your grid resolution at the wall cannot be chosen arbitrarily low, or your mesh convergence study will be spoiled by a complete failure of the turbulence model.

sam September 14, 2006 22:32

Re: Mesh independence study and Wall Y+
 
thats big problem. that implies that we have play simultanously with mesh indepdence study and first cell distance.

and .......................... main concern is that how to do it within the shortest possible time.

Mani September 15, 2006 08:17

Re: Mesh independence study and Wall Y+
 
well, my point was that you should >not< play with the first cell distance --> keep it as required, i.e. it should stay the same for all grid resolutions. what you can do is to fix the first cell size but vary the stretching (in cell-normal direction) to get coarser outer resolutions.

alternatively, you can start with an acceptable grid and increase (not decrease) the resolution, making the first cell smaller, not larger. this will work well if your turbulence model integrates to the wall, and as long as your initial grid already resolved the laminar sublayer it will be a valid grid resolution study. however, this might be problematic for wall function methods, because they often require the first cell to stay within upper and lower limits. so, find out if your method uses wall functions or not, and then choose the appropriate approach.


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