In "modeling turbulence" section of fluent Tutorial (Fluent6.2 User Guide), it reads as below:

Now, my question is:

How to make my grid adequate in practice?

Thanks for your specific suggestion.

99999999999999999999999999999999999999999999999999 999999999 11.12.1 Mesh Generation

If the flow is wall-bounded, and the wall is expected to significantly affect the flow,take additional care when generating the mesh. You should avoid using a mesh that is too fine (for the wall function approach) or too coarse (for the enhanced wall treatment approach). 99999999999999999999999999999999999999999999999999 999999999

First you have to decide if you are going to use a wall-function model or if you want to have fully resolved boundary layers. Wall functions should have the first grid point at y+ between say 30 and 200. A fully resolved grid should have the first grid point at about y+ 1.

You can estimate the actual size of y+ using one of the tools in:

http://www.cfd-online.com/Links/tools.html#yplus

For further information about y+´see:

http://www.cfd-online.com/Wiki/Dimen...wall_distance_%28y_plus%29

By the way, in the last few years commercial codes have started to use a new type of mixed wall treatments, which to some extent avoids the need to either have a resolved wall (y+ 1) or a wall-function mesh (y+ between 30 and 200). If you want to use this make sure that your model does support this and it would also be good to run some kind of tests before you trust the results from a mixed wall-treatment simulation too much.