first grid size and y-plus
 

hi
i am going to model a turbulent flow over a flat plate.
if i want to extract a given y-plus, how can i set the first grid size?
is there any formula to calculate the distance between wall and first grid in a given flow?

thank you in advance

The first grid height can be set easily when you're first meshing the problem (Gambit, ANSYS Meshing, etc..). After the mesh has already been created, fluent has some adaptation routines for refining the mesh near walls/boundaries, assuming they've been named. Have a look through the manual: Adapting Grids

As far as the required spacing, have a look here:http://geolab.larc.nasa.gov/APPS/YPlus/

Hi morteza,
note that y+ is a function of u*,nu and y. therefore you should compute average y+ on your plate. because u* varies along the plate , to obtain the same y+ over it , you ought to use non uniform grid that is not reasonable.
I think a try and error procedure over averaged y+ using adaptation tools in FLUENT can be helpful.

The y+ calculators are usually based on formulas for the flat plate. However you can successfully use them for curved surfaces (airfoils) using a bit of trial and error.

If you are interested in the underlying theory check:

Anderson - Fundamentals of Aerodynamics

hi deAR friend
the calculator that you have sent its link for me , is for which turbulence model?

It's turbulence-model-independent.

Please see the following:http://www.cfd-online.com/Wiki/Dimen..._wall_distance

RE:first grid size and y-plus
 

The y+ value is u*.y/nu, where y is the distance to the nearest wall. and this is your first grid size. u* you can take 1-5% of the bulk velocity and nu is the local kinematic viscosity and it depends upon the fluid is gas or liquid. But i take the same as the bulk fluid is in the computation. but it varies if flow is multiphase then nu will be different.

then if you want the y+ value 0.1 or less then this value then you can calculate the y value with above equation and this will be your first grid size.