I am interested to know how one can calculate y+ before doing a simulation. I have seen in papers on backward facing steps, that people use "wall coordinates" to define resolution. Can anyone explain how I can do this given a particular backward facing step problem?

Well, all you do is to assume that you would like to keep Y+ below 100 or so, then you calculate the first cell height based on that, which eventually decides the grid spacing.. This is done to get a good first-hand mesh which can resolve Y+ decently upon simulation. Well, it has one glitch though since the first hand calculations are done using global Reynold's number while it could be quite different locally. Still, it gives at least some help.

Here is the formula:

y = (Y+)* L/(Re * sqrt(Cf/2))

Here, L is characteristic length (let's say diameter in case of pipe, probably height in your case). Re is the Reynold's number and Y+ you would normally put as 100.

Cf/2 is different for different kind of geometries and can be obtained by a simple search from internet. For example for a pipe,

Cf/2 = 0.039 * (Re ^ (-0.2))

http://geolab.larc.nasa.gov/APPS/YPlus/ This site will help you too.

Thank you. Can you tell me where I can find information on CF (Coefficient of Friction??) on the web? I tried to find the info you presented above: Cf/2 = 0.039 * (Re ^ (-0.2)) for a pipe, but cold not find it anywhere. Thanks.