[alexcastello]

Dear Foamers,

I want to know how to build and generate the y and y plus folders?

Thank you
Alex

[gcengineer]

Use standard utility yPlusRAS (or YPlusLES) or third-party utility yPlus (or others with similar names) to create yPlus folder. Read OpenFOAM documentation for info on standard utility. Search CFD-Online for info on third-party utilities. (Note that there are many threads that discuss whether yPlusRAS calculates y+ or y*.)

Use foamToVTK to convert t/yPlus to VTK format for paraView.

Sorry, I don't what you mean by "y folder."

[alexcastello]

thanks a lot for your reply.
can you tell me why is Yplus necessary to use?

[gcengineer]

That's a big question.

y-Plus measures wall effects. "High-Reynolds number" turbulence models are inaccurate close to a wall where the turbulent Reynolds number (Re.t) become small and the effect of viscosity dominates the flow. In such case, a wall-function should be used with the high-Re model, or a "low-Reynolds number" turbulence model should be used instead of the high-Re model.

Consider Menter's explanation:

The near wall treatment is of equal importance in practical industrial CFD simulations as the formulation
of the turbulence model itself. Integration through the viscous sublayer is typically referred to
as 'low-Re' approach. The terminology 'low-Reynolds' number refers to the turbulent Reynolds
number [Re.t = k2/eps-nu = nu.t/nu], which is low near the wall. This is one of the most confusing terminologies in
turbulence modelling as many engineers relate it to the device Reynolds number. A more appropriate
terminology is 'Viscous Sublayer Model (VSM)'. In aeronautics, VSMs are typically employed and the
equations are integrated to the wall, with a grid resolution of the order of y+ ~ 1 near the wall. Such
demands on grid resolution cannot be enforced in general purpose codes/applications. This has been
recognised by many groups over the last decade and numerous y+-insensitive wall treatments have been
proposed (Kim et al. 2002, Esch and Menter 2003, Craft et al. 2004, Kalitzin el al. 2005, Popovac and
Hanjalic 2007). The goal is to avoid the stringent grid resolution requirements of VSMs and to ensure that
the predicted wall shear-stress and heat transfer is largely independent of the near wall (y+) resolution.
This is of course only reasonable if there are otherwise enough grid points in the boundary layer.
There are many different names for such methods. The author would like to propose the terminology
'y+-insensitive' wall treatment.

See also the entry for "Dimensionless wall distance (y plus)" in the CFD-Wiki.