What is exactly Y+? How do you choose(calculate) the size of the first cell? thanks for your help Al.

y+ is a non dimensional distance from a wall.

You choose the the size of the first cell to adequately resolve the flow and/or satisfy the requirements of the near wall treatment. This is principally a function of the turbulence model, the near wall treatment and the state of the flow adjacent to the wall. The requirments for an LES prediction are very different to that for a RANS k-e prediction. The requirements for a thin boundary layer can be significantly different to that for a thick boundary layer.

I would recommend reading something about near wall turbulence before tackling the CFD. A good introduction is contained in:

P. Bradshaw, "An introduction to turbulence and its measurement", Pergamon Press, 1971.

It is fairly widely held in libraries.

y+, the nondimensional distance from the wall, is defined as

y+=DeltaY*[(density^2 * Cmu^(1/2) * K)^(1/2)]/mu

for K-e turbulence models. DeltaY is the actual distance from the wall.

For the standard K-e model, y+ for the first cell should be 100 < y+ < 300. For the Low K-e model, y+ should be less than 1. Note that all of this depends on the code and model that you are using, it is just a general guideline.

David Creech

The CFX Subroutine Archive