[Roxy]

Hello,
My question is more about physical analyse than about software problems...
I am studying an internal air flow, and the models working best (getting results close from the experiments) are the Low Reynolds models, so I guess this is the type of my flow.
I've been reading about Low Reynolds models and they all say that you must use 15 to 25 Prism Layers. I found out that using less layers (7) but using a bigger Prism Layer Stretching (2) gets good results as well and uses less time to run the simulation.
Indeed, I understand that the important thing is to have a very thin first sub-layer... More than having a lot of layers.
What does it physically mean to need a thin first sub-layer?

Can anyone help me?
Thanks