Hi,

in a RANS of a wall bounded flow, where must I put the first point of my mesh.

I know that to do a good DNS or LES the first point must be located in the viscous sub-layer (y+ < 5) what leads to very fine mesh in the boundary layer (and obviously very expensive computations). Is it the same for RANS?

Or can we use the universal logarithmic law to reduce the density of nodes in the layer (without introducing approximated boundary condition)?

RANS implies the use of a turbulence model. Where you should place your first mesh point depends on which turbulence model you use. There are low-Re models which should have points down into the viscous sublayer and there are high-Re models which use wall-laws and thus only need one point in the log-region.