First cell height calculation
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While performing CFD simulations, it is critical that one capture boundary layer near the wall properly. In order to do that, the mesh should be generated in such a manner that it captures boundary layer properly. For turbulent flows, calculation of the Y plus value of the first interior node/gridpoint helps in doing that. This dimensionless distance is defined as
where 
is the friction velocity defined as
The wall shear 
can probably not be determined until after a simulation has been completed, so it is usually necessary to estimate a value, and then check after a simulation is complete.
