One of the most essential issues for the optimal performance of turbulence models is the proper resolution of the boundary layer. In this section, two criteria are suggested for judging the quality of a mesh:
Minimum spacing between nodes in the boundary layer
Minimum number of nodes in the boundary layer
These are simple guidelines for the generation of meshes that satisfy the minimal requirements for accurate boundary layer computations.
The goal is to determine the required near wall mesh spacing, 
, in terms of Reynolds number, running length, and
a 
target value. This target value depends on the flow
type and the turbulence model in use (that is, on the near-wall treatment
in use). See Scalable Wall Functions and Automatic Near-Wall Treatment for Omega-Based Models in the CFX-Solver Theory Guide for more information.
The estimates will be based on correlations for a flat plate with a Reynolds number of:
 | (4–17) |
with characteristic velocity 
and length of the plate 
.
The correlation for the wall shear stress coefficient, 
, is given by White [14]:
 | (4–18) |
where 
is the distance along the plate
from the leading edge.
The definition of 
for this estimate is:
 | (4–19) |
with 
being the mesh spacing
between the wall and the first node away from the wall.
Using the definition
 | (4–20) |
can be eliminated
in Equation 4–19 to yield:
 | (4–21) |
can be eliminated using Equation 4–18 to yield:
 | (4–22) |
Further simplification can be made by assuming that:
where 
is some fraction.
Assuming that 
, then, except for very small Re
x, the result is:
 | (4–23) |
This equation allows us to set the target 
value at a given 
location and obtain
the mesh spacing, 
for nodes in the
boundary layer.
A good mesh should have a minimum number of mesh points inside the boundary layer in order for the turbulence model to work properly. As a general guideline, a boundary layer should be resolved with at least:
 | (4–24) |
where N normal, min is the minimum number of nodes that should be placed in the boundary layer in the direction normal to the wall.
The boundary layer thickness 
can then be
computed from the correlation:
 | (4–25) |
to be:
 | (4–26) |
The boundary layer for a blunt body does not start with zero
thickness at the stagnation point for 
. It is, therefore, safe to assume that 
is some fraction of 
, say 25%. With this assumption, you get:
 | (4–27) |
You would, therefore, select a point, say the fifteenth off the surface (for a low-Re model, or 10th for a wall function model) and check to make sure that:
 | (4–28) |