The viscous flow near a wall with a no-slip condition is characterized by shear stress, which is related to the velocity gradient perpendicular to the wall. The distance z of a point from the wall, relative to the viscous length δν, is the relative wall distance z+, which can only be determined or verified after calculation:
The dimensionless wall distance z+ defines different flow regions: the viscous sublayer (z +< 5), the buffer layer (5 < z + <30), and the turbulent boundary layer (z+>30). In practice, the complex wall behavior is often approximated by wall functions. A distinction is made between "near-wall" or "low-Reynolds-number" models, which resolve the sublayer ( z+≈1), and "far-wall" or "high-Reynolds-number" models, in which the first cell should lie outside the sublayer (z+>11).
For accurately capturing pressures and forces, realistic modeling of flow separation is crucial. For sharp-edged structures, this occurs at the edges, while for curved surfaces, the process is more complex and requires advanced wall laws.
When using wall functions, the equivalent sand-grain roughness ks must be less than or equal to the distance of the first computational node from the wall. The exact position of this node depends on the numerical method used.
Shear stresses contribute to the total resistance and are especially important for the horizontal force coefficient in wing-like structures. For structures with flow separation, correctly capturing the shear layers in the separation regions is of greater significance.