The large-eddy simulation approach (LES or SGE in French) proposes an intermediate way between averaged approaches RANS and direct simulation DNS.
Unlike the RANS approach, LES gives access to fluctuating quantities without requiring the resolution of all spatio-temporal scales, as for DNS; the effect of small non-resolved scaled (“filtered” scales) on the resolved ones have to be modeled.
Since modeling is based on the assumption of isotropy for the smallest resolved scales (Smagorinsky model, WALE, structure-function ...), this should require the use of very refined meshes.
In industrial applications and despite the use of multi-million element meshes, this last criterion cannot be fulfiled near walls where flows are locally strongly anisotropic.
As a consequence, at the walls, where turbulence is produced and small coherent structures called streaks are generated (see figure), the mesh-cells are much larger than these structures.
The “universal” character of LES models being inappropriate to correctly resolve the flow in the near-wall region in such conditions, a specific wall-modeling has to be employed to reproduce the local effect of the wall-turbulence on the rest of the flow: this specific modelling is called wall functions.