How Do I Decide on a Wall Treatment?

There are clearly many applications where a wall-function-type mesh is adequate, and many applications where it is necessary to resolve the viscous-affected region. However, if the near-wall mesh resolution is not consistent with the modeling assumptions, significant errors can result.

The high-

wall treatment might save some computer storage by not requiring the computation of wall distance when used with certain high-Reynolds number models. It is suitable for use with models that do not explicitly damp the turbulence in the near-wall region (such as the Spalart-Allmaras high-Reynolds number model, the standard K-Epsilon model, the realizable K-Epsilon model, the standard K-Omega model, the SST K-Omega model and the BSL K-Omega model). While a good rule of thumb is that the wall-cell centroid should be situated in the logarithmic region of the boundary layer (

), it is unlikely that significant error will result from

values as low as 12. It is often inevitable that a few cells have a small value of

, especially near stagnation or separation. This is acceptable.

The low-

wall treatment makes no explicit modeling assumptions at all. It should only be used if the entire mesh is fine enough for

to be approximately 1 or less.

The all-

wall treatment is the most general, and should be used whenever available.

The above guidelines for both the high-

wall treatment and the low-

wall treatment reflect suitable lower limits on

. However, when wall-function-type meshes are used, the upper limit on

will be determined by many factors, particularly the Reynolds number and the presence of pressure gradients. Since the potential for errors increases with large values of

, it is generally prudent to aim for values between 30 and 50.

Not all wall treatments are available with every model or model variant. Some high-Reynolds number models do not contain the ability to attenuate the turbulence in the viscous-affected regions and therefore only include the high-

wall treatment. Some low-Reynolds number models are offered only with the low-

wall treatment, and the K-Epsilon two-layer models are only provided with the all-

wall treatment.