Page 103 - ansys

For standard or non-equilibrium wall functions, each wall-adjacent cell's centroid should be located
within the log-law layer, 30 < y+ < 300. A y+ value close to the lower bound (y+ ≈ 30) is most desirable.

Although the conventional wall functions can be used with fine near-wall meshes of y+ < 11.2, ideally
their use shoud be avoided. This is because the wall functions become much less accurate in this
region and the turbulence models employed inside the laminar sublayer might not be able to account
for the low-Reynolds-number effects. Both of these factors can considerably reduce the accuracy of
the calculation and cause convergence problems. The enhanced wall treatment should be used in such
cases.

6.2 Spalart-Allmaras Model

The Spalart-Allmaras model in its complete implementation is a low-Reynolds-number model. This
means that it is designed to be used with meshes that properly resolve the viscosity-affected region,
and the damping functions have been built into the model to attenuate the turbulent viscosity in the
viscous sub layer. The boundary conditions for the Spalart-Allmaras model have been implemented so
that the model is capable to work on coarser meshes that are suitable for the wall function approach.
In summary to achieve the best results with the Spalart-Allmaras model, one should use either a very
fine near-wall mesh (on the order of yp+ = 1 for the first near-wall cell center) or a mesh with y+ >= 30.

If you choose the Spalart-Allmaras model, the following options are available:

w Vorticity and Strain/vorticity -based production: For the Spalart-Allmaras model, you can
choose either Vorticity-Based Production or Strain/Vorticity-Based Production under Spalart-
Allmaras Production in the Viscous Model dialog box. If you choose Vorticity-Based Production,
ANSYS FLUENT will compute the value of the deformation tensor. These options will not appear unless
you have activated the Spalart-Allmaras model.

w Low-Re damping: If either of the Spalart-Allmaras or the Spalart-Allmaras Detached Eddy
Simulation models are used, you may enable a low-Reynolds damping to the turbulent viscosity by
enabling the Low-Re Damping option under Spalart-Allmaras Options in the Viscous Model dialog
box.

w Viscous heating (always activated for the density-based solvers)

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