Meshing consideration and wall functions

Meshing when using wall functions

Wall functions describe the flow velocity as a function of distance from the wall within the near-wall region. By using wall functions to approximate the mean velocity field in the near-wall region of the boundary layer, you avoid mesh requirements needed to resolve the viscous sublayer.

Use wall functions for first pass models as they are computationally less expensive than resolving the whole inner layer, and for turbulent flows on streamlined geometries without flow separation.

When you use wall functions to model flow and turbulence quantities near the wall, the flow solver provides near-wall boundary conditions for the momentum and turbulence transport equations without resolving the viscous sublayer. The wall function implementation accounts for each inner layer and also incorporates the effects of surface roughness.

When you request standard wall functions:

• For body-fitted fluid meshing, the centroid of the wall-adjacent control volume must be located in the upper part of the buffer layer or the lower part of the log-law region, y+≥30.

• For immersed boundary meshing, the element size of the 3D immersed boundary mesh close to the immersed boundary walls must be defined such as the estimated y+, using around half of this element size as a characteristic length, leads to y+≥30.

When you request hybrid wall functions, the previous requirement for y+ is relaxed. The centroid of the wall-adjacent control volume can be located anywhere from the wall to the lower part of the log-law region: y+>0. The hybrid wall functions are less sensitive to the first layer y+ values and provide a more robust performance upstream of the separation point in comparison with standard wall functions.

The following table summarizes the supported wall treatment options for each turbulence model.

Turbulence model	No slip wall	Standard wall function	Hybrid wall function	Comments
(None) Laminar Flow	Yes	No	No	Wall functions do not apply to laminar flows.
Fixed Turbulent Viscosity	No	Yes	No	N/A
Mixing Length	No	Always	No	Even if you select other options from the Wall Treatment list, this turbulence models always uses the standard wall function.
Standard K-Epsilon	No	Always	No	Even if you select other options from the Wall Treatment list, this turbulence models always uses the standard wall function.
RNG K-Epsilon	No	Always	No	Even if you select other options from the Wall Treatment list, this turbulence models always uses the standard wall function.
Realizable K-Epsilon	No	Always	No	Even if you select other options from the Wall Treatment list, this turbulence models always uses the standard wall function.
K-Omega	Yes	Yes	Yes	N/A
SST — Shear Stress Transport	Yes	Yes	Yes	N/A
SA — Spalart - Allmaras	Yes	Yes	Yes	N/A
LES — Large Eddy Simulation	Yes	Yes	No	N/A

You request the use of wall functions from the Wall Treatment list in the following dialog boxes and commands:

• 3D Flow page in the Solution dialog box

• Solid Blockage type of Flow Blockage simulation object

• Flow Surface simulation object

Note:

A single solution can only contain one type of wall functions: you cannot specify standard wall functions on some surfaces and hybrid wall functions on others. A combination of flow surfaces with and without wall functions is allowed.

Meshing when not using wall functions

For wall-resolved flow surfaces, you must create a very fine mesh close to the wall because the entire inner layer including the viscous sublayer, the buffer layer, and the log-law region is resolved. The centroid of the wall-adjacent control volume lies in the viscous sublayer such that y+ ≤ 1.0. In the boundary layer, you should use a mesh expansion ratio less than 1.2.

You should have at least 5 to 10 nodes in the viscous sublayer to capture fluid flow near walls for models with the following conditions:

• Large pressure gradients along the wall.

• Strong body forces, for example flow near a rotating body.

• Complex 3D geometry near walls.

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