[dinhanh]

Hi,

In RANS model such as k-epsilon model, we usually use the wall function approach at the first node away from the wall.
In 2D calculation, of course, the y+ is calculated base on the distance in normal direction (here we have one normal direction, one wall, assume y direction). However, in 3D wall bounded calculation, there is more than normal direction. So if we have rectangular calculation domain: 16 mm long in x direction, 4 mm in height (y direction) and 1 mm in z direction. Flow direction is assumed in x direction. So now, the problem is the y+ calculated from the wall in z direction may be smaller than that one from y direction.

1- How is the wall function handled?
2- I might be wrong that, Normally, the effect of shear stress term in y direction is more importance than z direction, isn's it?

[sbaffini]

The wall function is just a Black box which is invoked for a cell face which is a wall in order to obtain the wall stress to use as boundary condition.

Thus, for a given cell, it is invoked independently for each of its wall faces. For each of them you can identify a wall normal direction and the wall parallel velocity direction and magnitude. That's all you need.

[dinhanh]

Originally Posted by sbaffini

The wall function is just a Black box which is invoked for a cell face which is a wall in order to obtain the wall stress to use as boundary condition.

Thus, for a given cell, it is invoked independently for each of its wall faces. For each of them you can identify a wall normal direction and the wall parallel velocity direction and magnitude. That's all you need.

Thank you, sbaffini.

As I understand that the law of wall function will apply at the first cell central to all wall, then calculation velocity at that cell from the wall shear stress, doesn't it?

However for the boundary condition for the turbulent kinetic energy and turbulent dissipation rate, for example k=tau/constant, the value at cell central is affected by two wall boundary. So how is it value handed?

Also I got this information from Fluent manual:

"4.13.5.1.Two-Layer Model for Enhanced Wall Treatment
In ANSYS FLUENT’s near-wall model, the viscosity-affected near-wall region is completely resolved all
the way to the viscous sublayer. The two-layer approach is an integral part of the enhanced wall treatment
and is used to specify both


and the turbulent viscosity in the near-wall cells. In this approach, the
whole domain is subdivided into a viscosity-affected region and a fully-turbulent region. The demarcation
of the two regions is determined by a wall-distance-based, turbulent Reynolds number,, defined as
where y is the wall-normal distance calculated at the cell centers. In ANSYS FLUENT, is interpreted as the distance to the nearest wall"

What does the nearest wall mean? Is it smallest distance from cell central to walls?