Wall Treatment in the buffer Region by CFX
 

Dear all,

I am using CFX and k-omega SST, to simulate centrifugal fans at low velocities.
Having a yPlus equal to 1 is not affordable for my cases so I need to live with higher values.
Therefore my question is this, correct me if I am stating something wrong:

• when y Plus < 5 you get a wall resolved solution and you do not apply a wall function and it is the best case (k-omega);
• when y Plus > 30 and < 300, CFD code uses a wall function as in k-eps
• But what happens if I have yPlus in between 5 and 30 (buffer region)? Should I avoid this situation?

I am asking you this because some ANSYS experts told me that I can be fine with a yPlus < 300, and a good blending is performed even when yPlus is in the range between 5 and 30.
So I try to stay with a yPlus < 300, but it happens in my cases to have yPlus in this range between 5 and 30 and this situation is not very easy to be avoided.

Another question: when the wall function is applied, its velocity profile is superimposed to the solution or it just changes the wall viscosity in order to get the correct wall shear stress, still maintaining a linear velocity profile?

Thanks in advance.

Mauro

For the discretization question, I would suggest reading the theory guide section for "modeling the flow near the wall"

You can read in there what is blended, and how it is blended.

Definitely, the velocity profile is never imposed on the nodes of the mesh, it comes out of the solution given the boundary conditions for velocity.

Hope the above helps,

Thanks for the answer,

I will have look into it.

Perfect, thank you for the tip. What I meant is: if you solve for the velocity in the cell centroid of the first cell close to the wall, and you know that the velocity is zero at the wall, you could apply a nonlinear function (wall function) which is in good agreement with the real profile in order to capture the correct gradient. But I have read somewhere that this does not happen in every CFD code, and you only have a modification of the viscosity at the wall (by means of the wall function) in order to compute the correct wall shear stress, maintaining a linear velocity profile between the cell centroid of the first cell close to the wall and the wall (for all yPlus values).

The exact details of the implementation are proprietary for every code, and it depends on how you organize your discretization parts.

What matters at the end is that you introduce the correct flux into the control volume.

Recall that every finite volume method based CFD code integrates the equation around a control volume

Then, volume integrals are transformed into surface integrals (via Gauss divergence theorem) to compute the flux across the face. Therefore, the equation for a control volume is just a balance of fluxes and sources.

The relevant information is in that document, you may have to interpret it against your view of what each CFD code does.

Hi, I also have a doubt somehow related to this issue. I have done one simulation which involves turbulent air flow over a fin. I have used SST k-omega model for the simulation and I am getting a maximum Y+ value of 11 and an average Y+ value of 3.9. The simulations are performed in OpenFOAM 2112 software with wall functions. Can I rely on the result of this simulation?

There is many, many other things which must be correct beyond the near wall resolution for a CFD simulation to be correct.