|
[Sponsors] |
November 18, 2020, 10:46 |
Wall Treatment in the buffer Region by CFX
|
#1 |
New Member
Join Date: Nov 2020
Posts: 3
Rep Power: 5 |
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:
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 |
|
November 18, 2020, 11:58 |
|
#2 |
Senior Member
Join Date: Jun 2009
Posts: 1,788
Rep Power: 31 |
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,
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
|
November 18, 2020, 12:27 |
|
#3 |
New Member
Join Date: Nov 2020
Posts: 3
Rep Power: 5 |
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). |
|
November 18, 2020, 16:28 |
|
#4 |
Senior Member
Join Date: Jun 2009
Posts: 1,788
Rep Power: 31 |
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.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
|
January 26, 2024, 15:11 |
|
#5 |
New Member
Abhishek Dey
Join Date: Sep 2021
Posts: 3
Rep Power: 4 |
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?
|
|
January 28, 2024, 22:12 |
|
#6 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143 |
There is many, many other things which must be correct beyond the near wall resolution for a CFD simulation to be correct.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Error when solving p_rgh | bob94 | OpenFOAM | 0 | March 17, 2020 09:12 |
Problem with chtMultiregionFoam radiation boundary condition | baran_foam | OpenFOAM Running, Solving & CFD | 10 | December 17, 2019 18:36 |
On the LES near wall treatment | huangxianbei | FLUENT | 8 | August 27, 2014 12:13 |
Radiation interface | hinca | CFX | 15 | January 26, 2014 18:11 |
Error finding variable "THERMX" | sunilpatil | CFX | 8 | April 26, 2013 08:00 |