[Agit]

Hello FOAMers,


I am new to openFOAM (6 months) and have a few question about wallfunction and y+. I am using the current version openFOAM 6.


What I am trying to do:

I am using simpleFOAM with a kOmega-SST model for a 2D-case (flow around a rectangular cylinder with rounded edges) with high Re (1.42e+5)


My Querstions:

1. Is it possible to refine the cells until I reach my desired y+ value instead using layer addition in snappyHexMesh? I am having trouble getting a acceptable Mesh with layer addition, that's why I would try to set the surface level to refine my mesh until I get a proper y+ value next to the wall.

2. As far as I understand, with the kOmega-SST Model the turbulence near the wall can be calculated by the wall-function. This means, that I don't need to refine the mesh near the wall so excessively. My question is now: how do I estimate my y+ value to calculate the first cell-thickness? Should I use y+=75 for my first layer or as Tobias Holzmann stated in this thread (One more time: wall functions - SST

Thats why we can use wallFunctions which - in fact - are log functions (just check the source files). However, the log law is only valid in the range between [ 0 < y+ < 16.8] (I do not remember the exact maximum value; I refer to [1]).

Thanks in advance and please tell me, if I missed to mention some information.

[simrego]

Hi!


1. You can use refining but if it is possible you should align your cells to the flow direction, but with refining it is not so easy. (I think it is nearly impossible)
The real problem is the cell count. Since you split all cells into 8, then again, and again, you will have an enormous number of cells. So it won't be feasible.


2. You can use wall functions. There are also blended wall functions which will blend the function based on the y+ value in the cell.
With this little tool you can estimate the necessary cell size near the wall:
https://www.cfd-online.com/Tools/yplus.php
But this is mostly like a first guess, since the y+ value depends on local variables, which you don't know initially. So after the first run you will have to check it and adjust your mesh (and iterate).
The necessary y+ depends on your wall function. Low-Re, High-Re, or no wall function. Check the wall function you are using for the proper y+ value. Without wall function: y+<1 (with a high Re problem you will have a lot of cells, maybe it won't be feasible)

But if you can't have good y+ everywhere, don't panic, it's nearly impossible. If you have a bit high y+ far from your interest, it can be acceptable but then you should make some decision if you accept the results or not. It's up to you.


Tip: Layer generation with SHM can be pretty difficult. You should check cfMesh. This is included in OF-plus since (i think) v1706, but as i know you can use it as a standalone mesh generator tool. This will give you 100% layer coverage, really nice, good quality mesh, and much faster than snappy. But you can't create baffles, multiple regions, etc. So its knowledge is a bit limited.