Dale,
I double-checked this with our documentation maintainer and he said we don’t have anything about relaxation factors yet. The entire numerics section is high on the backlog, though. So it will come at some point in the near future, I don’t have an ETA though.
Johannes
Dale,
Im not sure if this is helpful but i have found a small section in The finite volume method in computational fluid dynamics book that might shed some light on this. Section 8.9 page 256. Im sure theres more information out there on recommended values.
Dan
Thanks Dan,
I believe that Section 8.9 only restates, in more mathematical terms, what the quote from LuckyTran states in my starting post of this thread…
Section 8.9 says “it should be noted that at convergence ϕC and ϕC* become equal independent of the value of relaxation factor used”
The data and runs that were provided in the beginning of this topic do not seem to confirm those statements.
I just re-read this topic and your above statement jumped out at me. I am surprised I did not respond at the time.
Any way, I have come to believe that changing quality factors does not make for differences in the results or convergence. They just make you feel better since they are the benckmark for illegal cell determination. I could be wrong about this and I would really like to be set straight on this if I am wrong 
I personally do not fully understand what is really going on here. However my guess is that the quality metrics only tell the algorithm during the meshing process, which cells to and go back and fix. So after the first round of snapping/ castillatin/ layer addition, ANY cell above ANY of the set quality metrics refer to the RELAXED snapping/castillation/layer addition settings specified in the mesh settings tree.
For example, the max non-ortho setting at 70 deg has 2000 illegal cells, this tells the algorithm to try and re-snap or move cells these 2000 cells based on the looser snapping rules set in the settings. It will most likely not be able to move all 2000 to a non-illegal position resulting in 1890 illegal cells at the end of meshing. However, 110 cells were moved to a position where non-orthogonality is below 70 deg … a good thing.
Now max non- ortho is set tot 75 deg, resulting in 154 illegal cells. Same process as above ensues and the end mesh result is 74 illegal cells. So now we have 80 cells in a non-illegal position but this non-illegal position is under 75 deg not under 70 deg which i would assume is not the better solution.
HAVING SAID THAT… now the question becomes, does the max-ortho setting of 70 deg instead of 75 deg move more illegal cells that have a very HIGH degree of non-ortho (85-90). As in the really bad cells that cause mesh and simulation failures.
It would be interesting to try and do meshes and simulations with variying amounts of Max non-ortho fromm 70 to 89 deg, ignoring the illegal cell count output, and seeing where the failure point is.
My opinion is still that a geometry will inherently produce non-orthogonal cells, and only changing the geometry, or its position in the meshing domain, will drastically effect the non-ortho count.
Dan
I was hoping my unscientific belief was wrong 
Sounds like your understanding at this point is much better than mine.
What about the cells that end up being between 70 and 75 degrees when you set to 75 and then mesh. The algo thinks the 70 to 75 ones are OK so it likely does not move them because illegal cells are only above 75. This is why I think increasing to 75 may be a losing battle.
If the algorithm could have gotten rid of the few nasty ones that really matter 85-90, it likely would have done so even with it is set to 70… Just rambling… Maybe it can never take an 85 cell reduced to <70 no matter what value we put in, or maybe to reduce the 85 cell it needs to put many 75 cells around it. I just can’t figure out why it would do its meshing any more diligently between different settings, but I guess it somehow does…
As a rule, bad cells (that affect results and convergence) are >70 so when we set to 75 we lose to ability to have a count of the ones still between 70 and 75.
Yes, it could take a lot of core hours to investigate this and I don’t have many right now…
My understanding is just a guess, nothing more haha.
Yes i agree that changing from 70 to 75 wouldnt help that much as the cells in between these values are left out. You could argue to go the other way and use 65 as the max value, adding more cells to the relaxed settings but this does not really solve the problem.
I am not even close to the experience level in meshing where i can make a good observation on this but from what i have done so far, changing the geometry has done the most.
Second place would be increasing mesh fineness
Third would be layer addition changes
pretty much anything that will make the algorithm snap cells to another location either on the surface or in space. This is why the mesh quality feature was a game changer for me. I used this extensively when checking meshes for really bad cells. then i could go back and add a camfer or fillet and the mesh snaps to a new location, solving the problem.
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I do not think you layered the locations where you were having the orthogonal issues, were they close to the geometry surface?
Imagine the layering deflation’s that would happen around them if they were…
Yes i layered just about the whole car. A lot of my problems came from the wheels and i did see layer deflation there. I did a lot of testing, both with geometry changes and messing with the mesh settings to try and reduce the Non-ortho cells there. While i did get rid of many bad cells, they were never gone. I also had a fairly constant mesh failure problem with three adjacent cells sharing the same face. but thats another story …