Well, it has been a few days and a lot of work but I have made some progress.
First, it was very difficult, as Barry had suggested, to make any sense of what mesh anomalies may cause that annoying little ‘Mesh Operation Event Log’ message that says ‘Mesh quality check failed. The mesh is not OK.’
I was only able to make 2 of the 5 meshes in my new SRJ study series meshes pass that, not very well documented, Quality check (SRJ1 and SRJ3).
Notwithstanding the fact that only two of my five SRJ meshes are ‘OK’ per the quality check, I believe that my new SRJ series of study meshes must be better quality than the SRF series of the last study that I have shown. The reason I say this is because I was able to get 4 converging ‘leastSquares’ gradient scheme simulations and 1 converging ‘fourth’ gradient scheme simulation from them. I was not able to get any of the previous ‘Not OK’ SRF meshes to converge with ‘leastSquares’ or ‘fourth’ gradient schemes.
In the below presented SRJ study, I am still not happy that Total Drag continues its downward trend. In fact, Total Drag also goes down about 5% for each of the last two increases in mesh number of volumes for the ‘leastSquares’ gradient scheme.
Barry, these results indicate to me that discretization schemes are NOT the likely the cause of continued divergence in the results, specifically Pressure Drag. I say that because ‘leastSquares’ Total Drag follows ‘Gauss Linear’ Total Drag in the same downward trend. Any other ideas?
Here is the SRJ Mesh Independence Study:
Here is how I was able to increase the quality of the SRF meshes to reach that of the SRJ meshes:
-
Set ‘Min tetrahedron-quality for cells’ to 6.1e-9. The default for this parameter is -1e+30, which actually turns off this quality assurance parameter. I am not sure why the default is off, other than it is ‘easier’ to layer a mesh with it ‘nearly’ off (see here). The problem is that with this parameter OFF, I think that leaves cells of low quality in the mesh, which in turn may generate the ‘Not OK’ quality failure notice and reduce the accuracy of your results. I think it is worth the time to refine the mesh better before you layer it rather than band-aid a layering failure by turning off this quality assurance parameter.
As far as why I chose the value I did, well, I tried values on both sides of this but I felt this was the highest value that reliably left my SRJ3 mesh quality notice as ‘OK’. I think this value may need to be tweaked further, probably on an individual mesh basis. -
Although #1 could likely have increased the quality of my meshes enough by itself so that higher order gradient schemes would be more likely to converge, I found a units discrepancy that I chose to fix for the SRJ mesh series.
The default value for the parameter ‘Min cell volume [in³]’ is 1e-13 while the same parameter in a project with SI units ‘Min cell volume [m³]’ is also 1e-13. I made the assumption that the correct default was the SI units value.
Therefore I converted 1e-13 m^3 to 6.1e-9 in^3 and I used this as my value for ‘Min cell volume [in³]’
I think this is a significant discrepancy and I hope someone looks into it…
Also, as an aside, I have discovered a Snappy meshing characteristic that disturbs me. If I make a Copy of any mesh, delete its result and then re-run it with the exact same set of parameter values, I end up with a mesh with a different number of cell volumes. I believe this can have many serious implcations and I have not really thought it through yet but does this concern anyone else???
Dale