SimScale CAE Forum

Very thin boundary layers and y+ on complex geometry


Yes, I think skewness could add the possibility that all 4 of your |||| cells could have face adjacent faces to the COI (but still possibly none of them could be) and in this case you have shown that we are seeing a pentahedron.

EDIT: I think the only way to say for sure that all 4 of your |||| cells are adjacent cells would be if the definition of adjacent cells includes faces with common edges…

To continue this discussion I think we need an answer on what does adjacent cells mean?

Which brings us back to which of the 7 permutations of pick 1,2 or 3 of common faces, edges or corner points, is the meaning of cell adjacency (especially WRT our volumeRatio)?

EDIT: If the adjacent cells definition includes all permutations, then an internal hexahedron cell (not touching the geometry or BMB surface) would have 26+ adjacent cells (I am not sure what the upper limit would be and I am not sure we need to know :wink: ). Sometimes I have trouble saying what my brain sees :wink:


Okay @DaleKramer I think I’m ready to have you take a shot. See my Run 21 on CAC Deflected for the current best shot at this. This is the best coverage mesh I’ve been able to get without much in the way of illegal faces. I decreased sim time to 450s as that’s about when the previous best mesh converged, but it may take more.

Getting this simulation complete will give us a pretty good comparison with the 6MM cell mesh with “better” Y+ values discussed earlier in this thread.


OK I have a couple sims running, just for a start I am using Dylan numerics on your Run 21 and a Run 1 with Dylan numerics on the 24M mesh in ‘For Dale’ sim. May not get different results yet but this is where I want to start :wink:

EDIT: Dylan numerics did not get as far as Default in these cases, so I am at my limit of trying to get a Y+=1 Full res mesh to complete a simulation.

Please, can the sim expert @PowerUsers_CFD 's now have a look at why these 2 meshes that @jhartung has laboured over for so many hours, will not finish a simulation…

CAC Deflected\Run 22 11.6M mesh

For Dale\Run 1 25.6M mesh


Hi everyone,

While you all are doing some fantastic work on snappy, I’ve proceeded to attempt the mesh generation on cfMesh on my PC. The problem is, I am unable to clean up the geometry sufficiently and cfMesh simply returns “too many errors”.

I’ll see what I can do but was wondering about how the geometry was created. Could you elaborate? @jhartung



I could never figure out what those errors were and the meshes that I saw were made with 0 illegal faces until we started relaxing some quality parameters for Full res mesh, hope you can figure them out…


Hi both,

For this run, first attempt to disable potentialfoam, then if still unstable, try using smooth solvers for all conditions (U, P, k, w). There should be no reason why higher order schemes are not usable, but if the sim continues to be unstable, change the gradient scheme to gauss linear for grad(U).

Similar comments for this.

It is important to get the sim running first before attempting higher order schemes or faster solvers.




As of Post #139, the ball is back in your court @jhartung :wink:


@DaleKramer I’m following along… are you suggesting I should try the above suggestions or you? Do you think any of them would reduce the memory requirements?

@Get_Barried CAD was built in Solidworks with special attention to minimizing small features and individual faces.


Yes you, I would have to figure them out as well :wink:

Personally I do not think a # cores issue here, I have done larger than these with 32 cores, maybe @Get_Barried could answer that too…


Hi both,

No that is not the issue. Core count does not affect solution stability. The decomposition algorithms are quite robust and only give negligible result errors, not instability in the solution.




This is one of those times where a Slack or Discord chat would be great.


Yes we did mention about it. I use slack and wouldn’t mind joining one. Maybe @jousefm can start one?


Hey Barry & Bryce!

Will create a channel and invite all of the PowerUsers :slight_smile:




I’m not sure I like Slack for this kind of thing. The beauty of a forum is that the posts persist, are editable and searchable, and the conversation stays (mostly) on one topic. I find important information gets lost in Slack conversations as posts move up the infinite scroll. Other users won’t be able to benefit from the conversation either. Not to mention it’s super distracting.


I agree with you in general terms but I just joined that new Powerusers Slack (and never used Slack before).

I can see where there is purpose in both ways of communicating and I hope that we can all divide the conversations between forum posts and Slack (and other) appropriately :wink:


Slack is good for figuring out what everybody wants for lunch.


Or discussing privately a potential forum item post between power users in order to ensure the post’s validity :wink:


May be a bit early to tell, but it looks like disabling potentialfoam made a big difference.


Yes, wow


@jhartung looks like you are making progress. While potentialFoam is great for reaching convergence, it can cause a few issues of its own.