SimScale CAE Forum

Optimizing mesh quality and Y+ layer formation on an FSAE car

Dale, I re-read this post and found this from Dylan to be potential problem in my meshing. He says that a HIGHER Region refinement will increase the surface refinement level but will this also happen if the region refinement is LOWER then the surface refinement?

dylan

Sep '18

@DaleKramer

Sorry my bad. In the refinement settings, you can define refinement levels on the fuselage, the wings, and landing gears, etc. You can also define refinement levels on a box that covers the plane, so that the cells within the box will have at least that refinement level. For example, with a background mesh of 0.5m generated from blockMesh, you can define level 8 refinement on the plane, and level 6 refinement on a box that covers the plane. Now, if you define a level 9 refinement on the box, then the entire plane will also have level 9 refinement, finer than what you need creating a lot of cells.

When doing mesh independency study, you can increase the level of refinement of that box to 7, but you don’t need to increase the level on the plane. If there are areas on the plane that are apparently not described by the surface mesh properly, then you refine it.

After a test of only the Rear Wing, using the following settings, it seems that the region refinement effected the surface refinement level by lowering it back down to level 6.

RW Test 1.0

Surface refinements

  • Wing surfaces at level 8 - 0.00375
  • Wing trailing edge surfaces at level 11 - 0.00046875m
  • Endplate surfaces at level 6 - 0.0015
  • Endplate and gurney flap edges at level 10 - 0.0009475

Region refinements

  • Close to model cartesian box - Level 6 - 0.015
  • Cell reduction cartesian box - Level 4 - 0.06
Inflate boundary layer settings - layer calculation Y+ Value
Layers 4
1st 0.000647 Y+ = 30
2nd 0.00106755
3rd 0.001761
4th 0.002906
Overall thickness 0.006381 Y+ = 296
Expansion ratio 1.65
Min thickness 0.001m
Final layer thickness (RATIO) 0.775

Inflate Boundary Layer settings - changes in Final layer thickness (RATIO)

  • Wing surfaces - 0.775
  • Wing trailing edge surfaces - 6.199
  • Endplate surfaces - 0.1937
  • Endplate and gurney flap edges - 3.06

Region refinement level was not lower than the surface refinement which causes the whole model to obtain region refinement level. This can be seen by observing the mesh and in the meshing log which shows only level 6 was achieved.

Layer mesh : cells:139885 faces:433057 points:153466

Cells per refinement level:

0 156

1 483

2 1262

3 3041

4 30329

5 7816

6 96798


RW test 1.1

By increasing the region refinement to level 8, the mesh conformed much better but lower level areas (endplate small edges, gurney flaps, wing trailing edges) are also changed to level 8 cell size- 0.00375

Another problem is just this rear wing mesh has 7.6 million cells - waay too much

Gurney flap not meshed very well - was set to level 10 - 0.0009475 surface refinement

I have also tried no region refinements, with a high level change between cells, but that resulted in the wing not even being meshed.

I am also trying just one universal boundary layer inflation, and will also try having the smaller faces with unequal level refinements.

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Can you give me a direct link to the RW test 1.1 mesh?

Sorry i just made it so i forgot to share /make it public. here it is

https://www.simscale.com/workbench/?pid=3614293863436765614&mi=spec%3Ab7681557-f19b-4416-9a66-c9433d72059c%2Cservice%3ASIMULATION%2Cstrategy%3A7

So the next tests, i think, confirmed my assumptions that the region refinement is changing surface refinements. Both test 1.3 and 1.4 have surface refinements higher then the region refinements (levels 10 and 11) but are still reduced to the region refinement level (level 8)

Dale, are you sure that the final thickness ratio is related to the surface refinement and not the region refinement cell sizes?

Either way, i am not getting the results i want, which is to have the mesh retain smaller cell sizes when i assign higher level surface refinements to specific faces, otherwise i am getting jagged edges. What is interesting is that in the whole model mesh i made, they did, the surface refinement size DID mesh to a higher level.

I am not sure what to do from here as i cant yet even test the Y+ changes i wanted. Would you reccomend sending some meshes through to simulation to see the Y+ values anyways.?

I also dont like the mesh skewing in between main wing and elements

Just a quick look at two sims i did of the Rear Wing test. The best meshing results so far are from test 1.1 and test 1.6. A comparison can be made of the Y+ results and it seems that the absolute layering of test 1.6 covers a bit better. However on both tests the forces and moments resulted in odd numbers for the pressure forces. Not sure if I made a mistake in some settings.

RW Test 1.0 (base test)

Surface refinements

  • Wing surfaces at level 8 - 0.00375
  • Wing trailing edge surfaces at level 11 - 0.00046875m
  • Endplate surfaces at level 6 - 0.0015
  • Endplate and gurney flap edges at level 10 - 0.0009475

Region refinements

  • Close to model cartesian box - Level 6 - 0.015
  • Cell reduction cartesian box - Level 4 - 0.06

Inflate Boundary Layer settings - Final layer thickness (RATIO)

  • Wing surfaces - 0.775
  • Wing trailing edge surfaces - 6.199
  • Endplate surfaces - 0.1937
  • Endplate and gurney flap edges - 3.06

RW Test 1.1 - Simulated

Only changes to Region refinements

  • Close to model cartesian box - Level 8 - 0.00375
  • Cell reduction cartesian box - Level 6 - 0.015

30 & 50

100 & 300

RW Test 1.6 - Simulated

  • Wing trailing edge surfaces min max even again at level 11
  • Endplate and gurney flap edges min max even again at level 10
  • Variable inflate boundary deleted
  • Inflate boundary layer settings same as test 1.5
  • ABSOLUTE Layering used

30 & 50

100 & 300

4 Likes

Hi Dan,

Now that the holidays are over, I will try to catch up to you…

I spent the whole day working on issues that have come up in this topic.

I have a lot to report…

I will start by quoting and responding quite a bit, so here goes ‘Quote and Response’ #1:

This is a huge problem, if your results are not making sense ! To properly see yPlus surface values, your sim run must be pretty well converged. Yours are not… Also, a ‘wings only’ yPlus investigation is a bit meaningless since the air flowing to the wings will not have been disturbed by the body of the car. Use the ‘wings only’ as just a test of how to get yPlus values that you seek… Only after the wings have been added to the body can you make the final yPlus tweaking…

Here is how silly the flow in your domain looks at 1000 iterations of the RW test 1.6 ‘Run 1’:

If you use Potential Foam Initialization, it will converge 3 or 4 time faster… I have a test running now to 5000 iterations with PotInitON…. I also added a ‘Coefficients’ results with a 1m reference length and a 1m^2 area… It is MUCH easier to see if the CL and CD are becoming stable than when the various pressure and viscous forces get stable… I will stop it when the CL and CD are about 1% stable over 500 iterations, THEN we can look at yPlus :slight_smile:
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AND FOR THE FIRST TIME IN PUBLIC, HERE IS MY ORSI by Moving Average Output screen that showed me when to stop that sim run :sunglasses::grinning::sunglasses::smiley::sunglasses::smiley::sunglasses:


As you can see, both CD and CL were 1% stable at 950 iterations, so that is when I COULD have stopped it if I were on the ball, OR if SimScale was watching it could have stopped it at 950 iterations EXACTLY (we can only wish for this :pray: )
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AND here is the converged flow pattern in your domain at y=0 (that now makes sense to me :wink: ):

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AND, here is my analysis of the meshing log table for the 1.6 mesh (VERY NICE RESULT BY THE WAY at 97.7%):

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AND here is the yPlusHistogram for the results of the now well converged sim run:

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NOW, is this what you EXPECTED for your ‘RW test 1.6’ yPlus values (theoretically, below 20 is not so good for wall function use) :question:

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Just to be able to see the need for a more complete convergence before the yPlus should have been looked at, here are the ‘Not well converged’ residual and force plots:


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Compared to the ‘Well converged’ residual and coefficients plots:


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And again, here are the ORSI values for the ‘Well converged’ coefficients:

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HOLY mother of Dale! this is amazing.

First off I hope you still know that this much help is not required of you. I appreciate it more then you know, but I do not want you to have to invest so much your time doing sim runs and analyzing everything. for me. This level of expertise is way beyond what i could have asked for and could have expected, especially when you are getting nothing in return. You should be paid for these services.

I assume you are doing this because you enjoy it but i will not be offended if you jump out completely at any time.
(end of rent)

Getting on to your response:

I did these two sim runs for two reasons;

  1. to see if the meshes are working and converging as they should - they are not, but i assume this has more to do with the sim settings then the mesh. I have used the same sim setting from last year. I have no idea what they do, just like i had no idea about the mesh settings when i started this thread, I intend to learn this now/soon.

  2. The intention was to just get the Y+ values stable on wing surfaces, I know these could change based on the turbulent incoming air of the car in front but i wanted to get good mesh quality first, then tweak after as you said. I am also concerned with how my meshing strategy effects the total cell count.

I definetly need to start with the post processing in Paraview, the amount you can do with this is impressive.

To be honest im not quite yet at the point of deciding to fine tune the Y+ settings through the boundary layer refinement. Im much farther behind you, where i am still trying to get the mesh to do what i want.

My main goal for these simulations, is to have decent mesh quality with low cell count. I am still stuck on:

  • surface refinement cells to region refinement transition

the only was i know of having higher region refinement levels next to the wing surface is to make many cartesian boxes. (the need for higher level region refinements is killing my total cell count.

  • lower level (endplate main surface) to higher level (wing surfaces or very small trailing edge faces)

I think that different levels on large vs small faces is not crucially important to increasing the cell count over mall areas but having super small cells on large surfaces (mono) is not needed and would effect this number. I seem to be having trouble getting different surface refinement levels even though i am telling each refinement to be different. The level 6 endplate main face is forced to increase its level even though i am not telling it to in the surface refinement. I think this is due to the region refinement being higher and forcing it to increase also. This is another problem i need solved. Once i can control these factors it will allow me to apply the mesh strategy to the whole car and i can get the quality and total cell count i require.

Once this is done, i can focus on Y+ settings and eventually understanding the simulation settings.

I look forward to your advice and am always grateful for your help

Dan

I think a lot of your issues with having to use Cartesian box region refinements will disappear if you use my Feature refinement trick at this topic link

Also check out my Cybertruck meshing here… It may give you clues too… I did not use any Cartesian region boxes in that mesh since I used the Feature refinement trick…

No way! i had already tests (which i think i deleted) without cartesian boxes. I had just increased the cells between layers t like 6 or 7 on one test and to 10 (the level distance between level 0 and my wing surface level) and i got the same results as you. Nothing showing up. Ideally my perfect mesh solution would have no boxes except for wake regions so that the cells could simply be reduced when they are a certain distance to the geometry. This would help a ton in terms of cell count and i could maybe if im lucky increase surface the refinement level on everything.

If this works as i envision it, along with possible absolute layering, i think this could work for my application needs!

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WAY BACK HERE WHEN YOU MADE THAT STATEMENT A WEEK AGO, I STARTED INVESTIGATING THE STRANGE ISSUE…

It did not make sense to me until I tried it and I thought I was stupid for not knowing why the mesh would not appear without the region refinements…

I finally discovered the Feature refinement trick and then I created that topic yesterday, where I tried to find out if there was a better way to get the mesh to ‘appear’ again… BUT it looks like so far the Feature refinement trick is working the best for me :slight_smile:

The gradient between refinement level changes is controlled by the ‘Cells between levels’ HEX Parametric algorithm meshing parameter… I am using 2 instead of the default 3 for my CyberTruck mesh (saves some cell count on layering the tire treads) :wink:

Also, you need to discover the ‘Distance to surface’ region refinement… I am using it on the CyberTruck mesh now…

Oh my gosh this is a game changer

I had known about this feature i just applied it to cartesian boxes. I never saw that it could be applied to the geometry. I feel so dumb, because this would have saved me a lot of frustration and mesh trials. Oh well, now a new round of meshes can be made and hopefully i can figure out how to get a good mesh.

Another side note about the pictures above. I do not like the leading edge on the element wing at all. It is extremely jagged. I am pretty sure that increasing the surface refinement level will not hlep here because the STL file size is too small. Currently I believe that it is at 0.013m but saving this again at a higher level will help. Another piece to this puzzle.

On a side note. I think i have made my decision, at least for wing surfaces, on the Y+ vlaue. After observing the meshes on the symmetry plane from the Rear wing, I have noticed that creating a boundary layer that has a max Y+ value over a certain amount, starts to warp the cells between the main and element wings. Shown below is from test 1.6 which is ok. if the boundary layer cells from both wings is too large Shown in seccond pic. Either the cells will distort too much or they will touch which i assume is no good.
image
image

However, using the distance region refinement on the geometry will allow for smaller cells in this region so maybe this distortion will not occur. The only downside of this is from what Dylan said about region refinements being a lower level then surface refinements and forcing the surface refinement down a level to whatever the region refinement was set to. I will be testing this as well.

As usual a lot to do and even more learned as the testing continues. Pumped to do this next round. I think im getting close.

Personally I would make all your trailing edges square (not sloped) and make sure they are slightly wider than the level that you are refining your TEs to… Reduces jaggies…

Also, try to stick with EXACTLY 1 meter Level 0 cell size in all axes… I even generally make my BMB locations exactly on 1 meter steps…

Have a look at how I modified your top wing in that link I sent you to. It has fewer faces and all surfaces never get closer than 0.5mm from each other and all faces never get narrower than 0.5mm…

Just to be sure you understand the trick, your Feature refinement that makes the mesh appear with be to 0.5mm which is the TE thicknesses…

You also need to make the mesh finer on your leading edges to smooth them out:
image

Can you give me a direct link to the mesh that this 2D image was extracted from :question: (I would like to see those weird cells in a 3D viewer):
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Also, I have isolated another issue where meshing fails with a Floating Point Exception error when either an Inside Cartesian Box region refinement OR a Distance from surface face Region refinement is added around a geometry to increase mesh fineness around the vehicle…

Here is the project that shows the isolated issue and SimScale support is working in it now…

Dale, this picture is from test 1.1. It is in the Rear wing test project i shared before but here it is again.

https://www.simscale.com/workbench/?pid=3614293863436765614&mi=spec%3Ab9873730-11a6-41b0-b076-c90c237cadc1%2Cservice%3ASIMULATION%2Cstrategy%3A14

I am working on more tests now and am making good progress, will show the results soon

Great…
By the way, you can copy the browser link when you are on any tree item to send me right to the test 1.1 mesh, like this.

It is good to put direct embedded links everywhere you show a picture for the lazy helpers like me :wink:

Ok Dale, i havent done one of these yet but i assume this is how it works …Just for you

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My preferred way to handle this is to split the airfoil surface so that the LE is a separate face at the yellow highlighted section so the you can mesh it at an EqualMaxMin level that is different than the less curved rear airfoil surfaces…
image

I took a mesh clip half way out to the endplate and they are less squished but you should worry about squished cells, these are OK but later you would see bad ones in the mesh quality metrics at the end of the mesh log… (views at symmetry plane are always worse looking for layering collapses and squished cells):


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and end of mesh log: