Advancing Layer Mesh Confusion
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Hi fellow fluid dynamics peeps,
I was making mesh of a 3d wing using the advancing layer method. However when looking at certain sections of my mesh I noticed an irregular behaviour of the near wall layers as shown in the attached photos. What could have caused this issue and is there a way to fix this? Attachment 93718 Attachment 93719 |
Are you sure that's real or just a consequence of the location of the cutting plane?
Also, hoooly molly, how many layers do you have? |
I am not sure, but when I look at different cutting planes I do not have this visual and irregularities.
hahah I got 40 layers. I wanted a prism layer total thickness of 0.05m so that I can have a very accurate boundary layer? I guess youre hinting it might be excessive? >.< |
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End of the day, if you have a machine powerfull enough to run it ok go for it. You could potentially have problems if your mesh is too small but hard to say beforehand if that will happen. Just try and check |
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Sorry for late reply, I am very new to the forums and I hadn't even noticed I got a response When you mean mesh quality check do you just mean to run it and check the simulations? I am new to star CCM software so I am not sure what features I have avaible. Your advice was very helpfull regarding number of layers! I decreased it and it gave me much more cells to play with around my mesh! I had initially thought it was vital to massively discretize the near layer prism cells (to capture the boundary layer) so if anything I was initially worried 40 wasn't even enough hahah >.< My work is also for professional applications so definitely speed was an issue I had, I only have like 3e6 cells to play with |
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- Run the mesh diagnostics in "MESH-DIAGNOSTICS" and see if you spot any problematic region. - Run the simulation and take a look at the residuals and see if everything converges as expected, if values like tke or tdr stay high then open a scalar and locate the problematic areas and improve mesh on that area. Quote:
I would also recommend use of "Custom surface mesh control" and reduce the number of layers in areas of little or no interest. That will allow you to reduce overall mesh count |
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I am no CFD dev expert, I was more of an end user but from what I know the results where good enough. I think in some areas they used some surface roughness settings to adjust CFD to reality for example. Sure you want to push to the limit but you also want a stable aero car at multiple yaws and car attitudes so you cannot design to the limit either. Additionally, parts are Wind Tunnel tested and validated before they hit the track and even with that some problems may happen. |
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I understand. This is very insightfull, however (and please correct me if I am wrong) I would imagine aspects such as boundary layer performance are not important for automotive applications (whereas for aerospace I would imagine they play a bigger role in performance), I doubt there are even much of a boundary layer with the exception of spoiler. However, I am going to take your advice, I am going to try and see how a lower prism layer count effects run times and accuracy. I am not sure I understand what you mean with Parts being wind tunnel tested and some problems may happen. Do you mean that Wind-Tunnel verification is inadequet? |
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Not saying it's not good but it's not perfect and real world testing is still needed for some things. |
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With regards to quantifying the effect of the prism cells on the solution, you could do a systematic study of the y+ and the number of layers. |
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I believe you said you're doing this in an industrial setting. I don't know what kind of industrial setting that is but in my case, I get to review CFD reports from various providers and about 90% they haven't done a mesh sensitivity study. So they have no idea about the error of their simulations. And because that stuff is for regulatory compliance it matters a lot. Then people come up with all sorts of excuses why they didn't do it or cannot do. And my answer is pretty much the same all the time... Excuses are not getting the CFD study approved. Simple as that. So, what I mean to say is, try to do a mesh sensitivity. And you can link the prism layer parameters to the base size of the mesh so your layers are refined along with the surface and the volume mesh. Now if the CFD you do is for internal consumption it may not matter that much, but if your work is going outside your organisation, someone might be asking you questions later on. |
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However before I can send it to the HPC, I wanted to get a mesh with a base size of 1 to be in somewhat an acceptable form such that, my supervisor, wont have to play with mesh parameters much. I must say I am suprised to hear from you people do not do mesh studies for external work. I cannot imagine what sort of excuses they have for not doing so. |
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To make your life easier, perhaps you already do that, you can link the base size to a characteristic dimension and have it as a multiple of that. Makes it easier to make changes later. As for excuses, they usually say that they don't have resources, that they solve the flow on laptops or workstations. And the problem is that often the mesh they present, the cell size, doesn't have much consideration for the scale of the phenomena they investigate.. [emoji24] |
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Damn, I guess not everyone in the profesionnal field has a grasp on y+ application? |
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So I wont judge them too hard yet cos Ill definitely be making my stupid mistakes as well. |
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However, when certification authorities are involved, I would expect CFD studies to be solid. (See what happened with the FAA and the 737MAX, which is not CFD related but some entities dropped the ball...) I think what is important is to think about the physics of the problem under consideration. Large scale phenomena, small scale phenomena and the associated timescales. Then, in an industrial setting, you decide how much detail is needed and what you can afford (money-wise, and also time-wise... and because time is money, actually money-wise :P) and then you build your model accordingly. Just don't cut corners, there are a few fundamental aspects that need to be there. |
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