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November 22, 2013, 06:25 |
Different types of mesh
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#1 |
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Meimei Wang
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Hi,
The structured mesh is strongly suggested to be used in CFD to have better accuracy in my school. So I never used unstructured mesh. But when I come to this forum and search for the papers and mesh pictures online, I found that a lot of people actually use unstructured mesh and also cartesian mesh. I'm wondering when(for what flow) can I use the unstructured mesh and cartesian mesh? How to check how reliable are they besides comparing with the structured mesh simulation results?
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November 22, 2013, 12:07 |
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#2 |
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majid
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Hi
structure mesh generation is hard and for a complex geometry it is very difficult. boundary layer mesh and structure is professional gird and anyone dont work it. unstructure grid is low accuracy and result of analys not validate but it is useful ! what is your geometry? |
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November 22, 2013, 15:43 |
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#3 |
Senior Member
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I guess it depends also on the type of software. For instance PowerFLOW from Exa uses structure grids…
Last edited by lovecraft22; November 24, 2013 at 16:22. |
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November 22, 2013, 19:58 |
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#4 | |
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Meimei Wang
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Quote:
But you know sometimes the unstructured grids can give very unreasonable prediction results. It is very hard to control the unstructured grids prediction accuracy and convergence might be very difficult. I can understand that some persons might still choose unstructured grids for simplicity. The question is how do they know how inaccurate is it before they use it? Any experience or theory to tell when tetra grids will give good prediction results? When I see pictures online that tetra grids are even generated for turbo 3D modeling and cartesian grids are used for car aerodynamics modeling, it's kind of hard for me to trust the result to a certain level. I don't have a certain geometry to model. This is a general question.
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November 22, 2013, 20:02 |
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#5 | |
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Meimei Wang
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Quote:
Some softwares have both options. The tetra grids generation is always much simpler. I never tried unstructured grids. The question is: how do you get a clue of the unstructured grids simulation accuracy before you run the simulations?
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November 23, 2013, 01:23 |
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#6 |
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majid
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for exmple I generate unstructured grid on wing,results of it was aproximate %30 diffrence with experimental data, with structured gird results was below %10 diffrence with experimental data. i work in private company and for commerical geomtry use unstructured mesh beacuse it is easy and results not bad...but for acadmic research and high accuracy result structured gird is need.
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November 23, 2013, 02:56 |
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#7 | |
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cfdnewbie
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Quote:
this statement is not generally true. In fact, there are a number of high accuracy research codes that are fully unstructured. Check out Nektran or Nek5000. Both have spectral accuracy on unstructured grids. Almost all FE schemes are unstructured, and their dirty cousins, the high order discontinuous Galerkins, as well. All high of those order or spectrally accurate. So especially in research, many many of the currently interesting codes are unstructured. |
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November 23, 2013, 06:18 |
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#8 | |
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Meimei Wang
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Quote:
By the way, you work for aircraft wing? If your inaccuracy is 30%, how do you use this prediction for design? The difference between 2 different designs is usually smaller than 30%, right? How do you compare their performances?
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November 23, 2013, 09:57 |
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#9 |
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I agree. 30% is pretty far in terms of accuracy. Probably you'll can catch the trends anyway but still…
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November 23, 2013, 15:59 |
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#10 |
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Meimei Wang
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For what flow will tetra grids give good prediction?
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November 24, 2013, 13:39 |
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#11 |
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Unfortunately I don't know the answer.
I guess what I said above is not completely true though. Let's have a look at these grids: This one, is what is generally denoted as an unstructured mesh This one is instead a structured mesh: Now, what would you call this one? According to the definition, it should be an unstructured one but still it doesn't look to me as the same thing as the first picture. Thanks! |
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November 24, 2013, 13:58 |
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#12 |
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cfdnewbie
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by definition, it is an unstructured mesh, as it needs to be treated internally as an unstructured data structure and as there are no lines of constant I,J,K.
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November 24, 2013, 14:19 |
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#13 |
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California
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Generally speaking, unstructured tetrahedral mesh by commercial software is for people who knows nothing or only little about mesh generation, CFD methods, and numerical methods, while structured hexhedral mesh is for people who knows much more about mesh generation, cfd methods and numerical methods.
For the difference between structured hexhedral grid and the unstructured tetrahedral mesh, please see the below: http://www.advanceddo.com/mesh_generation.html |
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November 24, 2013, 14:59 |
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#14 | |
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cfdnewbie
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Quote:
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November 24, 2013, 15:30 |
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#15 |
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Rick
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Hi,
As I know structured hexa mesh is very good when you have the flow aligned with your mesh. As you can understand when you have tetra meshes the flow will never be aligned with the mesh. Daniele |
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November 24, 2013, 23:39 |
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#16 | |
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Meimei Wang
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Quote:
The third one is cartesian grids. That shall not be classified to the structured grids.
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November 24, 2013, 23:40 |
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#17 | |
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Meimei Wang
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Quote:
So in what case will tetra grids give good prediction results compared with structured grids? Very very complex flow?
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November 24, 2013, 23:50 |
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#18 | |
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Meimei Wang
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Quote:
Okay. But when I look at the papers in the engineering field, like vehicle aerodynamics engineering, there are quite some papers researching on the numerical engineering design method by using tetra grids. Some of them even discusses the geometry optimization by using tetra or cartesian grids. And at the end of the paper, they even conclude that the numerical designing method can be well applied to that field because they have good comparison with experiment. This is kind of surprise to me because I was always told that only structured grids give good CFD prediction. So I'm interested in knowing more about unstructured grids. I'm wondering when could we trust the results from unstructured grids?
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November 25, 2013, 04:16 |
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#19 |
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cfdnewbie
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What is totally confused in this discussion is element type vs. mesh type. Element types make the quality of the mesh (and any interpolation / prolongation between nonconforminng ones), NOT the way the mesh is stored in a data structure. Think about an unstructured hex mesh!
Structured meshes are ok for simple geometries, or mildly complicated ones, but beyond that, meshing becomes a nightmare and you waste too many DOF in regions where you don't need them. Look at the airfoil example posted in this thread.... |
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November 27, 2013, 08:25 |
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#20 |
Member
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Greetings all,
By definition, it could be a structured cartesian mesh...Cartesians are just a particular case of structured meshes, among others, block structured, H- O- C- type, isn't it? Regards, |
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