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Laminar_turbulent_transition. Tetra or hexa?

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Old   November 21, 2012, 10:09
Default Laminar_turbulent_transition. Tetra or hexa?
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Roland Rakos
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Hello,

I complated two 3D (steady state) simulations to compare the influence of tetra and hexa mesh in case of calculation of laminar-turbulence transition. The SST turb.model and the gamma-theta transitional turbulence have been used.

Based on results, the y plus is same but the location of transition is different. The turb.kin.energy is higher in case of hexa mesh.

The boundary layer contains at least 12 elements and I tried to keep all of requirements in terms of meshing.

This simulation was just a test with a simple 3D geometry. I have to investigate the transition in case of a quite complicate geometry where it's likely that I can not apply structured hexa mesh. Therefore I would like to know whether it is possible to calculate the transition exactly with tetra mesh?

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Roland
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Old   November 21, 2012, 12:25
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Chris DeGroot
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A hexa mesh is in general better than a tetra mesh. However, I believe that with sufficient refinement (could be a LOT of refinement) it should be possible to get the same results out of a tetra mesh. In the limit of infinite mesh refinement the disretization error goes to zero, so it shouldn't matter about mesh type. That being said, you can't actually refine infinitely Keep refining your tetra mesh and I would expect it would converge upon the hexa result (assuming those results are sufficiently grid independent).
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Old   November 21, 2012, 19:06
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Glenn Horrocks
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First of all, good work on checking this first. It is important to know what the models limitations are. To many people on this forum seem to jump in and wonder why their model is inaccurate..... <END RANT >

It is difficult to achieve full mesh refinement with the more complex models, such as transition, multiphase, chemistry. Chris's comment is correct, you may just need to go finer again, but often you hit limitations of available resources and you just have to accept that your model has an error in it. So try to quantify the error if possible so you can put a reasonable estimate of error on the results.
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Old   November 23, 2012, 06:34
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Roland Rakos
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Hello,

thanks for your answers. I generated a very fine tetra mesh for my simple model yesterday. The result is better than the previous one. Of course there are problems and questions in the future too

Please help me to understand the maining of the "turbulence intermittency" and the "transition onset reynold number" variables. I read the cfx help, but I dont understand these wholly.

By the way, my main problem is that it is very difficult to generate the proper transition between the prism and the tetra elements. I try to attend the mesh expansion factor in spite of this the mesh transition and the mesh quality will not be the best...

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Roland
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Old   November 23, 2012, 06:47
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Glenn Horrocks
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There is some very good papers describing the turbulecne transition model - if you cannot find them in the literature then the CFX community site (on www.ansys.com) might have it, otherwise contact CFX support.

But in short transition onset Re number is the variable which keeps track of the Re number the flow will transition to turbulence given its history - smoother flows have higher Re transition numbers. Unfortunately it has been too long since I used that model and I cannot remember what the turbulence intermittency variable is -hopefully somebody can fill in for my memory.

Consider doing Richardson extrapolation to assist in the mesh refinement. It can be very useful in assessing whether you are in the convergent region of mesh refinement, and if you are it can extrapolate to the idea mesh. A very useful tool.

There are lots of meshing tutorials about and hopefully some of them cover the prism to tet transition. Again look on the web, the ANSYS community site and CFX support.
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Old   November 23, 2012, 10:56
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Alessio
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If I'm not wrong, intermittency (gamma) is the probability that a "point" is within a turbulent region..
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Old   November 25, 2012, 05:06
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Excellent, thanks for the clarification!
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Old   November 26, 2012, 07:48
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In addition to the y+, you should also check that mesh sizes over the wall in the flow direction are similar for both meshes. That's because mesh refinement in the flow direction is very important for the transition model since it affects how well it can calculate the transition position. The documentation has some guidelines on that.

I'm just saying this because when people create hexa meshes they usually take advantage of the higher aspect ratios allowed by hexahedrons, which could affect the results if you have sizes that differ from the ones in your tetra mesh.
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