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June 14, 2017, 11:02 |
Large eddy simulation
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#1 |
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dilaw meda
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Hi, i read that the large eddy simulation (LES) approach is very sensitive to the grid quality and there is no independence otherwise it becam a DNS approache, also it is very sensitive for the numerical methode employed. So how to choose the wright grid, the numerical methode, the initialization methode (hybrid or standard in fluent) and the time step size for calculation ?
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June 14, 2017, 11:20 |
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#2 | |
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Filippo Maria Denaro
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I suggest a reading of the book of Sagaut on the LES topics. A much more general presentation can be found in the book of Peric & Ferziger. I also suggest to have the book of Pope for some topics about turbulence. You can also find some brief notes I wrote https://www.researchgate.net/publica..._Part_I_and_II |
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June 14, 2017, 11:29 |
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#3 |
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Uwe Pilz
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> is very sensitive to the grid quality and there is no independence o
There is a huge difference between grid quality and spatial differentiation. LES may be sensitive to the quality, but that is true for RANS too. It is possible, however, to get a (nearly) grid independent solution with LES. You should taken into account what LES is developed for: Free shear fluid flow. In the near of walls, assumptions are made, but all in all to a lesser degree as with RANS. The most problematic cases are the ones where the energy transfer gets doesn't flow from the large to the small eddies, but in the opposite direction. Combustion or chemical reactions are such cases.
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Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950) |
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June 14, 2017, 11:37 |
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#4 | |
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Filippo Maria Denaro
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Yes, in case of confined flows you have two choice: a DNS grid resolution or a RANS-like apporach with wall-modelled BC.s However, energy backscatter is not a problem using dynamic modelling and scale similarity. |
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June 14, 2017, 11:39 |
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#5 | |
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Filippo Maria Denaro
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have a look here https://www.google.it/url?sa=t&rct=j...31b35g&cad=rja |
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June 14, 2017, 12:49 |
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#6 |
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dilaw meda
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Okay i will read it. A question About the grid , how to judge the grid, about number of elements or about mesh metric like skewness, orthogonal, element quality, aspect ratio ....
For examole two meshes with the same mesh metrics but difference in number of elements, which one is the best ? |
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June 14, 2017, 13:18 |
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#7 |
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Uwe Pilz
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> For example two meshes with the same mesh metrics but difference in number of elements, which one is the best ?
All in all you should avoid skew elements , very different element sizes and large changing in element size. This is especial true for regions you are interested in. A decent simulation includes an examination of dependence of the result from the mesh density. It would be best to simulating with half and double the mesh size. If this is not affordable, at lest a factor of 1,5 should be used.
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Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950) |
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June 14, 2017, 14:38 |
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#8 | |
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dilaw meda
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June 14, 2017, 14:44 |
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#9 |
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Uwe Pilz
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If you double the node density and get a difference of the main results of let's say 10%, than you have to assume that a even finer mesh would get another 10% and a very very fine mesh again some percents. You are not in a region I would call mesh independent.
Differences of no more than "single percents" (1-3%, maximal 5) should be aspired. If you need to vary some parameters of the geometry or b.c, the chemisty or what else you don't need to perform the mesh analysis for all cases. It is sufficient to investigate a typical one.
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Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950) |
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June 14, 2017, 14:50 |
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#10 | |
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dilaw meda
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June 14, 2017, 15:00 |
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#11 | |
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Filippo Maria Denaro
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Consider then if you need to resolve the boundary layer across the walls. Just as an example, I solved a problem in a domain of 100m^3 at Re=10^3 using 100^3 grid points. Again, have a look the the paper I linked. |
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June 14, 2017, 15:02 |
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#12 |
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Uwe Pilz
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A big domain ist not a big problem if you don't need to resolve small details. That means, if there is only a slow velocity. Regions with high velocity need to mesh finer.
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Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950) |
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June 14, 2017, 15:08 |
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#13 | |
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dilaw meda
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June 14, 2017, 15:14 |
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#14 | |
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Filippo Maria Denaro
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Well, you need to refine enough the grid around each jets. I think you need to work initially with some millions of nodes for the whole domain. But if you need to study only the effect of the jet, you could consider a simplified model, for example a unique jet with a periodic domain. |
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June 14, 2017, 15:17 |
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#15 | |
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dilaw meda
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June 14, 2017, 15:47 |
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#16 |
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dilaw meda
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An other question, if i want lets to compare lets say a configuration of 7 jets to a configuration of 3 jets to find wich configuration gives tje best mixing of air based on température and velocity mesurements, what parameter to fix ( based on diameter of the nozzles and velocity) i can use do the comparaison ? For example to compare a jet wich exit from a nozzle A and an other jet from an other nozzle B, the searchers did the comparaison with keeping the same reynolds number at the nozzles exit. So if the nozzle are not the same, velocity must also not be the same too keep Re equal. But in the case of multiple jets when the number of jet is not the same, how to compare ?
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June 15, 2017, 02:32 |
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#17 | |
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ivan
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l/L = (10/Re)^(-1/2) Where l is your elements characteristic lenght and L is a characteristic lenght of your domain. For a 4x4x2m room, with Re ~10^4, you could need easely several tens of millons of elements. Sent from my SM-T810 using CFD Online Forum mobile app |
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June 15, 2017, 02:41 |
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#18 | |
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ivan
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Those indicators are more related with divergence problems that with the grid independence. Grid independence is mostly dependent of the grid size. Using finer meshes in LES you will obtain more and more details, BUT average quantities should converge to stable values. It means, you can state grid independence calculating U and u' in some relevant points. U is the time-average and u' the standar deviation of the velocity, both at some point(s). Sent from my SM-T810 using CFD Online Forum mobile app |
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June 15, 2017, 03:10 |
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#19 | |
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Filippo Maria Denaro
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Well, actually the Taylor microscale is the largest of the dissipative lenght scale, therefore you would resolve almost with a DNS grid resolution...Theoretically, the filter lenght (in other words the grid size) must lie in the inertial range of the spectrum, a region almost without effect of dissipation. |
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