epsilon and omega free stream BC's
in this page:
http://www.cfd-online.com/Wiki/Turbu...ary_conditions there are different formulas for epsilon and omega,I want to test with both kOmegaSST and k-epsilon-realizable(does anyone knows how to set it in OF?) how can find out which is appropriate for OF with your experience? for epsilon we have: or or and for omega we have: or or |
Hi again
I saw in page 25 of this thesis that he has used :(I managed this LaTeX formula at last!) the thesis: Simulation and validation of compressible flow in nozzle geometries and validation of OpenFOAM for this application is this work and change in formula by adding l is acceptable? http://s2.picofile.com/file/77781039...rbeit.pdf.html |
Hi Ehsan,
Quick note - here's another link to said thesis: http://www.rw.ethz.ch/alumni/thesis/...sterarbeit.pdf From what I can figure out, the author of the thesis went along with the spirit of this whole turbulence stuff. If you look at the first sections of the wiki page you've mentioned earlier, you'll see in "Modified turbulent viscosity" that the "turbulent length scale" is used for the "modified turbulent viscosity". This is possibly what influenced the author's decision. As for the wiki, it does have some more helpful hints, namely the one about "Eddy viscosity ratio": http://www.cfd-online.com/Wiki/Eddy_viscosity_ratio In essence, if you are able to calculate the "turbulent length scale", it's easier to calculate through it, instead of through the "Eddy viscosity ratio". In case you haven't seen how to calculate the relevant parameters, here's a quote: Quote:
Best regards, Bruno |
Hi Ehsan,
I'm also interested in some free-stream turbulence problems. I run a simulation of an object in uniform flow and I would like to perform a sensitivity study on the inlet turbulence intensity level. I've tested several TI levels (3% to 7%), but in all cases the TI drops to about 1% a few cells after the inlet. So whatever TI level I put at the inlet, my object is always in a 1% TI level. Have you experienced something similar with your simulations? If not, what is the way to maintain TI through the domain? About my case: the velocity profile is steady and uniform, the mesh is uniform, I use k-omega SST turbulence model. Best regards, Eloïse |
Hi Eloïse,
A few questions regarding your case:
Bruno |
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sorry for delay,how did you notice that TI in internal cells becomes low? |
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1. It is a 3D case 2. I ran the case in laminar and haven't had any specific problem. Do you think of something specific which should be checked in the laminar case? I'll have a look at the paper, thanks for the link. 3. It is a steady-state simulation 4. The TI decreases in the very first cells after the inlet. Around the object, the TI increases again. (see attached image, which is a cut of the domain) Ehsan: that's how I detected the issue. Regards, Eloïse |
How did you define turbulence intensity field?is there any postProcessing utility for that?
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Eloïse |
how you have defined TI in inlet?
you should set it to a constant so that can define k in inlet not inverse. |
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Eloïse |
is velocity constant in inlet?if it varies during run and you have set a fixed k and omega it might makes some error,I don't know how you set your BC's,could you attach them?
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Greetings to all!
@Eloïse: Well, the only way to maintain the turbulence intensity is if you introduce small obstacles, such as wires or something like that. Because there are a few details in play here and all of them are related to the solver doing what it was told to do. More specifically:
Which reminds me... have you ever seen those old fluid dynamics/mechanics videos showing how turbulence work and so on? I can't remember where I kept the link for them... Anyway, one of them shows the effect of a wire inducing turbulence. Best regards, Bruno |
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@Bruno: Thanks for the advices on fvSchemes. I'll have a look at that too. I guessed that if there is nothing to create velocity gradients and maintain turbulence, then the turbulence would fade out. Or at least that's how I understood my problem. I asked about it here because I was wondering if others observed the same and if something could be done about it :) |
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