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k-omega SST: what is the Reference length scale? |
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#1 |
New Member
Andrea Stedile
Join Date: Feb 2020
Posts: 11
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Hi all,
I'm trying to investigate the flow of air around a car with the RAS k-omega SST. The OpenFOAM guide: https://www.openfoam.com/documentati...omega-sst.html says that the following formula can be used to estimate omega, that is the turbulence specific dissipation rate: ![]() where L is the reference length scale, and cmu is equal to 0.09 I wanted to ask: what exactly is L? In my case, would it be the length of the car? I saw that OpenFOAM has a motorbike scenario and tutorial.
![]() If we substitute for k, omega and Cmu, we get 3.058 m, which is different than the valur of lRef (the length of the motorbike). Therefore, I'm not anymore convinced that L is the length of the vehicle. Can anyone help me? Regards, Andrea |
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#2 |
New Member
Join Date: Feb 2020
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The exact reference length is not that important afaik. The important thing is that your nu/nu_t ratio (laminar viscosity to turbulent viscosity) is small enough. This paper by Spalart & Rumsey gives some recommendations in Section C for external aerodynamics: https://ntrs.nasa.gov/archive/nasa/c...0070035069.pdf
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#3 |
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Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
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It's not the same length scale.
The length scale in forceCoeffs is related to the definition of your force coefficients like lift, and that one is a geometric length scale. The length scale in the formula for omega is a turbulent length scale for your boundary. Say if your oncoming flow comes from a fully developed flow in a duct with diameter D is a small fraction of D (usually estimated to be about L~0.07D). It's a boundary condition, you need to "know" what it is. It could be anything because not all flows come from fully developed ducts. |
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#4 | |
New Member
Andrea Stedile
Join Date: Feb 2020
Posts: 11
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Quote:
Do you have any idea on how to estimate the turbulent length scale for my case, which concerns the study of the air flow around a car, or even many cars? Can I use a value similar to the one used for the flow around the motorbike? If I do so, the yielded results seem convincing and reasonable. But I am wondering if there is a more "scientifical" sound way to do it. Also, do you know why the OpenFOAM developers have specifically chosen 3.058 as a value for the turbulent length scale? I can't seem to find the reason anywhere on the web. |
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#5 |
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João
Join Date: Aug 2020
Posts: 3
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Hi everybody.
I'm studying the flow around a circular cylinder in the drag crisis region, using the kOmegaSST turbulence model. I'm struggling to find a plausible estimation for the turbulence lenght scale in the literature. What would be a good option? Thanks. |
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#6 |
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Edgar Alejandro Martínez Ojeda
Join Date: Jul 2019
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I made a tutorial on YT.
Here's the meshing part: https://www.youtube.com/watch?v=zWCPRhWHPqs And here is the case setup: https://www.youtube.com/watch?v=3yw6nf-bN20 Go to the video's description to get the link to the files. |
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#7 | |
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Lorenzo Mazzei
Join Date: Dec 2010
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Quote:
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#8 | |
Senior Member
Farzad Faraji
Join Date: Nov 2019
Posts: 206
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Dear Steddy
did you find any answer for Code:
Do you have any idea on how to estimate the turbulent length scale for my case Thanks, Farzad Quote:
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#9 | |
Senior Member
Farzad Faraji
Join Date: Nov 2019
Posts: 206
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Using the correct formula form here, I reversely solved motorBike problem and I got
Code:
Turbulent intensity = 0.02 turbulent length scale = 0.14 which is the 10% of Lref Thanks, Farzad Quote:
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Tags |
lref, motorbike, reference length, reference length scale, sst |
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