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Turbulence model under laminar conditions

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Old   June 13, 2017, 13:56
Default Turbulence model under laminar conditions
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Michael
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Hi all,

I am running a thin film hydrodynamic bearing model and am having some issues with the turbulence model. When you include a turbulence model under laminar conditions, should the results be equal to that of the case with no turbulence model.

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Old   June 13, 2017, 19:41
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Glenn Horrocks
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It depends on the turbulence model. k-omega based turbulence models degenerate to zero turbulent viscosity when k=0, so they will be pretty close. k-epsilon based models cannot handle k=0 so they do not degenerate to laminar flow well. There are modifications to the k-e model to help this, but then things start getting even more complex.

In short, if your flow is near transition then a k-omega based model handles it much better. If you use the SST model you also have the benefit of a turbulence transition model. But note the turb transition model has not been tuned for thin film bearings so its predictions could be considerably wrong.
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Old   June 14, 2017, 10:19
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I am seeing a drop of 1/4-1/3 drop in peak pressure using k-omega model. The Reynolds number of the flow as reported by a 3rd party software is 250 so the flow should be laminar. Does this sound strange?

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Old   June 14, 2017, 10:29
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Try modelling it in laminar. Also as Ghorrocks mentioned, k-w SST may not be tuned to your specific task (thin film)
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Old   June 15, 2017, 04:03
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Basically all eddy viscosity models will artificially induce turbulence where there is not (in laminar flows). It is just how they work. One of the basic assumptions of k-w, k-e, etc models is that the flow is fully turbulent. Even if you set the zone as a laminar zone, you will just kill the turbulence production. Eddy viscosity models does not predict transition right, you shouldnt trust in any result where the turbulence is low. If it is a thin domain, with low Re, why dont you just use laminar flow? Your Re number appears to be way below the critical one for a thin layer.

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Old   June 15, 2017, 06:56
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If you have a laminar flow and you run it with a laminar flow model and a k-omega turbulence model the results will be just about the same for most cases. This is because k-omega models degenerate to laminar flow models when k=0. They usually do add a bit of dissipation so the effective viscosity is slightly higher than a laminar model, but for many cases this is small with the result that a laminar model and k-omega based model will give essentially the same result.

So if you know the flow is laminar then it is best to use a laminar model, but if you are not sure then use k-omega (or one of its derivatives).
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Old   June 18, 2017, 15:33
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In the problem that I am trying to investigate, I am trying add features to the the stationary surface which may induce turbulence. I posted it in another thread but I am seeing a pressure drop occur that should be a pressure rise. This pressure drop occurs well into the laminar regime. I have tried the zero order model, the k-omega model, and the BSL Reynolds stress model and am seeing the same behavior from all three of them.
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