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Old   August 18, 2013, 10:14
Default turbulent or laminar flow
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Hi all,

I model flow through a triangular duct. At the inlet of the domain the flow seems to be turbulent with Re=4600. At the outlet the Re is decreased to 900, because of increasing cross-section. In my opinion relaminarization takes place. Am I right?
In addition to the SSt-Model I used the transition model. Both methods give similar results for pressure drop and other quantities, that I am interested in.
Is there any way to determine exactly wether a flow is turbulent or laminar (comparable quantities for sst and transition model)? I used the eddy viscosity ratio to compare both approaches. But a maximum value of 46 seems realy too less for turbulent flow to me, isn't it?

What model should I use?

thanks in advance
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Old   August 18, 2013, 11:12
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Filippo Maria Denaro
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Quote:
Originally Posted by student2008 View Post
Hi all,

I model flow through a triangular duct. At the inlet of the domain the flow seems to be turbulent with Re=4600. At the outlet the Re is decreased to 900, because of increasing cross-section. In my opinion relaminarization takes place. Am I right?
In addition to the SSt-Model I used the transition model. Both methods give similar results for pressure drop and other quantities, that I am interested in.
Is there any way to determine exactly wether a flow is turbulent or laminar (comparable quantities for sst and transition model)? I used the eddy viscosity ratio to compare both approaches. But a maximum value of 46 seems realy too less for turbulent flow to me, isn't it?

What model should I use?

thanks in advance
If I understand correctly your problem, the duct has an increasing area along the streamwise axis. In such case I don't think relaminarization can be possible, the flow would separate from walls and what I expect is still a turbulent flow.
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Old   August 18, 2013, 11:38
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Hi,

thank you for your quick reply. Yes, there is an increasing area. But how can the flow still be turbulent with Re=900?

Kind regards
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Old   August 22, 2013, 10:43
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Hi,
I think a positive pressure gradient (if it is high enough) can lead to relaminarization process. Moreover, flow separation is not necessarily an indication of turbulence. Separation can happen in laminar flows as well.
But, in your case you can check the shape of velocity profile at near wall regions. Laminar and turbulent flows have distinct velocity profiles.
Another point is that since you have a separated flow, shear stresses between main stream and circulating flow increase and may produce turbulence. so you can check K and epsilon (or turbulent viscosity) at these regions.
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Old   August 22, 2013, 11:50
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Filippo Maria Denaro
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These can be of some help

http://arxiv.org/pdf/physics/0409037.pdf

http://arxiv.org/pdf/1307.7965v1.pdf
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Old   August 24, 2013, 05:33
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Hi,

thank you very much. These Information help a lot. I had a look at the eddy viscosity ratio and I think the flow is still turbulent.
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