# turbulent or laminar flow

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 August 18, 2013, 10:14 turbulent or laminar flow #1 New Member   Join Date: Jun 2013 Posts: 14 Rep Power: 6 Sponsored Links 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

August 18, 2013, 11:12
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Filippo Maria Denaro
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Quote:
 Originally Posted by student2008 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.

 August 18, 2013, 11:38 #3 New Member   Join Date: Jun 2013 Posts: 14 Rep Power: 6 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

 August 22, 2013, 10:43 #4 Senior Member   Hamid Zoka Join Date: Nov 2009 Posts: 212 Rep Power: 11 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.

 August 22, 2013, 11:50 #5 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,416 Rep Power: 39 These can be of some help http://arxiv.org/pdf/physics/0409037.pdf http://arxiv.org/pdf/1307.7965v1.pdf

 August 24, 2013, 05:33 #6 New Member   Join Date: Jun 2013 Posts: 14 Rep Power: 6 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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