turbulent or laminar flow

student2008 · August 18, 2013, 10:14

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

FMDenaro · August 18, 2013, 11:12

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.

student2008 · August 18, 2013, 11:38

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

Hamidzoka · August 22, 2013, 10:43

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.

FMDenaro · August 22, 2013, 11:50

These can be of some help

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

http://arxiv.org/pdf/1307.7965v1.pdf

student2008 · August 24, 2013, 05:33

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.

August 18, 2013, 10:14	turbulent or laminar flow	#1
student2008 New Member Join Date: Jun 2013 Posts: 14 Rep Power: 12	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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August 18, 2013, 11:38		#3
student2008 New Member Join Date: Jun 2013 Posts: 14 Rep Power: 12	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
Hamidzoka Senior Member Hamid Zoka Join Date: Nov 2009 Posts: 282 Rep Power: 18	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
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,764 Rep Power: 71	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
student2008 New Member Join Date: Jun 2013 Posts: 14 Rep Power: 12	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.