# Is it a turbulent flow or laminar flow?

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 January 7, 2015, 21:43 Is it a turbulent flow or laminar flow? #1 Member   Join Date: Dec 2009 Location: China Posts: 79 Rep Power: 15 Hi guys, I am trying to simulate the flow through a heat sink with several fins. the geometry model is shown as the below picture. 1.jpg According to the length of the fin, the Reynolds number is 9e+4, which is less than the number of transition. so, the flow should be laminar? I'm not sure if I should treat it as a laminar flow, and then use laminar model to simulate the flow? What's more, the flow starts from a fan. Is it resonable that the flow has become turbulent when it leaves the fan blade? so I sould simulate it by using a turbulent model. Thanks in advance.

 January 19, 2015, 20:42 #2 Member   Join Date: Dec 2009 Location: China Posts: 79 Rep Power: 15 Could anyone give me any suggestion? Or.. is there anything I didn't descrbe clearly?

January 19, 2015, 21:00
#3
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Troy Snyder
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Quote:
 Originally Posted by ringtail Hi guys, I am trying to simulate the flow through a heat sink with several fins. the geometry model is shown as the below picture. Attachment 36392 According to the length of the fin, the Reynolds number is 9e+4, which is less than the number of transition. so, the flow should be laminar? I'm not sure if I should treat it as a laminar flow, and then use laminar model to simulate the flow? What's more, the flow starts from a fan. Is it resonable that the flow has become turbulent when it leaves the fan blade? so I sould simulate it by using a turbulent model. Thanks in advance.
In which direction (x,y,z) is the freestream flow directed? Is the freestream flow
inline with the fins or perpendicular.

If there is any question about laminar or turbulent, you may want to run laminar
flow as an initial limiting case. The laminar case should produce the lowest heat
flux values without the additional energy transport due to the motion of turbulent
eddies.

 January 20, 2015, 02:55 Is it a turbulent flow or laminar flow? #4 New Member   Firas Join Date: Nov 2014 Posts: 3 Rep Power: 10 Laminar flow will be suitable for this case without caring of near fan treatment ....

January 21, 2015, 00:34
#5
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Venkatesh Devaraj
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Quote:
 Originally Posted by Firas abd ali Laminar flow will be suitable for this case without caring of near fan treatment ....
Yes, thats true but as the flow leaves from the fan, it has already been disturbed and it is quite reasonable to treat it as a turbulent flow in this case.
Note: Freestream turbulence should be taken into account before deciding whether it a laminar or turbulent flow. Reynolds number doesn't alone play the role.

January 22, 2015, 19:24
#6
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p ding
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Quote:
 Originally Posted by ringtail Hi guys, I am trying to simulate the flow through a heat sink with several fins. the geometry model is shown as the below picture. Attachment 36392 According to the length of the fin, the Reynolds number is 9e+4, which is less than the number of transition. so, the flow should be laminar? I'm not sure if I should treat it as a laminar flow, and then use laminar model to simulate the flow? What's more, the flow starts from a fan. Is it resonable that the flow has become turbulent when it leaves the fan blade? so I sould simulate it by using a turbulent model. Thanks in advance.
turbulent is prevalent in nature. Your criterion of the transition from laminr to turbulent only make sense for the simple plate flow or pipe flow. The flow is turbulent unless your heat sink if micro fin and micro channel.

January 22, 2015, 20:32
#7
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Hi, Troy
Thank you~~The flow the inline with fin.
Yes, I agree with you. The heat transfer in laminar case is smaller than turbulent one. But I wanna know the real boundary layer thickness on the fins, so it's essential to know the flow model in advance. And I think the best suitable gap between fins, shoud be the double of the boundary layer thickness.
Quote:
 Originally Posted by tas38 In which direction (x,y,z) is the freestream flow directed? Is the freestream flow inline with the fins or perpendicular. If there is any question about laminar or turbulent, you may want to run laminar flow as an initial limiting case. The laminar case should produce the lowest heat flux values without the additional energy transport due to the motion of turbulent eddies.

January 22, 2015, 20:37
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If the free stream is disturbed by the fan, hasn't it become turbulent flow in front of the fin? so, the flow around the fins should be turbulent since the turbulent flow won't transit to laminar flow even I don't care about the flow near the fan. But I'm not sure about my opinion.

Quote:
 Originally Posted by Firas abd ali Laminar flow will be suitable for this case without caring of near fan treatment ....

January 22, 2015, 20:47
#9
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yes, Venkatesh , I totally agree with you. And I want have a deeper discussion about this topic.
How should we make a appraisal of the disturb if it's big enough to triggle the transition? and how should we assess the turbulent intensity of the free stream after it leaves the fan? (you know, it may be the key parameters to set the bc in FLUENT)
Quote:
 Originally Posted by Venkatesh Devaraj Yes, thats true but as the flow leaves from the fan, it has already been disturbed and it is quite reasonable to treat it as a turbulent flow in this case. Note: Freestream turbulence should be taken into account before deciding whether it a laminar or turbulent flow. Reynolds number doesn't alone play the role.

January 22, 2015, 20:52
#10
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the heat sink is about 400 mm long , 300 mm width and 200 mm high.
the gap between the fins is about 5 mm.
Quote:
 Originally Posted by ztdep turbulent is prevalent in nature. Your criterion of the transition from laminr to turbulent only make sense for the simple plate flow or pipe flow. The flow is turbulent unless your heat sink if micro fin and micro channel.