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About Some Concepts:Laminar flow, turbulent flow, steady flow and timedependent flow 

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March 2, 2013, 13:24 
About Some Concepts:Laminar flow, turbulent flow, steady flow and timedependent flow

#1 
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Jing Shi
Join Date: Feb 2013
Posts: 20
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Hi everyone,
I have some confusion between these basic conceptsLaminar flow, turbulent flow, steady flow and timedependent flow. I thought laminar flow was steady flow, while turbulent flow was connected with timedependent flow before, but I just found it should be wrong. Can I understand those concepts in the following way now: Laminar flow and turbulent flow are distinguished in the scale of space, while steady flow and timedependent flow are distinguished in the aspect of time; both laminar and turbulent flow could be either steady or timedependent? And another question is: For turbulent flow,"timeaveraged" properties are used in RANS equations, what is the scale of that time? Any discussions are appreciated. Regards, Jing 

March 2, 2013, 13:44 

#2 
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Join Date: Dec 2011
Location: Madrid, Spain
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Hi Jing.
Just to clarify: you're right in that laminar flow can be either steady or unsteady. However, turbulent flow is always unsteady. Turbulence is an inherently unsteady process since it involves rapid variations of the thermofluid properties. Turbulent flows can, nevertheless, be statistically steady, in the sense that the mean flow features do not vary over time. In RANS you are modelling all the turbulent scales so I think the time scale of the averaging procedure should be the characteristic time associated with the slowest eddy. Maybe some expert around here can tell you more about this stuff. Cheers, Michujo. 

March 2, 2013, 13:54 

#3 
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Lefteris
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Laminar is a flow in which the fluid flows in parallel layers while turbulence is a stochastic phenomenon. Steady is a flow where the properties reach a steady state after some time and they do not vary any more while in unsteady flow the properties vary in time although there might be a periodicity in the variation.
Solving a turbulent flow using RANS models means that you're solving a steady flow.
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Lefteris 

March 2, 2013, 14:11 

#4  
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Jing Shi
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Quote:
Cheers, Jing 

March 2, 2013, 14:18 

#5  
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Jing Shi
Join Date: Feb 2013
Posts: 20
Rep Power: 5 
Quote:
Cheers, Jing 

March 2, 2013, 15:02 

#6 
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Filippo Maria Denaro
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