# About Some Concepts:Laminar flow, turbulent flow, steady flow and time-dependent flow

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 March 2, 2013, 13:24 About Some Concepts:Laminar flow, turbulent flow, steady flow and time-dependent flow #1 New Member   Jing Shi Join Date: Feb 2013 Posts: 20 Rep Power: 4 Hi everyone, I have some confusion between these basic concepts-Laminar flow, turbulent flow, steady flow and time-dependent flow. I thought laminar flow was steady flow, while turbulent flow was connected with time-dependent 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 time-dependent flow are distinguished in the aspect of time; both laminar and turbulent flow could be either steady or time-dependent? And another question is: For turbulent flow,"time-averaged" 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 Senior Member   Join Date: Dec 2011 Location: Madrid, Spain Posts: 133 Rep Power: 6 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 thermo-fluid 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 Senior Member   Lefteris Join Date: Oct 2011 Location: UK, Greece Posts: 187 Rep Power: 5 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. __________________ Lefteris

March 2, 2013, 14:11
#4
New Member

Jing Shi
Join Date: Feb 2013
Posts: 20
Rep Power: 4
Quote:
 Originally Posted by michujo 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 thermo-fluid 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.
Hi Michujo, thank you for your clear explanation for the concepts, and I think I get what you mean of "Turbulence is always unsteady, but can be statistically steady".

Cheers,
Jing

March 2, 2013, 14:18
#5
New Member

Jing Shi
Join Date: Feb 2013
Posts: 20
Rep Power: 4
Quote:
 Originally Posted by Aeronautics El. K. 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.
Hi, why you say "Solving a turbulent flow using RANS models means that you're solving a steady flow",I didn't get it and couldn't agree with this point.

Cheers,
Jing

March 2, 2013, 15:02
#6
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,640
Rep Power: 23
Quote:
 Originally Posted by Jing Hi, why you say "Solving a turbulent flow using RANS models means that you're solving a steady flow",I didn't get it and couldn't agree with this point. Cheers, Jing

RANS average is intrinsically steady when defined by

<f>(x) = lim (1/T) Int [0, T] f(x,t) dt
T-> inf

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