CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

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

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 2, 2013, 13:24
Question 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: 8
Jing is on a distinguished road
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
Jing is offline   Reply With Quote

Old   March 2, 2013, 13:44
Default
  #2
Senior Member
 
Join Date: Dec 2011
Location: Madrid, Spain
Posts: 134
Rep Power: 11
michujo is on a distinguished road
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.
michujo is offline   Reply With Quote

Old   March 2, 2013, 13:54
Default
  #3
Senior Member
 
Lefteris
Join Date: Oct 2011
Location: UK
Posts: 203
Rep Power: 11
Aeronautics El. K. is on a distinguished road
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

Aeronautics El. K. is offline   Reply With Quote

Old   March 2, 2013, 14:11
Default
  #4
New Member
 
Jing Shi
Join Date: Feb 2013
Posts: 20
Rep Power: 8
Jing is on a distinguished road
Quote:
Originally Posted by michujo View Post
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
Jing is offline   Reply With Quote

Old   March 2, 2013, 14:18
Default
  #5
New Member
 
Jing Shi
Join Date: Feb 2013
Posts: 20
Rep Power: 8
Jing is on a distinguished road
Quote:
Originally Posted by Aeronautics El. K. View Post
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
Jing is offline   Reply With Quote

Old   March 2, 2013, 15:02
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,950
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by Jing View Post
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
FMDenaro is offline   Reply With Quote

Old   October 5, 2018, 05:26
Default
  #7
New Member
 
Nimisha
Join Date: Oct 2018
Posts: 1
Rep Power: 0
Nimisha is on a distinguished road
RANS equation stands for Reynold's Average Navier Stokes equation and is essentially a time averaged mathematical model a.k.a representing steady flow.
To solve for turbulence, instead of employing the complete Navier Stokes equations, the simplified RANS model is used with additional equations to support it by adding additional parameters which empirically model turbulence effects (eg : k-epsilon turbulence model). These addition equations model the fluctuations in flow properties with time.
I hope my understanding is correct but please correct me if I am wrong.
Nimisha is offline   Reply With Quote

Old   October 5, 2018, 17:55
Default
  #8
Senior Member
 
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 3,629
Rep Power: 45
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by Nimisha View Post
RANS equation stands for Reynold's Average Navier Stokes equation and is essentially a time averaged mathematical model a.k.a representing steady flow.
To solve for turbulence, instead of employing the complete Navier Stokes equations, the simplified RANS model is used with additional equations to support it by adding additional parameters which empirically model turbulence effects (eg : k-epsilon turbulence model). These addition equations model the fluctuations in flow properties with time.
I hope my understanding is correct but please correct me if I am wrong.

It is totally 100% incorrect to say that RANS solves for turbulence (maybe not exactly 100%, at least 99%). The variables you are solving for are the mean velocity, the time-averaged velocity which is defined as containing no turbulence in it. You are solving for the mean velocity which has been influenced by turbulence (which you model). But never do you solve for the fluctuating velocities. You have to model them because you cannot solve for the mean flow without them.

You can also imagine that I have a laminar flow in where there are no fluctuations but I apply Reynolds-averaging and solve the RANS equations anyway. How can you solve for turbulence in this situation? There isn't any...


The issue is you need to differentiate between a turbulent flow (i.e. a flow that is turbulent and contains turbulence) vs actually addressing the turbulent fluctuations themselves. RANS solves for the mean variables, which are influenced by the turbulence. But the mean variables cannot possibly be any farther from the turbulent fluctuations.
LuckyTran is offline   Reply With Quote

Old   October 5, 2018, 18:02
Default
  #9
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,950
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by LuckyTran View Post
It is totally 100% incorrect to say that RANS solves for turbulence (maybe not exactly 100%, at least 99%). The variables you are solving for are the mean velocity, the time-averaged velocity which is defined as containing no turbulence in it. You are solving for the mean velocity which has been influenced by turbulence (which you model). But never do you solve for the fluctuating velocities. You have to model them because you cannot solve for the mean flow without them.


You can also imagine that I have a laminar flow in where there are no fluctuations but I apply Reynolds-averaging and solve the RANS equations anyway. How can you solve for turbulence in this situation? There isn't any...



Actually, the issue is much more complex. This is because you can get a RANS solution without solving the RANS equations supplied by some approximate turbulence model. Just consider to have a DNS solution and perform explicitly the time-averaging. You get the "exact" RANS solution and it totally contains the contribution of turbulence (i.e., the fluctuations) to the averaged field.
FMDenaro is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Extrusion with OpenFoam problem No. Iterations 0 Lord Kelvin OpenFOAM Running, Solving & CFD 8 March 28, 2016 12:08
Define Profile, time dependent flow variation query Sdezmond Fluent UDF and Scheme Programming 5 March 3, 2013 12:13
Orifice Plate with a fully developed flow - Problems with convergence jonmec OpenFOAM Running, Solving & CFD 3 July 28, 2011 06:24
Transient simulation not converging skabilan OpenFOAM Running, Solving & CFD 12 September 17, 2007 18:48
time averaged heat transfer in oscillating flow Matthieu Ubas Main CFD Forum 2 November 5, 1999 15:20


All times are GMT -4. The time now is 09:37.