|
[Sponsors] |
July 10, 2009, 03:33 |
About the physics of turbulence...
|
#1 |
New Member
Join Date: Jul 2009
Location: near Marseille, France
Posts: 7
Rep Power: 16 |
I still would like to have some further explanations about the formula (turbulence intensity in a fully developped pipe flow) which I quoted in my previous post.
- How comes the turbulence intensity decreases when mean velocity gets bigger? Would it be that fluctuations are smoothed when a flow becomes faster? - What range of Re this expression if valid for? I cannot believe turbulence intensity exceeds 6% for laminar flows....! Thank you in advance |
|
July 10, 2009, 05:28 |
|
#2 |
New Member
Join Date: Jul 2009
Location: near Marseille, France
Posts: 7
Rep Power: 16 |
I guess I won't be more successful than this poster whose question was asked more than a year ago.......
http://www.cfd-online.com/Forums/flu...intensity.html Please, make an effort, I cannot believe that nobody here is able to answer that simple question |
|
July 10, 2009, 16:55 |
|
#3 |
Senior Member
Join Date: Apr 2009
Posts: 154
Rep Power: 17 |
The formula is an approximation of turbulent intensity from pipe flow empirical correlations, and is valid only for developed turbulent flow. The Re is actually Re_Dh:
http://www.cfd-online.com/Wiki/Hydraulic_diameter The actual definition of turbulent intensity is velocity fluctuation divided by the mean flow velocity. So, to answer your first question, the empirical correlations tell us that velocity fluctuation growth is small compared to the increase in mean flow velocity. |
|
July 11, 2009, 05:15 |
|
#4 |
Senior Member
Henrik Rusche
Join Date: Mar 2009
Location: Wernigerode, Sachsen-Anhalt, Germany
Posts: 281
Rep Power: 18 |
||
July 11, 2009, 05:18 |
|
#5 |
New Member
Join Date: Jul 2009
Location: near Marseille, France
Posts: 7
Rep Power: 16 |
Hello f-w
I'm okay with this correlation valid for fully developped turbulent flows only, I just wondered how low could the Reynolds actually get. The transition is known to occur between 2000 and 3000, so I guess I can still use the formula for ? Anyway, my simulation is rather not sensitive to turbulent boundary conditions, so I may use the formula without commiting too much error. I was just surprised such a low Re could yield to 5% turbulence intensity... You're right, that is just the competition between <u'²> and U0 turning in favor of U0 when Re grows. My question was kind of metaphysical: why? Thank you for your precisions! |
|
July 11, 2009, 05:26 |
|
#6 |
New Member
Join Date: Jul 2009
Location: near Marseille, France
Posts: 7
Rep Power: 16 |
Thank you henrik
Actually f-w had made me realized my dumb mistake in confusing <u'²> and I..... Your formula clearly shows <u'²> grows with u (thus Re), which seems logical. But after all, this still does not answer what is the physical reason for I decreasing with u... Are the fluctuations "smoothed" when flow goes faster, as I said in my first post? |
|
July 11, 2009, 18:33 |
|
#7 |
Senior Member
Join Date: Apr 2009
Posts: 154
Rep Power: 17 |
As long as your Re_Dh is two times the expected transitional value (~2300 for pipes), you're fine. Otherwise, some additional considerations must be taken for an accurate analysis, where transition might be delayed.
Turbulent intensity is undefined for laminar flows, so careful how you use it. It does grow with decreasing Re, and can get pretty large with complex geometries where you can have large fluctuations with low mean velocities (like in forest flows). As henrik pointed out, fluctuations can be rewritten (with the help of a correlation) as a function of mean velocity. At higher mean velocities, you can imagine having more powerful eddies, which in turn mix all the gradients (velocity, pressure, temperature, ...) to a greater degree. |
|
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Question on Turbulence Intensity | Eric | FLUENT | 1 | March 7, 2012 04:30 |
Discussion: Reason of Turbulence!! | Wen Long | Main CFD Forum | 3 | May 15, 2009 09:52 |
Code release: Flow Transition and Turbulence | Chaoqun Liu | Main CFD Forum | 0 | September 26, 2008 17:15 |
Natural convection - Inlet boundary condition | max91 | CFX | 1 | July 29, 2008 20:28 |