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

Turbulence Intensity dependent on Re

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

Like Tree1Likes
  • 1 Post By scoufield

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   January 30, 2018, 07:44
Default Turbulence Intensity dependent on Re
  #1
New Member
 
Alois
Join Date: Jan 2018
Location: Alemania
Posts: 4
Rep Power: 3
scoufield is on a distinguished road
Hello Guys,

i try to prepare a cfd-Simulation with FloEFD using the k-\epsilon-Model.

When i calculate the Intensity [ I=0,16*Re^-\frac{1}{8} ]
i get a small Intensity for a huge Re-Number.
Normaly i should use higher Intensity for a higher Re-Number ?
https://www.simscale.com/forum/uploa...fd9f6cc954.png

Thanks

Sco
scoufield is offline   Reply With Quote

Old   January 31, 2018, 05:18
Default
  #2
Senior Member
 
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 3,575
Rep Power: 44
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Turbulence intensity tends to decrease with increasing Reynolds number, yes. That doesn't mean the turbulence is less. The turbulent kinetic goes k~(UI)^2
LuckyTran is offline   Reply With Quote

Old   January 31, 2018, 06:10
Default
  #3
New Member
 
Alois
Join Date: Jan 2018
Location: Alemania
Posts: 4
Rep Power: 3
scoufield is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
Turbulence intensity tends to decrease with increasing Reynolds number, yes. That doesn't mean the turbulence is less. The turbulent kinetic goes k~(UI)^2
I calculate the turbulent kinetic with k~ 2/3 *(u*I)^2
so we get also a lower Turbulence kinetic when the Re-Number rise.

I mean the approximation Formula doesnt fit with the picture in the link.
Whats the reason for that ? In the pic we get higher Intensity when we choose a higher Re-Number. The Formula results in the opposite ..

Thanks
BlnPhoenix likes this.
scoufield is offline   Reply With Quote

Old   January 31, 2018, 08:15
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,937
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
The function I decreases with Re but I think one should also consider the increasing in the reference velocity U.
FMDenaro is offline   Reply With Quote

Old   January 31, 2018, 08:52
Default
  #5
Senior Member
 
Join Date: Aug 2014
Location: Germany
Posts: 290
Rep Power: 9
BlnPhoenix is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
The function I decreases with Re but I think one should also consider the increasing in the reference velocity U.
From this equation [ I=0,16*Re^(-1/8) ] high turbulent cases exist for lower Reynolds Numbers! If we say I = 20% means high turbulent cases. So this is very confusing, i agree with OP.

Last edited by BlnPhoenix; January 31, 2018 at 10:40.
BlnPhoenix is offline   Reply With Quote

Old   January 31, 2018, 09:35
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,937
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by BlnPhoenix View Post
From this equation [ I=0,16*Re^(-1/8) ] high turbulent cases exist for lower Reynolds Numbers! If we say I = 20% means high turbulent cases. So this is a very confusing, i agree with OP.

I know this formula appears in Fluent, being cited as an experimental correlation valid in the core of a fully developped duct flow. Whitout knowing the assumptions use to express the correlation I cannot say more. Maybe it is valid only in a specific range of Re numbers, as you can see for Re->+Inf and Re->0.
FMDenaro is offline   Reply With Quote

Old   January 31, 2018, 15:28
Default
  #7
Senior Member
 
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 3,575
Rep Power: 44
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
There already is a clear measure of how turbulent something is and that's Reynolds number.

Keep in mind you need to specify two things (out of three). Usually you take a velocity scale (and often people choose the turbulence intensity, implicitly assuming the velocity scale is the mean velocity) and a length scale. You need to consider what the is happening to the length scale and/or time-scale with increasing Reynolds number and remember that turbulence intensity needs to be paired with the right velocity scale.

The turbulence intensity alone doesn't give the complete picture (plus it only makes sense if you specify also the mean velocity associated with it). If you think turbulence intensity is a measure of "high turbulence" then consider the low velocity (or low Reynolds number) limit where any velocity fluctuation results in I=>Inf. E.g. a butterfly slowing moving its wings or a falling leaf can generate Inf % I. Clearly that tends towards laminar flow and there isn't any turbulence.
LuckyTran is offline   Reply With Quote

Old   February 5, 2018, 08:04
Default
  #8
New Member
 
Alois
Join Date: Jan 2018
Location: Alemania
Posts: 4
Rep Power: 3
scoufield is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
There already is a clear measure of how turbulent something is and that's Reynolds number.

Keep in mind you need to specify two things (out of three). Usually you take a velocity scale (and often people choose the turbulence intensity, implicitly assuming the velocity scale is the mean velocity) and a length scale. You need to consider what the is happening to the length scale and/or time-scale with increasing Reynolds number and remember that turbulence intensity needs to be paired with the right velocity scale.

The turbulence intensity alone doesn't give the complete picture (plus it only makes sense if you specify also the mean velocity associated with it). If you think turbulence intensity is a measure of "high turbulence" then consider the low velocity (or low Reynolds number) limit where any velocity fluctuation results in I=>Inf. E.g. a butterfly slowing moving its wings or a falling leaf can generate Inf % I. Clearly that tends towards laminar flow and there isn't any turbulence.
I also use the intensity & the length scale. But I thought the length scale is a parameter which i calculate out of the geometry. (L=0.07*D) Where D is the Pipe Diameter. So the length scale should not depending on Re or velocity ?
I already used the mean velocity for calculate the Re-Number.

The example with the butterfly and the leaf is right. And i think there should be a specific range of re numbers which you can use for the approximation formulae. Maybe FMDenaro is right. Thanks for your answers guys..

Sco
scoufield is offline   Reply With Quote

Old   February 5, 2018, 16:45
Default
  #9
Senior Member
 
Simbelmynė's Avatar
 
Join Date: May 2012
Posts: 367
Rep Power: 10
Simbelmynė is on a distinguished road
Just want to add that there is a Wiki on the topic here at CFD-Online:

https://www.cfd-online.com/Wiki/Turbulence_intensity

It is noted that the expression you use is from the Ansys manual and that no reference to it's origin is given.

Also, (@LuckyTran) please have a look at the definition of High- to low-turbulence cases.

For OP, I suggest that you do a small parametric study to test the dependence on the k and eps boundary conditions for your solution. In many cases there is only a weak dependence if the flow has time to develop inside the domain.
Simbelmynė is offline   Reply With Quote

Old   February 6, 2018, 03:24
Default
  #10
Senior Member
 
Join Date: Aug 2014
Location: Germany
Posts: 290
Rep Power: 9
BlnPhoenix is on a distinguished road
I once read somewhere on this forum that turbulence intensity gets smaller with higher Re number, because the increased mean flow velocity somehow damps out the velocity fluctuations. So, i don't know if this is a general experimental observation for turbulence..
BlnPhoenix is offline   Reply With Quote

Old   February 6, 2018, 04:54
Default
  #11
Senior Member
 
Simbelmynė's Avatar
 
Join Date: May 2012
Posts: 367
Rep Power: 10
Simbelmynė is on a distinguished road
To me that sounds counter intuitive. Also, higher Re does not necessarily mean higher mean flow velocity.
Simbelmynė is offline   Reply With Quote

Old   February 6, 2018, 05:04
Default
  #12
Senior Member
 
Join Date: Aug 2014
Location: Germany
Posts: 290
Rep Power: 9
BlnPhoenix is on a distinguished road
Yes, i agree it sounds counter intuitive. But it's the logic behind the posted equation for fully developed pipe flow. In an adiabatic pipe flow, increased mean velocity is equal to decreased pipe diameter, which results in higher Re number. The obeservation is: velocity fluctuations get smaller, so what else than some damping behavior can explain this..?
BlnPhoenix is offline   Reply With Quote

Old   February 7, 2018, 02:41
Default
  #13
New Member
 
Alois
Join Date: Jan 2018
Location: Alemania
Posts: 4
Rep Power: 3
scoufield is on a distinguished road
Quote:
Originally Posted by BlnPhoenix View Post
Yes, i agree it sounds counter intuitive. But it's the logic behind the posted equation for fully developed pipe flow. In an adiabatic pipe flow, increased mean velocity is equal to decreased pipe diameter, which results in higher Re number. The obeservation is: velocity fluctuations get smaller, so what else than some damping behavior can explain this..?
I read something interesting:

"The issue here is that the Reynolds number is not a measure of "how turbulent" the flow is or will become. It is a ratio representing the relative importance of inertial forces to viscous forces in the flow."

Reference https://www.physicsforums.com/thread...ensity.676226/

Sco
scoufield is offline   Reply With Quote

Reply

Tags
floefd, intensity, turbulence

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
Turbulence Intensity vs Reynolds number jason580315 ANSYS 3 March 1, 2017 06:21
Question on Turbulence Intensity Eric FLUENT 1 March 7, 2012 05:30
High Turbulence Intensity Problem bwg FLUENT 1 January 13, 2010 14:09
Turbulence intensity dependent from the Re-Number linch Main CFD Forum 1 October 2, 2009 02:21
About Turbulence Intensity (Pipe flow assimilated) gRomK13 Main CFD Forum 1 July 10, 2009 04:11


All times are GMT -4. The time now is 02:23.