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

Inlet boundary conditions for turbulence changing

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 24, 2013, 04:11
Default Inlet boundary conditions for turbulence changing
  #1
Member
 
Join Date: Nov 2011
Posts: 81
Rep Power: 5
robboflea is on a distinguished road
hi all,

I noticed a thing in fluent which I don't understand and I'll be happy if somebody could explain it to me.

I was working on a CFD simulation for a turbine vane with transitional SST turb model and I was setting the inlet turbulence boundary conditions in the form of turbulence intensity and length scale.

My idea was that these values were used in fluent and converted to the corresponding values of k and w which were actually imposed as boundary conditions at the inlet.
However, after my CFD has converged I found out, by plotting the contours of k and w at the inlet that their value is not even close to the value they should have from the turbulence intensity and length scale.

If I specify directly k and w as boundary conditions then they are kept constant at the inlet.

So my question is: how are the turbulence intensity and length scale information at the inlet handled in fluent? Why do they give way to different values of k and w?

I hope I've been clear enough.
Thanks for your answer.

Cheers,

Rob
robboflea is offline   Reply With Quote

Old   September 24, 2013, 11:33
Default
  #2
Senior Member
 
Andrew Kokemoor
Join Date: Aug 2013
Posts: 119
Rep Power: 5
Kokemoor is on a distinguished road
Do the measured values match what it calculates if you use "Calculate from..." on the initialization tab?

How are you calculating k and omega from intensity and length scale? I believe k=\frac{3}{2}(UI)^2 and \omega=\frac{\sqrt{k}}{\lambda_T} where I is the intensity and \lambda_T is the length scale. Are you using a moving reference frame? If so, try it with both the absolute and relative velocity for U; it's probably absolute, but I'm not certain.

Last edited by Kokemoor; September 24, 2013 at 11:41. Reason: Correction: k=3/2(UI)^2, not 1/2(UI)^2
Kokemoor is offline   Reply With Quote

Old   September 24, 2013, 11:38
Default
  #3
Member
 
Join Date: Nov 2011
Posts: 81
Rep Power: 5
robboflea is on a distinguished road
Quote:
Originally Posted by Kokemoor View Post
Do the measured values match what it calculates if you use "Calculate from..." on the initialization tab?
Nope, they do not match them.

Quote:
Originally Posted by How are you calculating k and omega from intensity and length scale? I believe [math
k=\frac{1}{2}(UI)^2[/math] and \omega=\frac{\sqrt{k}}{\lambda_T} where I is the intensity and \lambda_T is the length scale. Are you using a moving reference frame? If so, try it with both the absolute and relative velocity for U; it's probably absolute, but I'm not certain.
I didn't use any formula myself, I just referred to the calculated values from Fluent in the "calculate from inlet" function in the initialize tab.
I am not using a moving reference frame, everything is stationary.
robboflea is offline   Reply With Quote

Old   September 24, 2013, 13:28
Default
  #4
Senior Member
 
Andrew Kokemoor
Join Date: Aug 2013
Posts: 119
Rep Power: 5
Kokemoor is on a distinguished road
If you use those formulas, do you get results matching either the data or the initialization computation?
Kokemoor is offline   Reply With Quote

Old   September 25, 2013, 10:02
Default
  #5
Member
 
Join Date: Nov 2011
Posts: 81
Rep Power: 5
robboflea is on a distinguished road
nope...they don't match any of them
robboflea is offline   Reply With Quote

Old   October 8, 2013, 05:14
Default
  #6
Member
 
Join Date: Nov 2011
Posts: 81
Rep Power: 5
robboflea is on a distinguished road
also I did notice that, after the calculations are completed the inlet values of turbulent intensity are completely different from the values I specified at the inlet.

Anybody has any idea on why is this happening?
robboflea is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Implementation of boundary conditions for FVM Tom Main CFD Forum 7 August 26, 2014 05:58
Water subcooled boiling Attesz CFX 7 January 5, 2013 04:32
Using Pressure Inlet and Velocity Inlet Boundary Conditions Together abhinavgupta88 FLUENT 0 September 25, 2012 11:45
inlet velocity boundary condition murali CFX 5 August 3, 2012 08:56
RPM in Wind Turbine Pankaj CFX 9 November 23, 2009 05:05


All times are GMT -4. The time now is 17:50.