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

Turbulence intensity BC

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   June 28, 2006, 11:11
Default Turbulence intensity BC
  #1
Ralf Schmidt
Guest
 
Posts: n/a
Hi!

I have a simple geometry (free stream around a cylinder). The inlet BC is vel.inlet with turbulence parameters are turbulence intensity and hydraulic diameter (diameter of the cylinder).

Now when I plot the values of turbulence intensity at the inlet, they are 1. not all the same over the face (they vary about +- 5%) and 2. slightly lower (about 10%) than the values I set as BC. What happend?? The reference velocity is the free stream vel.

Any Idea?

Ralf
  Reply With Quote

Old   June 29, 2006, 19:16
Default Re: Turbulence intensity BC
  #2
freeman
Guest
 
Posts: n/a
you might want to look up in the USER manual in that regards. Remember that, the turb. intensity at the inlet depends on Ub or tke. so if the vel. is not uniformaly distributed then you wouldn't expect the intensity to be uniform. what I'll suggest is that plot the vel, tke and based on that you tell what's going on.

all the best
  Reply With Quote

Old   June 30, 2006, 02:02
Default Re: Turbulence intensity BC
  #3
Ralf Schmidt
Guest
 
Posts: n/a
Thanks for reply!

I checked the points you mentioned:

there is a constant inlet velocity BC of 21 m/s BUT in the plot, the vel. is NOT uniformaly distributed, it is fluctuating about +-0.005% of the inlet vel.

How could that happend?? Because of mass balance? I do a incompressible calculation.

The Turbulence kinetic energy, turb. viscosity, omega and the turbulence intensity have the same fluctuations. BUT they are much larger!!

How could a vel. fluctuation of 0.009% cause a Tu.Intensity fluctuation of about 5% ???

What happend?

Ralf

  Reply With Quote

Old   June 30, 2006, 05:17
Default Re: Turbulence intensity BC
  #4
freeman
Guest
 
Posts: n/a
if I may ask, what is the shape of the inlet, circular or rect?

anyway, to check if you're doing right thing, check your mass balance to see if mass_in=mass_out through the report menu. I think what happened was tha if you specified normal velocity at surface, Fluent interpolate it over the surface by dividing it according to the number of nodes so you wouldn't expect all the nodes to have the same values, however the mass-flow will be correct and the total integration of the vel will be equal to what you specified. You could cross check that through that through the 'REPORT menu' and also the user manual.

as for higher variations in the turb quantities are obvious, since they are in higher order of the vel. eg. tke=(...)vel^2. That's not a problem.

I hope that works. if I may ask, how are the general results look like? outlet plane results? are you comparing with expt or what? if you do, how did it deviate from it?
  Reply With Quote

Reply


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 postprocessing Mohsin FLUENT 2 October 3, 2016 15:18
Question on Turbulence Intensity Eric FLUENT 1 March 7, 2012 05:30
About Turbulence Intensity (Pipe flow assimilated) gRomK13 Main CFD Forum 1 July 10, 2009 04:11
Discussion: Reason of Turbulence!! Wen Long Main CFD Forum 3 May 15, 2009 10:52


All times are GMT -4. The time now is 12:35.