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

Unstable behaviour after long period of stablility

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 17, 2011, 08:21
Exclamation Unstable behaviour after long period of stablility
  #1
New Member
 
Join Date: Mar 2011
Posts: 3
Rep Power: 6
plunge11 is on a distinguished road
Hi,
I am trying to model a turbulent aerated jet entering normal to the surface of a tank of water, based on an experiment which was carried out. In the experiment, the jet is aerated by means of air bubbles flowing into the water through holes with a diameter of 0.5 mm, prior to the jet's entry into the tank.


I have modelled this jet using multiphase flow, using the "mixture" option since I assumed that the flow should be identified as "bubbly" flow. I have used "second order upwind" spatial discretization where the option was available, "QUICK" for the volume fraction, and “least squares cell based” for the pressure. For a flow in which the volume air-fraction is 0.27, the solution converges in just over 17,000 iterations, with the under-relaxation factors remaining as default. However, for a volume air-fraction of a higher value of 0.45, the calculation seemingly refuses to converge, continuing to display highly fluctuating behaviour even after many thousands of calculations - the 0.44 model was run for over 48,000 calculations without showing any sign of convergence. Because of a persistent message about a turbulent viscosity limit and some divergent behaviour in epsilon, k, and x-momentum, I reduced the under-relaxation factors from their defaults in the early stages of the calculation for 0.44. However, although this allowed the calculation to run without these messages, it did not move toward convergence.
In the case of an air content of 0.36, I first ran the calculation with the equations for slip velocity and volume fraction turned off and using the default under-relaxation factors. This ran ok so, at 400 iterations, I turned these two equations back on and reduced the under-relaxation factors to the following: pressure (0.2), density (0.5), body forces (0.5), momentum (0.4), slip velocity (0.1), volume fraction (0.3), turbulent kinetic energy (0.4), turbulent dissipation rate (0.4), turbulent viscosity (0.5). In this case, the calculation ran fine and appeared to be on its way to converging until around 5150 iterations at which point it began to display the same kind of fluctuating behaviour as the calculation for a jet air content of 0.44 displayed. I therefore now have no idea what to do to get the solution to converge.
I have attached files showing the vector results for the iteration behaviour and contours of a jet with air-fraction 0.44 at around 20,000 iterations, and the results for a jet air-fraction of 0.27.
My questions are:
1.Have I used the wrong multiphase model?
2.Is there a direct link between the changes in the under-relaxation factors and the calculation behaviour? Is there any effect on the final solution if under-relaxation factors are changed?
3.Is there another method I should use to prevent unstable and fluctuating behaviour and other than changing the under-relaxation factors?


I would really appreciate any help that anyone could give - I'm completely at a loss as to what I should do next
Attached Images
File Type: jpg T29,e1.6,u03.4,a0.44,itr20983,iterations.jpg (75.8 KB, 29 views)
File Type: jpg T29,e1.6,u03.4,a0.44,itr23106.jpg (57.8 KB, 28 views)
File Type: jpg T29,e1.6,u03.4,a0.27.jpg (98.0 KB, 22 views)
plunge11 is offline   Reply With Quote

Old   April 6, 2011, 09:15
Default
  #2
New Member
 
Merend
Join Date: Jan 2011
Posts: 12
Rep Power: 6
pz1337 is on a distinguished road
Interested, did you find a solution, especially regarding " turbulent viscosity limit" ?

Thanks!
pz1337 is offline   Reply With Quote

Reply

Tags
convergence issues, unstable behaviour

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
critical error during installation of openfoam Fabio88 OpenFOAM Installation 21 June 2, 2010 03:01
Problems in compiling paraview in Suse 10.3 platform chiven OpenFOAM Installation 3 December 1, 2009 08:21
OpenFOAM-1.5-dev svn revision 1438: libOpenFOAM does not compile in SP 4xF OpenFOAM Bugs 3 October 16, 2009 05:35
Missing math.h header Travis FLUENT 4 January 15, 2009 12:48
Convergence issues with long period sloshing flows Brian FLUENT 1 February 1, 2006 09:41


All times are GMT -4. The time now is 05:41.