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

Convergence tips

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 25, 2005, 12:02
Default Convergence tips
  #1
John
Guest
 
Posts: n/a
Hello,all

In CFX10 help file, I found exactly the same convergence tips for multiphase modelling(bubble column) as in CFX5:

(ANSYS CFX 10 Best Practices Guide for Multiphase, page 132): "Sometimes, physical instabilities (such as recirculation zones) can result in slow or stalled convergence. In these cases, you can still obtain an indicator of convergence for a global quantity by creating a monitor point at some point in the domain. As a result, you can determine whether values for selected global quantities (such as gas hold-up) are meaningful."

Does this mean that the convergence critira(eg. RMS) is not so important(or critical) for multiphase problems?

I asked the same question in a previous post, but I still fell confused about these words.

The answer to this question is important to my current work. I would appreciate your inputs!

Regards!

John

  Reply With Quote

Old   August 26, 2005, 09:49
Default Re: Convergence tips
  #2
John
Guest
 
Posts: n/a
It may be a naive question but I hope someone can explain it. In literatures I did see some people use liquid height level(eg. for sieve trays) or holdup as indicator parameters to terminate the run.

Regards!

John

  Reply With Quote

Old   August 28, 2005, 00:41
Default Re: Convergence tips
  #3
Jeff
Guest
 
Posts: n/a
Multiphase simulations are, by nature, tansient. However, the transients can be very localized. Usually this is due to small flow oscillations or oscillating bubble streams. When this happens, the RMS residuals will drop to a certain level and then show a repeating oscillating pattern. The residuals can't be reduced any further because there simply isn't a non-oscillating steady-state solution. You have two recourses:

1) after the residuals have "bottomed out", plot velocities at various locations, and total gas volume (hold up). When these values become steady, or reach a repeatable oscillating "pseudo-steady-state" pattern, you're probably converged.

2) Switch the simulation to transient, and plot the same monitor points as for the steady-state run until they reach a pseudo-steady-state behavior. I prefer this method, since I can show that my transient residuals are 10^-5 or lower, and that a stable solution has been reached through the monitor point values.

Hope this helps.

Jeff
  Reply With Quote

Old   August 28, 2005, 01:15
Default Re: Convergence tips
  #4
John
Guest
 
Posts: n/a
Nice comments. Thanks again, Jeff

I have recently switched to transient simulation, and observed same convergence level as you mentioned(sometime can be as low as 10^-6).

What I am worry about is the simulatously conducted tracer simulation. I still haven't tried much, but some initial results showed that the tracer curves are not smooth.

Could you suggest a best way of tracer simulation for transient multiphase flow?

Best regards!

John
  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
Convergence Centurion2011 FLUENT 24 May 9, 2015 08:02
Force can not converge colopolo CFX 13 October 4, 2011 22:03
tips to speedup the convergence of CHT problem with gravity derisala FLUENT 1 February 24, 2011 03:28
Convergence of CFX field in FSI analysis nasdak CFX 2 June 29, 2009 01:17
RSM with Eulerian, tips for convergence? Paul FLUENT 3 April 9, 2008 13:25


All times are GMT -4. The time now is 19:21.