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

how to determine the length scale for DPM?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 26, 2003, 06:42
Default how to determine the length scale for DPM?
  #1
xiang
Guest
 
Posts: n/a
When doing the particle trajectory calculation, the result varied whenever the length scale value was reduced. How could I know if the length scale value is small enough to generate the accurate trajectory?
  Reply With Quote

Old   February 27, 2003, 02:22
Default Re: how to determine the length scale for DPM?
  #2
T. H. Kim
Guest
 
Posts: n/a
hi.
  Reply With Quote

Old   February 27, 2003, 05:37
Default Re: how to determine the length scale for DPM?
  #3
xiang
Guest
 
Posts: n/a
any suggestions? Thanks.
  Reply With Quote

Old   February 27, 2003, 05:46
Default Re: how to determine the length scale for DPM?
  #4
Christian
Guest
 
Posts: n/a
Isn't a correct result the result that is independant of grid, length scale ect. ? (a kind of convergence criteria)

If yes, you would have to reduce the length until the result stop changing.

But, what result are you talking about ?
  Reply With Quote

Old   February 27, 2003, 06:02
Default Re: how to determine the length scale for DPM?
  #5
xiang
Guest
 
Posts: n/a
I am looking at the fate of the injected particles. I have set the fate of particles hitting different boundaries as escape, trap. I started the DPM after the flow field solution was converged. The result I am talking about is that the reported numbers of particles escaped or trapped. Whenever I changed the length scale value in the discrete phase modeling panel, the reported numbers of different fate varied.According to the FLUENT user's guide, small length scale will give more accurate result,which means that result is not independent of the length scale. Am I right? I have reduced the length scale to the extent that the calculation of particle trajectory couldn't be completed with the max. number of steps of 1e+09. Still i couldn't have the result stop changing. Hope my question is clear this time. Thanks!
  Reply With Quote

Old   February 27, 2003, 11:44
Default Re: how to determine the length scale for DPM?
  #6
Allan Walsh
Guest
 
Posts: n/a
When using CFD, it is easy to focus on numbers - like the fraction of particles trapped or escaped. Everyone likes to have straight forward criteria for making decisions.

However, the most important information you might be getting from your simimulation is why are the particles trapped or why do they escape? And what happens in your simulation when you change the length scale such that it changes the fate of the particle? Do you have a lot of back eddies and recirculation?

Based on my experience with large particles (i.e. typical diameters 0.001 to 0.005m) in large domains (10m by 10m by 30m)your max no. of steps, 1e9, is huge. I normally take about 2500 as max.

Do your results look reasonable? Can you use more particles such that each one represents a smaller fraction of the overall condensed phase?

I'm not sure there is any general parameter that sets the step length for different problems. What is the time step between particle calculations in your case? And don't trust that printed on the screen while tracking particle history - look in the code.

While we are at it - if you really want the "right" numerical result - have you considered whether the particle phase effects the gas phase and whether you need to recalculate the gas phase after a particle phase calculation. And, are the differences that you calculate with a different time step large enough that they are significant compared to other uncertainties in boundary conditions and solving techniques?

Hope this helps.
  Reply With Quote

Old   February 27, 2003, 20:04
Default Re: how to determine the length scale for DPM?
  #7
xiang
Guest
 
Posts: n/a
Dear Walsh,thank you very much for your detailed reply. I think I should follow your suggestions to find out what really happend in my problem. There are a lot of things for me to learn and understand using FLUENT.
  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
Integral Length Scale vs Turbulent Length Scale Scott Nordsen FLUENT 2 January 20, 2014 01:17
Turbulence length scale and integral length scale rizhang CFX 1 September 10, 2009 06:38
How to determine eddy length scale? Zaidun CFX 1 March 27, 2006 19:12
LES inlet length scale f.neilson Main CFD Forum 0 January 31, 2005 13:26


All times are GMT -4. The time now is 20:07.