CFD Online URL
[Sponsors]
Home > Forums > FLUENT

Why is Fluent unable to deal with my model at the micron scale?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   December 8, 2011, 04:47
Default Why is Fluent unable to deal with my model at the micron scale?
  #1
New Member
 
Join Date: Dec 2011
Posts: 9
Rep Power: 5
dixylo is on a distinguished road
My model is a micro turbo rotor spinning in a cylindrical wall. When I scaled the grid by setting the scale factors of xyz to be 0.00001 or lower, the iteration would stop in one step, saying "solution is converged"
At first i thought something was wrong with my grid and settings, so i created a cylindrical pipe model where liquid flows in with a speed magnitude equal to that of the radius through the bottom and out through the top, but this model has the same problem that when the scale factors of xyz are lower than a certain value (0.0009 here), the iteration stops within one or a couple of steps.
however, what has puzzled me is that when the scale factors are greater than a certain value (1e-5 for my rotor model and 0.0009 for the pipe one), the problem doesn't show up and Fluent gives a good solution.
so can Fluent deal with models as small as tens of microns?
dixylo is offline   Reply With Quote

Old   December 8, 2011, 06:04
Default
  #2
Senior Member
 
Amir's Avatar
 
Amir
Join Date: May 2009
Location: Shiraz, Iran
Posts: 738
Blog Entries: 1
Rep Power: 13
Amir is on a distinguished road
Quote:
Originally Posted by dixylo View Post
My model is a micro turbo rotor spinning in a cylindrical wall. When I scaled the grid by setting the scale factors of xyz to be 0.00001 or lower, the iteration would stop in one step, saying "solution is converged"
At first i thought something was wrong with my grid and settings, so i created a cylindrical pipe model where liquid flows in with a speed magnitude equal to that of the radius through the bottom and out through the top, but this model has the same problem that when the scale factors of xyz are lower than a certain value (0.0009 here), the iteration stops within one or a couple of steps.
however, what has puzzled me is that when the scale factors are greater than a certain value (1e-5 for my rotor model and 0.0009 for the pipe one), the problem doesn't show up and Fluent gives a good solution.
so can Fluent deal with models as small as tens of microns?
Hi,

Did you disable convergence criteria in "solve->monitor->residual" ?
It better to use double precision version for your purpose and also, you can change length dimension to "mm" for example but as you said, this problem may exist even in double precision solvers if the variables would be in the order of round off error which is about 1.0e-16. As there are many simulations with FLUENT for micro channels in literature, I don't think that this can make any issue for your case. But if you couldn't solve this problem, maybe a dimensional analysis can help.

Bests,
__________________
Amir
Amir is offline   Reply With Quote

Old   December 8, 2011, 14:39
Smile
  #3
New Member
 
Join Date: Dec 2011
Posts: 9
Rep Power: 5
dixylo is on a distinguished road
Quote:
Originally Posted by Amir View Post
Hi,

Did you disable convergence criteria in "solve->monitor->residual" ?
It better to use double precision version for your purpose and also, you can change length dimension to "mm" for example but as you said, this problem may exist even in double precision solvers if the variables would be in the order of round off error which is about 1.0e-16. As there are many simulations with FLUENT for micro channels in literature, I don't think that this can make any issue for your case. But if you couldn't solve this problem, maybe a dimensional analysis can help.

Bests,
I didn't disable convergence criterion (I kept it as "absolute"). Most settings were kept as default. I tried double precision solvers as soon as i met this problem, but they didn't work either. As you said, there are many simulations with FLUENT for micro channels in literature, this is what has puzzled me. If you are interested in this problem and will spend a little time, you may create a similar model, for instance, a cylinder with a height of 5 and a radius of 1, and the bottom as VELOCITY-INLET boundary type, the top as OUTFLOW. Scale it with scale factors 1e-6. Other settings can be done as they should be. And i think you would probably see "solution is converged" in one iteration step. anyway, thank you for your reply and your suggestion.
dixylo is offline   Reply With Quote

Old   December 8, 2011, 15:31
Default
  #4
Senior Member
 
Amir's Avatar
 
Amir
Join Date: May 2009
Location: Shiraz, Iran
Posts: 738
Blog Entries: 1
Rep Power: 13
Amir is on a distinguished road
Quote:
Originally Posted by dixylo View Post
I didn't disable convergence criterion (I kept it as "absolute"). Most settings were kept as default. I tried double precision solvers as soon as i met this problem, but they didn't work either. As you said, there are many simulations with FLUENT for micro channels in literature, this is what has puzzled me. If you are interested in this problem and will spend a little time, you may create a similar model, for instance, a cylinder with a height of 5 and a radius of 1, and the bottom as VELOCITY-INLET boundary type, the top as OUTFLOW. Scale it with scale factors 1e-6. Other settings can be done as they should be. And i think you would probably see "solution is converged" in one iteration step. anyway, thank you for your reply and your suggestion.
Hi,

I meant toggle off "check convergence" in "solve->monitor->residual"; in this manner you never face "solution is converged"!!! just let it continue to iterate.

Bests,
__________________
Amir
Amir is offline   Reply With Quote

Old   December 9, 2011, 00:39
Smile
  #5
New Member
 
Join Date: Dec 2011
Posts: 9
Rep Power: 5
dixylo is on a distinguished road
Quote:
Originally Posted by Amir View Post
Hi,

I meant toggle off "check convergence" in "solve->monitor->residual"; in this manner you never face "solution is converged"!!! just let it continue to iterate.

Bests,
Sorry, I'm a newb so i didn't quite understand what you'd meant as well as some of those variables and settings. Your suggestion did work, and i think what i should do now is optimize the grid. Thank you very much for you kind help.
dixylo 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
Use of k-epsilon and k-omega Models Jade M Main CFD Forum 11 December 21, 2013 05:50
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 17 August 22, 2009 04:59
self-built model embeded in FLUENT Jiedong FLUENT 0 January 22, 2009 03:57
Modeling in micron scale using icoFoam m9819348 OpenFOAM Running, Solving & CFD 7 October 27, 2007 01:36
Can FLUENT model flashing liquids? SWK FLUENT 4 December 13, 2005 22:55


All times are GMT -4. The time now is 08:58.