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

Airflow in a chamber

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 27, 2010, 09:57
Default Airflow in a chamber
  #1
New Member
 
Join Date: Nov 2009
Posts: 7
Rep Power: 7
Issa is on a distinguished road
Hi,

Please be informed that I'm using CFX to simulate airflow in a chamber. The chamber is ventilated by displacement ventilation with air velocity of 0.1 m/s and there are some heat sources in the chamber. When i used steady state simulation, i didn't get a converged solution because the flow is mainly buoyancy driven flow. So, I decided to go for transient simulation. But my question is what is the appropriate time step and total time should I use for transient simulation? Thank you

Regards,
Issa
Issa is offline   Reply With Quote

Old   January 27, 2010, 14:53
Default
  #2
New Member
 
Join Date: Nov 2009
Posts: 7
Rep Power: 7
Issa is on a distinguished road
Hi,

Still waiting for a reply. Kindly, I need your help

Regards,
Issa
Issa is offline   Reply With Quote

Old   January 27, 2010, 16:46
Default
  #3
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,945
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Time step size selection is discussed in the documentation. In summary you want to have 3-5 coefficient loops per time step. You can use an adaptive time step technique to find this quickly if you like.

Have you tried the tips here to get convergence in steady state?
http://www.cfd-online.com/Wiki/Ansys...gence_criteria

But I suspect you are right in that the result is transient which means you only real approach is a transient model. HVAC flows, especially ones with heat sources tend to have large scale transient behaviour.
ghorrocks is offline   Reply With Quote

Old   January 27, 2010, 17:02
Default
  #4
New Member
 
Join Date: Nov 2009
Posts: 7
Rep Power: 7
Issa is on a distinguished road
Hi ghorrocks,

Thank you for your reply. Regarding the documentation, where i can find it? Thank you.

Regards,
Issa
Issa is offline   Reply With Quote

Old   January 28, 2010, 03:48
Default
  #5
New Member
 
Join Date: Nov 2009
Posts: 7
Rep Power: 7
Issa is on a distinguished road
Hi ghorrocks,

Thank you for your reply. Kindly, where i can find the documentation? I would like to know what is the required time step and the total time for transient simulation. Thank you.

Regards,
Issa
Issa is offline   Reply With Quote

Old   January 28, 2010, 08:37
Default
  #6
Senior Member
 
Attesz's Avatar
 
Attesz
Join Date: Mar 2009
Posts: 355
Rep Power: 8
Attesz is an unknown quantity at this point
In CFX help!
Attesz is offline   Reply With Quote

Old   January 30, 2010, 10:27
Default
  #7
Senior Member
 
Join Date: Jul 2009
Posts: 211
Rep Power: 9
kingjewel1 is on a distinguished road
Quote:
Originally Posted by Issa View Post
Hi ghorrocks,

Thank you for your reply. Kindly, where i can find the documentation? I would like to know what is the required time step and the total time for transient simulation. Thank you.

Regards,
Issa
You can set your timestep based on the air residence time within your room. Typically your Courant number should not be greater than 20.
Personally I run the simulations for a few thousand iteration on a Timescale factor of 5 then switch to physical.
kingjewel1 is offline   Reply With Quote

Old   January 30, 2010, 11:12
Default
  #8
New Member
 
Join Date: Nov 2009
Posts: 7
Rep Power: 7
Issa is on a distinguished road
Dear kingjewel1,

Thank you for your reply. Kindly be informed that I'm using transient simulation not steady state. For steady state i tried physical timescale but the solution didn't converged. Actually for my case the flow is buoyancy driven flow. So, it is too difficult to get converged solution using steady state simulation. For transient simulation I would like to know what is the appropriate time step and the total time for the simulation. Actually I don't know what is the residence time? So what to do?

Regards,
Issa
Issa is offline   Reply With Quote

Old   January 30, 2010, 16:43
Default
  #9
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,945
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
I have already told you how to set time step in a transient simulation.

Quote:
Time step size selection is discussed in the documentation. In summary you want to have 3-5 coefficient loops per time step. You can use an adaptive time step technique to find this quickly if you like.
The total time for the simulation is problem dependent. It probably has a large scale transient structure so you will want a few oscillations of this structure so you can get a reasonable overall picture.

Quote:
Personally I run the simulations for a few thousand iteration on a Timescale factor of 5 then switch to physical.
Kingjewel - most CFX steady state simulations converge with 100-200 iterations. So why are you running thousands? Is your physics complicated?
ghorrocks is offline   Reply With Quote

Old   January 30, 2010, 17:10
Default
  #10
Senior Member
 
Join Date: Jul 2009
Posts: 211
Rep Power: 9
kingjewel1 is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
I have already told you how to set time step in a transient simulation.



The total time for the simulation is problem dependent. It probably has a large scale transient structure so you will want a few oscillations of this structure so you can get a reasonable overall picture.



Kingjewel - most CFX steady state simulations converge with 100-200 iterations. So why are you running thousands? Is your physics complicated?
Sorry I did actually mean hundreds. Thanks for the correction.
kingjewel1 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
[GAMBIT] Dynamic Meshing of a combustion chamber donarundas ANSYS Meshing & Geometry 1 December 2, 2009 08:13
I need design planes of combustion chamber HITACHI GE AdidaKK Main CFD Forum 0 July 2, 2009 14:04
Coal particles in combustion chamber Luis Filipe Fabiani CFX 2 March 10, 2009 16:37
Flow and Heat transfer in a Thawing chamber B.Prabhu FLUENT 0 May 12, 2002 15:13
A moving object within a chamber Whitaker FLUENT 5 October 10, 2000 22:02


All times are GMT -4. The time now is 09:37.