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

deciding time duration in transient simultion

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 3, 2012, 09:43
Default deciding time duration in transient simultion
  #1
New Member
 
Join Date: Jan 2012
Posts: 9
Rep Power: 5
moniker is on a distinguished road
I am doing a transient simulation for my model ,
I have decided to use time steps as 0.0632 seconds,how do I decide the time duration ?I first gave time duration as 65 and time steps as 0.0632 but the
solver ran continously ..has'nt converged at all
moniker is offline   Reply With Quote

Old   March 4, 2012, 05:05
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,646
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
How have you decided 0.0632s is the right time step size? If it is not converging it probably is not the right size.

And as for the time duration - what are you modelling? An inkjet printer ejection (total time about 10 microseconds), or magma motion in a magma chamber (turn over time 140 million years). I have done work in both and guess what? They both use different total simulation times.
ghorrocks is offline   Reply With Quote

Old   March 4, 2012, 07:17
Default
  #3
New Member
 
Join Date: Jan 2012
Posts: 9
Rep Power: 5
moniker is on a distinguished road
I am modelling the flow of air through the intake manifold of a 4cyl diesel engine ,naturally aspirated ,upto before valve..now I calculated timestep by taking the whole length of manifold as distance and inlet velocity of air , that value I took multiplied by (1/20) to get 0.0632 sec ..my length is 0.349 m and intake velocity is 27.61m/s ..1/20 because that is the time required for a particle of fluid to traverse the length of object
moniker is offline   Reply With Quote

Old   March 4, 2012, 17:41
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,646
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Hmmm, I don't think you have much idea of how to set a time step

My PhD thesis was on IC engine modelling. Have a look at the time step size I used:
http://hdl.handle.net/2100/248
ghorrocks is offline   Reply With Quote

Old   March 5, 2012, 08:47
Default
  #5
New Member
 
Join Date: Jan 2012
Posts: 9
Rep Power: 5
moniker is on a distinguished road
Thanks for that thesis..i have got new doubts...how is the CFL stability criterion related to timestep..and I am not able
to comprehend what a timestep is and what are the parameters that affect it.
moniker is offline   Reply With Quote

Old   March 5, 2012, 18:16
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,646
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Quote:
how is the CFL stability criterion related to timestep
CFX is an implicit solver so it is not relevant.

Quote:
and I am not able to comprehend what a timestep is and what are the parameters that affect it.
If you do not understand the basics you do not have much hope of doing a useful analysis. You need to do some studies or reading into numerical methods and CFD before you can start doing simulations.
ghorrocks 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
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 18 March 3, 2015 06:36
Upgraded from Karmic Koala 9.10 to Lucid Lynx10.04.3 bookie56 OpenFOAM Installation 8 August 13, 2011 04:03
Transient Time Step for supersonic flows CP CFX 1 November 12, 2007 17:22
Modeling in micron scale using icoFoam m9819348 OpenFOAM Running, Solving & CFD 7 October 27, 2007 00:36
unsteady calcs in FLUENT Sanjay Padhiar Main CFD Forum 1 March 31, 1999 12:32


All times are GMT -4. The time now is 23:25.