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

How to decide time step size

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 6, 2010, 08:08
Default How to decide time step size
  #1
Member
 
tom
Join Date: Feb 2010
Posts: 46
Rep Power: 7
caohan is on a distinguished road
Hi all,
In a unsteady simulation, one thing always confus me which is how to decide time step size and how many time steps is enough, hope someone can explain for me
Regards
Han
caohan is offline   Reply With Quote

Old   September 6, 2010, 23:25
Default
  #2
New Member
 
Toby Cheung
Join Date: Aug 2010
Posts: 3
Rep Power: 7
tobytoby is on a distinguished road
Hi Caohan,

Actually it does not have any requirement for the time step, yet I can share some exp. of myself to you.

1. base on the velocity equation: velocity = distance /time step
where the velocity is refered to the design velocity in your model; and the distance is the length between the inlet and the first contacted wall. So, it gives a rough reference for designing the time step.

2. Guess the time step first and modify it by experience (more trials within the same model). This saying seems rubbish .... I know ... yet it is always true. Jsut set a smaller iteration size first, and extend it after you confirm a suitable time step.

Gook luck to you,
Toby
tobytoby is offline   Reply With Quote

Old   September 8, 2010, 02:54
Wink
  #3
New Member
 
cerisier
Join Date: Sep 2010
Posts: 5
Rep Power: 6
pepi33 is on a distinguished road
Hi all,

Firt of all, sorry for my poor english.

For me, it depends on what phenomenon you want to see.
For exemple if you want the very low scale near the wall region, you have to use this :
it is still the same concept but here time step= syze of the smaller mesh (near the wall region)/Local velocity in this smaller mesh
For exemple, at this moment i'm using a time step of 0.0001s.

Concerning the convergence of the time of the time step you have to use this concept (i think):
For your first case you have to calculate for a hihgt time (such as 1s, or more it depends on your model) . For all iterations you plot a characteristic (pressure for exemple) on one representative point (or more) of your mesh.
Basically, if your mesh an turbulence model is good you will have a sine curve (pressure/time) less and less tall (difference between +/-). Concerning to your criterion, wou will able to say at this time step it is enough converge for me.

I hope it will help you,
Best regards,
pepi
pepi33 is offline   Reply With Quote

Old   September 8, 2010, 03:30
Default
  #4
Senior Member
 
Philipov's Avatar
 
Svetlin Philipov
Join Date: Mar 2009
Location: United Kingdom
Posts: 176
Rep Power: 8
Philipov is on a distinguished road
Time step must be lower then ratio of the minimum cell size and the expected velocity at that region. I am saying expected velocity because in general you may have reactions (for example combustion) ...
How many time steps? It depends on the total time you want to calculate
number of time steps=(total time)/(timestep) or something like this....
Philipov 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
Time step size, number of time steps and max iterations per time step guido_88 FLUENT 4 August 30, 2012 14:49
critical error during installation of openfoam Fabio88 OpenFOAM Installation 21 June 2, 2010 03:01
Modeling in micron scale using icoFoam m9819348 OpenFOAM Running, Solving & CFD 7 October 27, 2007 00:36
Transient simulation not converging skabilan OpenFOAM Running, Solving & CFD 12 September 17, 2007 17:48
Time Step Size for Airfoil Model - Fluent Prateek Main CFD Forum 0 November 4, 2004 19:42


All times are GMT -4. The time now is 20:14.