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

The result depends on the time step

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

Like Tree3Likes
  • 2 Post By FMDenaro
  • 1 Post By arjun

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 27, 2017, 15:30
Post The result depends on the time step
  #1
New Member
 
Join Date: Aug 2017
Posts: 4
Rep Power: 8
Mira is on a distinguished road
Hi,
I am use buoyantboussinesqpimplefoam solver. I found that when the time step changes, the result of numerical calculations changes too. How to understand at what step in time the correct calculation? How to make so that at change of step on time calculations results did not vary? If some one know how to solve the problem it will be very helpful. Thank you.
Mira is offline   Reply With Quote

Old   September 28, 2017, 00:55
Default
  #2
Senior Member
 
piu58's Avatar
 
Uwe Pilz
Join Date: Feb 2017
Location: Leipzig, Germany
Posts: 744
Rep Power: 15
piu58 is on a distinguished road
The time step and the mesh have to check at every simulation. Lower the time step until the changes in your solution are very small. Then it is time step stable.
In the same way you should use finer and finer meshes until the changes are as small as you need.
__________________
Uwe Pilz
--
Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)
piu58 is offline   Reply With Quote

Old   September 28, 2017, 04:50
Post
  #3
New Member
 
Join Date: Aug 2017
Posts: 4
Rep Power: 8
Mira is on a distinguished road
Thanks for the quick reply! I used the standard tutorial "hot room" and decreased the step from 0.1 to 0.01, 0.001 and 0.0001. I got different results at time step 0.001 and 0.0001. The results at time step 0.1 and 0.01 was the same, But at the time step 1 other results were obtained. I'll try to reduce the size of the grid. Thank you!
Mira is offline   Reply With Quote

Old   September 28, 2017, 19:47
Default
  #4
Senior Member
 
Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 1,278
Rep Power: 34
arjun will become famous soon enougharjun will become famous soon enough
PISO solution is time step dependent.
arjun is offline   Reply With Quote

Old   September 30, 2017, 18:47
Post
  #5
New Member
 
Join Date: Aug 2017
Posts: 4
Rep Power: 8
Mira is on a distinguished road
Quote:
Originally Posted by arjun View Post
PISO solution is time step dependent.
Do you know why?
Mira is offline   Reply With Quote

Old   September 30, 2017, 19:54
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,793
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Generally explicit/implicit FD/FV depend on time step in the sense that the local truncation error is a function of the time and space steps.
arjun and SaufiSangi like this.
FMDenaro is offline   Reply With Quote

Old   October 1, 2017, 19:46
Default
  #7
Senior Member
 
Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 1,278
Rep Power: 34
arjun will become famous soon enougharjun will become famous soon enough
Quote:
Originally Posted by Mira View Post
Do you know why?
major contributor is non linear term in convection. There are other small issues that affect the results to different degrees (for example one might not update the gradient with every corrector to save time while in implicit methods they are computed every iteration)
SaufiSangi likes this.
arjun is offline   Reply With Quote

Old   September 12, 2019, 11:27
Default
  #8
New Member
 
Abd Essamade Saufi
Join Date: Apr 2017
Posts: 15
Rep Power: 9
SaufiSangi is on a distinguished road
Quote:
Originally Posted by arjun View Post
major contributor is non linear term in convection. There are other small issues that affect the results to different degrees (for example one might not update the gradient with every corrector to save time while in implicit methods they are computed every iteration)
Hi foamers! I study droplets evaporation in OpenFOAM and I also recently noticed that my results are very time step dependent. I can reach time step convergence for Courant numbers like 2, 1, 0.5. I also get nice results. As soon as I decrease the time step of one/two orders of magnitude the results become really strange and totally different. I agree that the problem is the convection term, because if I perform the same simulations suppressing the velocity field (or with a very slow one) the time step dependency does not appear anymore. In literature I never found the time step to be a serious numerical problem (such as the mesh), as long as the code is stable and the Courant number lower than 1.

Do you guys have a satisfying explanation of this? Is it really the PISO method? Can it be due to the numerical diffusion of the convective term, that sums up "more errors" if more time steps are used? I really don't know what to think!


Hope to hear from you guys,

Sangi
SaufiSangi is offline   Reply With Quote

Reply

Tags
time step size

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
courant number increases to rather large values 6863523 OpenFOAM Running, Solving & CFD 22 July 5, 2023 23:48
Transient simulation not converging skabilan OpenFOAM Running, Solving & CFD 14 December 16, 2019 23:12
p_rgh initial residual no change with different settings manuc OpenFOAM Running, Solving & CFD 3 June 26, 2018 15:53
pimpleDyMFoam computation randomly stops babapeti OpenFOAM Running, Solving & CFD 5 January 24, 2018 05:28
Help for the small implementation in turbulence model shipman OpenFOAM Programming & Development 25 March 19, 2014 10:08


All times are GMT -4. The time now is 23:26.