CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Running, Solving & CFD

local-time stepping (LTS)

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 5, 2020, 22:35
Lightbulb local-time stepping (LTS)
  #1
New Member
 
Diego Jerez
Join Date: Feb 2020
Posts: 11
Rep Power: 4
djerezg is on a distinguished road
Hi to all,

anyone can help me to understand better how "local-time stepping (LTS)" works?

I know its a kind of pseudo transient method but actually i dont know how this method choose his delta t for each iteration (for example in ansys fluent you know how the method set this delta t.)

If some one have some document or information, i'd be very grateful.

Greetings!
djerezg is offline   Reply With Quote

Old   April 6, 2020, 12:13
Default
  #2
Senior Member
 
Join Date: Aug 2015
Posts: 494
Rep Power: 12
clapointe is on a distinguished road
This information was provided when it was first incorporated : https://openfoam.org/release/2-0-0/steady-state-vof/. Basically, the solution is accelerated to steady state because a time step "field" is computed, so each cell has its own timestep instead of one global timestep. The timestep field is smoothed (and changes between iterations are damped) to maintain stability.

Caelan
clapointe is offline   Reply With Quote

Old   April 6, 2020, 12:45
Default
  #3
New Member
 
Diego Jerez
Join Date: Feb 2020
Posts: 11
Rep Power: 4
djerezg is on a distinguished road
Quote:
Originally Posted by clapointe View Post
This information was provided when it was first incorporated : https://openfoam.org/release/2-0-0/steady-state-vof/. Basically, the solution is accelerated to steady state because a time step "field" is computed, so each cell has its own timestep instead of one global timestep. The timestep field is smoothed (and changes between iterations are damped) to maintain stability.

Caelan
TY for you answer!
i already knew that info, but how this time step field for each element is calculated? or just take the maximun time to keep the courant number in each element?

Greetings!
djerezg is offline   Reply With Quote

Old   April 6, 2020, 13:01
Default
  #4
Senior Member
 
Join Date: Aug 2015
Posts: 494
Rep Power: 12
clapointe is on a distinguished road
At the bottom of the link there are paths to source files -- so you know that there is one to compute the time step "field" on the solver end and other files in src to facilitate. So check out interFoam for a modern version and we see a setRDeltaT.H file (actually, there is a general collection of VoF-related files in the multiphase solvers folder). This is where the (inverse) time step field is computed. It is commented to designate smoothing, etc.

Caelan
clapointe is offline   Reply With Quote

Old   April 6, 2020, 19:50
Default
  #5
New Member
 
Diego Jerez
Join Date: Feb 2020
Posts: 11
Rep Power: 4
djerezg is on a distinguished road
Quote:
Originally Posted by clapointe View Post
At the bottom of the link there are paths to source files -- so you know that there is one to compute the time step "field" on the solver end and other files in src to facilitate. So check out interFoam for a modern version and we see a setRDeltaT.H file (actually, there is a general collection of VoF-related files in the multiphase solvers folder). This is where the (inverse) time step field is computed. It is commented to designate smoothing, etc.

Caelan
I tried to read that code, but was difficult to me understand very well that info.
Can you (or anyone who knows and want to) help me to understand better that code with words, please?

Greetings!
djerezg is offline   Reply With Quote

Old   April 6, 2020, 20:26
Default
  #6
Senior Member
 
Join Date: Aug 2015
Posts: 494
Rep Power: 12
clapointe is on a distinguished road
The comments give us the gist of what is happening in setRDeltaT.H. Working through essentially gives us -- compute "timesteps" for each cell based off a max courant number, smooth this timestep field (in space) to improve stability, and then damp this timestep field (in time, as in limit changes) to further improve stability. As far as I'm aware, these steps are included in each instance of setRDeltaT; other damping may be applied, e.g. for the multiphase solvers based on interface location, or for reacting solvers based on heat release. Hope this was a bit clearer.

Caelan
clapointe is offline   Reply With Quote

Old   April 6, 2020, 21:09
Default
  #7
New Member
 
Diego Jerez
Join Date: Feb 2020
Posts: 11
Rep Power: 4
djerezg is on a distinguished road
Great! that explain a lot.

Now im wondering, what is the criteria in Ansys Fluent, because when you use pseudo transient method in a steady simulation, you cant choose a Courant number. In that case, the under-relaxation is controlled through the pseudo time step size, but there is not relation with courant number, or in that case just use a time for each iteration until the program converge?

thank you for your help, is very usefull for me.

Greetings!

Edit: I found this https://www.afs.enea.it/project/nept...underrelax-eqn
I hope this i'd be usefull for someone else.

Cheers!

Last edited by djerezg; April 7, 2020 at 11:54.
djerezg is offline   Reply With Quote

Reply

Tags
lts, pseudo, transient

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
Setting up Lid driven Cavity Benchmark with 1M cells for multiple cores puneet336 OpenFOAM Running, Solving & CFD 11 April 7, 2019 00:58
Star cd es-ice solver error ernarasimman STAR-CD 2 September 12, 2014 00:01
Micro Scale Pore, icoFoam gooya_kabir OpenFOAM Running, Solving & CFD 2 November 2, 2013 13:58
pisoFoam with k-epsilon turb blows up - Some questions Heroic OpenFOAM Running, Solving & CFD 26 December 17, 2012 03:34
[blockMesh] BlockMeshmergePatchPairs hjasak OpenFOAM Meshing & Mesh Conversion 11 August 15, 2008 07:36


All times are GMT -4. The time now is 07:36.