Time Step confusion (SOLVED)

Numerical methods and mathematical models of Elmer
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Gary R
Posts: 162
Joined: 12 Apr 2012, 07:23
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Location: Long Beach CA, USA

Time Step confusion (SOLVED)

Post by Gary R »

This would seem to be a very simple thing but is causing me mass confusion. I can't seem to get the whole time step thing straight in my mind. Referring to the graphic below please explain to me the meaning of "Time Step Interval" and "Time Step Size" in ElmerGUI and "Time", "Start Time", End Time" and "No. Frames" in Paraview.

0.......1....................................N
|-------|-------|-------| ... |------|
Where 0 is the start, 1 is the length of each step and N is the total time for the whole process.

I can't seem to get these settings right.

Gary R
Last edited by Gary R on 24 Mar 2018, 03:21, edited 1 time in total.
mzenker
Posts: 1999
Joined: 07 Dec 2009, 11:49
Location: Germany

Re: Time Step confusion

Post by mzenker »

Hi,

in ElmerGUI, you can set "timestep intervals" and "timestep sizes" (note the "s" at the end). Generally in Elmer, "timestep intervals" is the number of timesteps, "timestep sizes" is the size of the timesteps. So for a total length of 10 s in steps of 0.1 s, you must set

timestep intervals = 100
timestep sizes = 0.1

The plural is there because you can also specify more than one timestep size.

timestep intervals(2) = 5 10
timestep sizes(2) = 0.1 0.001

means that you have 5 steps of 0.1 s, followed by 10 steps of 0.001 s.
In ElmerGUI, you need to write

size 2; 5 10
size 2; 0.1 0.001

in the "timestep intervals" and "timestep sizes" fields.
See the Elmersolover manual for more information.

Now for the correspondance between Elmer and ParaView, the state at t=0 is normally not stored by Elmer. So you will have the first vtu file for the state after the first timestep, and you will not know to which time it corresponds since the time itself is not stored in the vtu file. A remedy to that is to use a vtu time collection. For that, you have to explicitely add the resultoutput solver and write

vtu time collection = logical true

in the free text field (see also the Chapter on Result Output in the Elmer Models Manual). Elmer then writes a .pvd file which you can open with ParaView. Then you have the correct time for each step.

HTH,

Matthias
Gary R
Posts: 162
Joined: 12 Apr 2012, 07:23
Antispam: Yes
Location: Long Beach CA, USA

Re: Time Step confusion

Post by Gary R »

Thank you very very much.

Gary R
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