Eigen Modes

Post processing utility for Elmer
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arend
Posts: 4
Joined: 06 Aug 2010, 15:40

Eigen Modes

Post by arend » 06 Aug 2010, 15:55

Hello,
I am looking for how to visualise the eigen modes (eingenvectors) on the Elmer Post.
The solver generated the eigen values but I don't know how to set the eigen mode / associed displacement view in the Elmer Post.
tks,
Arend

Code: Select all

...
Solver 1
Equation = "Stress Analysis"
Eigen Analysis = Logical True
Eigen System Values = Integer 5
...

raback
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Re: Eigen Modes

Post by raback » 06 Aug 2010, 16:26

Hi Arend

Eigenmodes are treated as timesteps. So you should have five "timesteps" in your .ep file.

-Peter

arend
Posts: 4
Joined: 06 Aug 2010, 15:40

Re: Eigen Modes

Post by arend » 06 Aug 2010, 17:01

Hi Peter,
Yes, the .ep file shows five #time steps. The question is how to display it. The color mesh variables shows only the displacement x, y, z and abs.
tks

raback
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Animating timedependent and harmonic data

Post by raback » 06 Aug 2010, 17:37

Hi,

Here is a copy-paste from some older discussion:

ElmerPost assumes that the node coordinates are defined in the "nodes" matrix
such that nodes=[x y z]. Now if you want to have the displacement/deflection
included in the nodes you should alter it accordingly i.e. x=x0+dx etc. For
that we 1st save the original nodes:

math n0=nodes

a) The simplest case is if you have just one timestep/eigenvalue read:
In the math window of the main window

math nodes=n0+Displacement*scale

and press the phsychedelic button in the upper righthand corner. Here "scale"
is a scaling factor of your choice

b) for many steps

open "Timestep control" panel, write the following in
the "Do after Frame" field:

math nodes=n0+Displacement(0:2,time($t))

and press the "Loop" button.

c) for time-harmonic / eigenmode case
You need to create time-harmonic behavior on-the-fly in the main math window
using a small loop. For example over 2*pi:

do i 0 20 { math phi=2*pi*$i/20; math
nodes=n0+Displacement(0:2,time(0))*sin(phi)*0.02; display}

or if you have a complex function, like in this case a complex pressure,

do i 1 50 { math phi=2*pi*$i/50; math nodes=n+Displacement*sin(phi)*1.0e-5;
math pres=Pressure.1*cos(phi)-Pressure.2*sin(phi); display; savempg append }

In this case its a good idea to a priori set the scaling and fix it since
otherwise the colorscale will be chanching all the time. For sinusoidal
movement the interval sould be [-max(Displacement_abs),max(Displacement_abs)].

animations
=======
To make an animation using the "savempg" module on the right-hand-corner do
1) Start with the name and codec specified:
2) Put within the loop the command "savempg append" and loop over the
timesteps (like in the example)
3) Stop

Or on older way, if the modules are not available you could (in Unix/Linux)
take a number of screenshots
$ xwd -name "ELMER POST GRAPHICS" -out fig$i.xwd

The possibilities are endless ;-)

BR, Peter

arend
Posts: 4
Joined: 06 Aug 2010, 15:40

Re: Eigen Modes

Post by arend » 06 Aug 2010, 17:55

Peter,
To show a displacement in x,y and z is not the issue.
In my application I selected 5 main modes (Eigen System Values = Integer 5), how to display each one ?
So many tks,
Arend

arend
Posts: 4
Joined: 06 Aug 2010, 15:40

Re: Eigen Modes

Post by arend » 06 Aug 2010, 21:00

Peter,
I found an example ( ElasticPlateEigenmodes) in the new Elmer Tutorial .pdf document (my last tutorial doc. version did not have this one).
Now is clear the time step procedure.
Thanks and sorry the mess,
Arend

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