[SOLVED] Solving FEM system

[Catherine]

Hi,

I am trying to find the new configuration of vertices of a mesh after a deformation using FEM. I have an .obj file containing the mesh and know the external forces applied to each node as well as the Young modulus and Poisson ratio. Is it possible to use SOFA to find the new vertex positions and, if so, how do I go about calculating them?

Thank you,
Catherine

[Hugo]

Dear Catherine,

Welcome on the SOFA forum!
Yes it is possible. Physics simulation is what SOFA is made for.
Do you have already a SOFA scene ?

Hugo

[Catherine]

I have got a scene displaying my mesh but I am unsure of how to proceed from there.

Catherine

[Hugo]

Dear Catherine,

Please have a look at :

• the introduction paper
• the documentation (Main principles are described)
• and the tutorial scenes that you can start with (in SOFA, examples/Tutorials/ folder.

Unfortunately SOFA is not yet super newcomer-friendly but this is our 2018 mission. Let me know if you would be interest in a specific training session for SOFA.

Best regards,

[Catherine]

Thank you, I will have a look at those documents.

Catherine

[Catherine]

Hi,

I read the documents and have done several of the tutorials. Here is the .xml file that I have used for my scene so far: https://www.dropbox.com/s/xrogzmvkfebjtn0/Tutorial3.scn?dl=0&m= However, rather than deforming due to gravity or interaction from the user, I have a vector that contains the external force acting on each node in each frame and would like these values to be used by the Euler/CG solvers. Is there a way to feed in these vectors at each frame and visualise the corresponding deformation?

Thanks,
Catherine

[jnbrunet]

Hi Catherine,

You can look at the LinearForceField component. If “Volume” is the name of your MechanicalObject, you could write:

<LinearForceField points="@Volume.indices" forces="0 -1 0 0 -5 0 0 -10 0" times="0 10 20" />

This will set the forces to (0,-1,0) at time 0, (0, -5, 0) at time 10 and (0, -10, 0) at time 20. Between those times, the force will be linearly interpolated.

If you want to apply the forces on a subset of your domain, you can specify the indices of your dofs in the points attribute of the LinearForceField. For exemple,

<BoxROI name="f_indices" box="0 0 0 10 10 10"/>
<LinearForceField points="@f_indices.indices" forces="0 -1 0   0 -5 0  0 -10 0" times="0 10 20" />

will set the forces contained in the box (xmin, ymin, zmin, xmax, ymax, zmax) = (0,0,0,10,10,10)

If you need more than this, then I would suggest you to look at python scenes.

Let me know if you need more details.
Jean-Nicolas

[Catherine]

Hi Jean-Nicolas,

Thank you for your response! However, I am looking to apply a different force vector to each node in the mesh rather than the same force everywhere. How do Ido this?

Thanks,
Catherine

[jnbrunet]

Hi Catherine,

Unfortunately this is not possible with the LinearForceField component. I’m not sure if there is another component that would do that. If you are comfortable enough with c++, you could try to extend the LinearForceField class to add this feature.

Otherwise, I would suggest to look at python scenes, you can have a look at the various examples in the SofaPython plugin directory (sofa/applications/plugins/SofaPython/examples). You could, for instance, use a ConstantForceField component and change the values inside its vector of forces at each step with your python script.

Let me know if you still struggle with this.
Jean-Nicolas

[Author]

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