BeamSolver3D: wave propagation

Numerical methods and mathematical models of Elmer
palyvar
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BeamSolver3D: wave propagation

Post by palyvar »

Hi,
I am a newbie in FEM and I am attempting to simulate elastic wave propagation using transient simulation and BeamSolver3D. However, the waves do not travel. I was able to make it work using StressSolve and with a full 3d model of the beam but I am wondering if it is possible using 1D beams. I am attaching a GIF of the simulation result (U), the SIF file and an archive of the whole setup (sif + mesh).
Attachments
anim2.gif
anim2.gif (569.09 KiB) Viewed 90 times
circle_waves.tgz
(7.33 KiB) Downloaded 2 times
case.sif
(1.81 KiB) Downloaded 3 times
Last edited by palyvar on 27 Jan 2023, 00:38, edited 3 times in total.
kevinarden
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Re: BeamSolver3D: wave propagation

Post by kevinarden »

I can only conclude that the beam solver does not include the dynamic forces of acceleration and velocity. The motion on the far side of the circle is moving but only due the the rings stiffness as opposed to the solid model which is excited by dynamic forces. I plot the displacement of the far side node.
beam model
beam.png
(76.54 KiB) Not downloaded yet
solid model
elementp2.png
(70.32 KiB) Not downloaded yet
I test this by leaving the load on only for the first 5 steps, then turn it off, the problem should still continue to oscillate, the beam model does not
but the solid model does.
solidring.png
(72.09 KiB) Not downloaded yet
palyvar
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Re: BeamSolver3D: wave propagation

Post by palyvar »

Thank you very much for your response! Then, I'll simply continue only with the solid model.

I was confused because the model's documentation says something about effects of inertial forces in transient simulation, but the effects are probably different from my expectations (the Density parameter has observably some impact).
kevinarden
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Re: BeamSolver3D: wave propagation

Post by kevinarden »

I think by inertial in the beam solver they may mean gravity turned into a static body force, not acceleration.
raback
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Re: [Solved] BeamSolver3D: wave propagation

Post by raback »

Hi

I looked at the code and at least the M(d^2u/dt^2) terms are there for transient simulation.

I looked at the sif file and one thing that caught my eye was the sloppy linear system tolerances. If your timestep is very short with sloppy tolerances it could be that the previous solution fulfills the equation and nothing is done. I would recommend using "direct" instead of "iterative" for 1D meshes as they result to very narrow bandwidth.

-Peter
kevinarden
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Re: [Solved] BeamSolver3D: wave propagation

Post by kevinarden »

I used direct, mumps, on my studies. And P:2 elements on the solid models.
kevinarden
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Re: [Solved] BeamSolver3D: wave propagation

Post by kevinarden »

I have tested an tried everything I can think of, but the solid model and the bean model yield very different results. The solid model appears to be a more expected result.
solid.zip
(13.24 KiB) Downloaded 3 times
beam.zip
(6.21 KiB) Downloaded 3 times
kevinarden
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Re: [Solved] BeamSolver3D: wave propagation

Post by kevinarden »

I take test case
https://github.com/ElmerCSC/elmerfem/tr ... Cantilever

and change from steady state to transient and the solution fails immediately

ERROR:: ComputeChange: Numerical Error: Norm of solution appears to be NaN
raback
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Re: BeamSolver3D: wave propagation

Post by raback »

Hi

How about if you set for the BeamSolver section:

Code: Select all

Use Global Mass Matrix = True
-Peter
kevinarden
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Re: BeamSolver3D: wave propagation

Post by kevinarden »

Yes that fixes the problem, solutions look good now.
Thanks.
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