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longitudinal mode of bulk

Posted 6 mars 2013, 19:48 UTC−5 Structural Mechanics Version 4.3a 3 Replies

Yuhao Liu
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Hi All,

I am quit new to structure mechanics. I am trying to find the longitudinal eigenmode of a bulk. I started with 3D simulation. I could see the fundamental mode(X direction strain, deformation gradient xX ). but for higher modes there are a lot of junk modes(flexural mode and other mode mixed) How could i eliminate them and only see the longitudinal modes?

I am also trying to apply harmonic force at one boundary to excite the longitudinal modes to see the resonance. I used frequency domain and boundary load for the simulation. how could i see the longitudinal resonance behavior by sweeping the frequency?

I tried the simulation also in 2D. There are still shear waves.

The boundary condition is free-free or fixed-fixed. I think this wouldnt change the resonance behavior a lot because the reflection coefficient for longitudinal wave is -1 or 1. Only phase changes are different.


Thank you!
3 Replies Last Post 2 avr. 2014, 18:49 UTC−4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 7 mars 2013, 00:55 UTC−5
Hi

you will always get "all types of modes", Except if you block then with some BC, i.e. you apply 1 or two symmetry planes for a cantilever and fix one end then you will probably get only the longitudinal mode as the first one. But by doing so you change the mode shapes too so it might not give you totally what you want.

my way is to turn on the participation mass normalisation and plot out the square of the participation masses and then pick the first mode with highest mass along the desired direction. Then run a frequency domain scan just around that frequency (+ adding some damping to avoid a singularity at the resonance

Clear if you do not fix your object of all, you start with the 6 first eigenmodes of free free, and you change the mode and frequency behaviour compared to fixed-free or fixed-fixed

--
Good luck
Ivar
Hi you will always get "all types of modes", Except if you block then with some BC, i.e. you apply 1 or two symmetry planes for a cantilever and fix one end then you will probably get only the longitudinal mode as the first one. But by doing so you change the mode shapes too so it might not give you totally what you want. my way is to turn on the participation mass normalisation and plot out the square of the participation masses and then pick the first mode with highest mass along the desired direction. Then run a frequency domain scan just around that frequency (+ adding some damping to avoid a singularity at the resonance Clear if you do not fix your object of all, you start with the 6 first eigenmodes of free free, and you change the mode and frequency behaviour compared to fixed-free or fixed-fixed -- Good luck Ivar
Yuhao Liu
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Posted: 1 decade ago 7 mars 2013, 02:13 UTC−5
Thank You for your numerous help! I just solved the problem... I was too lazy to see all the frequency response.. The eigenmodes that I want were just in the solution set.




Hi

you will always get "all types of modes", Except if you block then with some BC, i.e. you apply 1 or two symmetry planes for a cantilever and fix one end then you will probably get only the longitudinal mode as the first one. But by doing so you change the mode shapes too so it might not give you totally what you want.

my way is to turn on the participation mass normalisation and plot out the square of the participation masses and then pick the first mode with highest mass along the desired direction. Then run a frequency domain scan just around that frequency (+ adding some damping to avoid a singularity at the resonance

Clear if you do not fix your object of all, you start with the 6 first eigenmodes of free free, and you change the mode and frequency behaviour compared to fixed-free or fixed-fixed

--
Good luck
Ivar



Thank You for your numerous help! I just solved the problem... I was too lazy to see all the frequency response.. The eigenmodes that I want were just in the solution set. [QUOTE] Hi you will always get "all types of modes", Except if you block then with some BC, i.e. you apply 1 or two symmetry planes for a cantilever and fix one end then you will probably get only the longitudinal mode as the first one. But by doing so you change the mode shapes too so it might not give you totally what you want. my way is to turn on the participation mass normalisation and plot out the square of the participation masses and then pick the first mode with highest mass along the desired direction. Then run a frequency domain scan just around that frequency (+ adding some damping to avoid a singularity at the resonance Clear if you do not fix your object of all, you start with the 6 first eigenmodes of free free, and you change the mode and frequency behaviour compared to fixed-free or fixed-fixed -- Good luck Ivar [/QUOTE]
Ruiyuan Ma
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Posted: 1 decade ago 2 avr. 2014, 18:49 UTC−4
Hi,

I am studying simulation of longitudinal/shear elastic wave in nano-material right now.
Could you please give me some suggestion about how to get dispersion curve of Longitudinal, transverse elastic wave in 3-d nano-material? eg: cubic or cylinder shape.

Thanks for your time.
Ruiyuan Ma
Hi, I am studying simulation of longitudinal/shear elastic wave in nano-material right now. Could you please give me some suggestion about how to get dispersion curve of Longitudinal, transverse elastic wave in 3-d nano-material? eg: cubic or cylinder shape. Thanks for your time. Ruiyuan Ma

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