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Finding displacement for each eigen/resonant frequencies modes
Posted Jul 3, 2017, 10:44 a.m. EDT Structural Mechanics Version 5.2a 2 Replies
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Hi,
I have been modeling a square shaped diaphragm structure and want to calculate the displacements corresponding to each resonance frequencies. I have a 100um*100 um structure with atmospheric pressure/load of 100kPa acting from top and intrinsic stress is 0.01Mpa. I have given a damping of isotropic loss factor with Q as 10 (just assumption) and applied a harmonic perturbation boundary load/point load at center with 25mN from bottom of diaphragm. Since there is already a constant load of 100kPa applied it requires a prestressed frequency analysis. I found my resonant frequencies with study prestressed eigen frequency. Now corresponding to those eigen frequencies I want to calculate displacements so I did prestressed frequency domain analysis.
When I plotted the graph between displacement and frequency, it shows me resonance peaks for 1 st and 5 th harmonics. I don`t know why it does not show for the rest frequencies. I tried zooming in but it just gives a constant slope and no peaks. I don`t know how I will calculate the displacement then.
Is there any way around to obtain displacements corresponding to each eigen frequencies??
Also, the legends shows some normalized value and not the exact displacement, in one of the examples for Bracket analysis, it says " Note that the displacement values are normalized and have no physical significance. The normalization method can be changed in the Eigenvalue Solver node, available under the Solver Configuration node". I couldn`t find it. Help me locate this.
Please help.
I have been modeling a square shaped diaphragm structure and want to calculate the displacements corresponding to each resonance frequencies. I have a 100um*100 um structure with atmospheric pressure/load of 100kPa acting from top and intrinsic stress is 0.01Mpa. I have given a damping of isotropic loss factor with Q as 10 (just assumption) and applied a harmonic perturbation boundary load/point load at center with 25mN from bottom of diaphragm. Since there is already a constant load of 100kPa applied it requires a prestressed frequency analysis. I found my resonant frequencies with study prestressed eigen frequency. Now corresponding to those eigen frequencies I want to calculate displacements so I did prestressed frequency domain analysis.
When I plotted the graph between displacement and frequency, it shows me resonance peaks for 1 st and 5 th harmonics. I don`t know why it does not show for the rest frequencies. I tried zooming in but it just gives a constant slope and no peaks. I don`t know how I will calculate the displacement then.
Is there any way around to obtain displacements corresponding to each eigen frequencies??
Also, the legends shows some normalized value and not the exact displacement, in one of the examples for Bracket analysis, it says " Note that the displacement values are normalized and have no physical significance. The normalization method can be changed in the Eigenvalue Solver node, available under the Solver Configuration node". I couldn`t find it. Help me locate this.
Please help.
2 Replies Last Post Jul 3, 2017, 11:36 a.m. EDT
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Posted:
7 years ago
My guess is that you are applying the point load at a location where that particular eigenmode has zero displacement.
> The normalization method can be changed in the Eigenvalue Solver node, available under the Solver Configuration node"
In 5.3 look under Solver Configurations > Solution 1 > Eigenvalue Solver 1
(if #1 is the solution number)
D.W. Greve
DWGreve Consulting
> The normalization method can be changed in the Eigenvalue Solver node, available under the Solver Configuration node"
In 5.3 look under Solver Configurations > Solution 1 > Eigenvalue Solver 1
(if #1 is the solution number)
D.W. Greve
DWGreve Consulting
Please login with a confirmed email address before reporting spam
Posted:
7 years ago
Thanks Greve for the response.
I even tried applied harmonic perturbation /load even all around the dimensions of square diaphragm (to make sure all modes are actuated) but nothing seems to work, it still shows the same result. The 2nd, 3rd and 4th harmonics are still not visible with resonance peaks. However, if I apply harmonic load to any corners to actuate a particular frequency mode it does give a 2nd resonant peak but I think that calculation will be erroneous as the harmonic load is not uniformly distributed to actuate the whole structure at once.
That makes my displacement calculation difficult.
And for the eigen value solver node, I have located it but not sure "what to" change in the settings tab to avoid normalization of displacement? Also am using 5.2a version.
Please guide.
I even tried applied harmonic perturbation /load even all around the dimensions of square diaphragm (to make sure all modes are actuated) but nothing seems to work, it still shows the same result. The 2nd, 3rd and 4th harmonics are still not visible with resonance peaks. However, if I apply harmonic load to any corners to actuate a particular frequency mode it does give a 2nd resonant peak but I think that calculation will be erroneous as the harmonic load is not uniformly distributed to actuate the whole structure at once.
That makes my displacement calculation difficult.
And for the eigen value solver node, I have located it but not sure "what to" change in the settings tab to avoid normalization of displacement? Also am using 5.2a version.
Please guide.