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How to model an undamped vibration

Posted Sep 5, 2011, 6:56 p.m. EDT Low-Frequency Electromagnetics, Acoustics & Vibrations, Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 4.2 2 Replies

Bruno Rango
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Hi,

I want to model the undamped vibration of a cable (using truss element and the time dependent solver) but in spite I don't consider any damping, after a few minutes the vibration results totally damped while it should continue vibrating at its natural frequency.

The cable is pinned at both ends. I'm using the BDF time stepping method and I chose a time step which is about 1/100 of the period of vibration corresponding to the first natural frequency. The cable is under the load of own weight only.

I attach a picture of the displacement graph at the middle of the cable.

I made the same model in ALGOR and I got the undamped vibration I'm looking for.

Anyone knows where is the problem?

Thank you,

Bruno.
2 Replies Last Post Sep 6, 2011, 10:40 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Sep 6, 2011, 2:32 a.m. EDT
Hi

I'm not sure, but check the doc, about the solver and the "numerical" damping that is sometimes used.
If you are looking in the "time domain" wit a transient solver, it could also be the time stepping, if you are in auto mode it could start to skip over the vibration and you would ahave a type of aliasing, use "intermediate" and manually set the tme steps to correctly resolve the highest frequency could take quite some time to solve though ;),
I prefer using the harmonic solvers for frequency analysis

--
Good luck
Ivar
Hi I'm not sure, but check the doc, about the solver and the "numerical" damping that is sometimes used. If you are looking in the "time domain" wit a transient solver, it could also be the time stepping, if you are in auto mode it could start to skip over the vibration and you would ahave a type of aliasing, use "intermediate" and manually set the tme steps to correctly resolve the highest frequency could take quite some time to solve though ;), I prefer using the harmonic solvers for frequency analysis -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Sep 6, 2011, 10:40 a.m. EDT
There is numerical damping in the BDF method that increases as with step size. However, if you keep the step size always less than 1/100 of the period of the frequency of interest the damping should not be as severe as your figure shows. Make sure that you limit the maximum time step the solver can take to be that value (in the Time Stepping tab of the Time-Dependent Solver node).

Nagi Elabbasi
Veryst Engineering
There is numerical damping in the BDF method that increases as with step size. However, if you keep the step size always less than 1/100 of the period of the frequency of interest the damping should not be as severe as your figure shows. Make sure that you limit the maximum time step the solver can take to be that value (in the Time Stepping tab of the Time-Dependent Solver node). Nagi Elabbasi Veryst Engineering

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