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sine excitation
Posted 05.06.2013, 07:19 GMT-4 Structural Mechanics Version 4.0 4 Replies
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I am performing a 2D frequency analysis in the Structural Mechanics Module.
I want to apply a prescribed displacement on the left boundary and to see which waves propagate inside the medium. The prescribed displacement is a sine wave of 5 cycles with a certain frequency.
Could anyone tell me how to apply this boundary condition?
If I perform a time-dependent analysis, I can define a piecewise function using t (time) as argument, but this doesn't work in the frequency analysis.
I want to perform a frequency analysis mainly for two reasons:
1. The medium is a thin layer and the time-dependent analysis doesn't give the expected solution. The resulting waves are undersampled even if I use the proper time-step and maximum element size.
2. I would like to avoid reflections from the boundaries and PML doesn't work in the time-domain.
Any suggestion is more than welcome!!!
Best regards
Ramona
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first, I think you should apply your sine wave as force as follow: sin(2*Pi*f[1/s]*t), unless you want to modulate your excited wave with Hanning window then you will need to multiply with Hanning function.
second, transient analysis(time-dependent) is more suitable than frequency analysis your case especially for thin layer elements where the expected wave is type of guided waves like Lamb wave.
Good Luck
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Note that in frequency domain, the time dependence is implicit. This means that if you write a load as 17[N], it actually means 17[N]*cos(2*pi*f).
In frequency domain, the assumption is however that it is a steady state situation, which contradicts your statement about "5 cycles".
In a time domain analysis the 'Low-Reflectiing Boundary' boundary condition can be used to suppress reflections of outgoing waves. It was introduced in version 4.3
Regards,
Henrik
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thanks for your suggestions.
I apply my sine wave as a fixed displacement as follow: A[m]*sin(2*Pi*f[1/s]*t). I want to simulate a rigid body vibrating in touch with the thin layer.
If I apply only one pulse (see the attached figure called one_pulse) I can see this pulse propagating inside the thin layer (I have ploted the y-displacement in function of time for different distances from the source). If I apply a burst of 5 cycles (I have changed only the number of pulses) I can't see these 5 cycles propagating inside the medium. It looks like a bad sampling (see the attached figure burst_5cycles).
For both simulations I have used a time step of 1/(20*freq) and a maximum element size of wavelength/20.
Many thanks.
Ramona
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thanks for the suggestions about the frequency domain.
I am mainly interested in the time-dependent analysis.
I wanted to perform the frequency analysis only in order to use the PML. I am using the Structural Mechanics Module of Comsol 4.0, can you suggest a technique to apply non-reflecting boundary conditions in the time-dependent analysis?
I would like to understand which wave modes propoagate inside the medium. I was thinking to perform an eigenfrequency analysis but in this case the bodt is free and without load. I would like to apply a source of displacement and constraints on the edges.
Many thanks.
Ramona