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Prescribed Displacement in Frequency Domain Study
Posted 17 lug 2020, 00:26 GMT-4 Acoustics & Vibrations Version 5.5 7 Replies
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Dear All,
I hope you are doing well and safe.
I have a beam fixed from one end and experiencing a harmonic displacement excitation as: y (t) = A sin (wt) [Figure 1 is attached]. As peer the software manual, what I have understood is that; in the frequency domain study, all I need to put as an input to the software, is the amplitude only. For example, if I want to run harmonic Force/Velocity/Displacement/Acceleration, I just need to give the software the amplitude part of the equation only.
However, when I try to run the simulation under Prescribed Displacement given an amplitude of[ - 3 cm], the results are not reasonable at all: 1. the results at all frequencies are constant, 2. I should see a peak at the resonant frequency, but there is no peak which means there is something wrong [Figure 2 is attached]. On the other hand, when I put a force excitation given the amplitude only, the results are reasonable and correct.
It is perfectly working with force excitation, but not with any of prescribed Velocity/Displacement/Acceleration.
May I know where I went wrong? And why the results are not correct as am treating the prescribed displacement excitation as force excitation.
Many Thanks, Islam
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Islam,
a prescribed displacement BC rigidly enforces this displacement regardless of the frequency.
Cheers Edgar
-------------------Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
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But, I want to apply a displacement with amplitude A and frequency is variable.
Exactly the same as force BC in frequency domain, I was able to examine a range of frequency. How to do the same, but instead of applying Force, I want to apply displacement, velocity or acceleration.
May you please help, I've been stuck in this step for days. Many thanks.
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Maybe you want to tell the forum what the objective of your work is. Applying rigid constraints to the model won't likely get you anywhere.
-------------------Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
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I want to generate the Transmissibility Curve of my beam system: X/Y vs f .. so I must apply base excitation on my system (i.e., forced vibrations). Thus, how to do so?
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Prescribed displacement means...prescribed displacement. So of course the displacement will be independent for frequency. AND the velocity and acceleration are both directly related to the frequency in sinusoidal steady state (jomegadiplacement for velocity, -omegaomegadisplacement for acceleration) so there is no new information to be obtained.
If you want a transfer function you want a ratio between a forced quantity and a response. That could be displacement divided by force. Or vice versa. Or force divided by velocity...
Probably what you want is to apply a force and calculate the resulting displacement as a function of frequency.
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what do you explain: when I added prescribed velocity, at all frequency I get zero displacement? the system doesn't move at all in the frequency domain.
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If the comments above are not sufficient to point you in the right direction do the following:
Post your .mph file after removing solutions (https://www.comsol.com/forum/thread/265451/prescribed-displacement-in-frequency-domain-study?last=2020-07-18T01:29:14Z#reduceFileSize)
State exactly what transfer function you want to calculate.