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Amplitude of driven vibrations
Posted 4 lug 2011, 13:53 GMT-4 Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 4.1 2 Replies
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Imagine a hollow wineglass shape that has a fixed boundary condition at the stem of the wineglass and free everywhere else. This has various modes (the rim vibrates in various ways), e.g. outward at North and South and inward at East and West rim (by "outward" and "inward" at the rim I mean, for a snapshot in time). Let's call that one Mode 1.
Another mode which happens to be orthogonal would be outward at Northeast and Southwest and inward at Northwest and Southeast. Let's call that one Mode 2.
Another mode with a lower modal number is where North goes outward and South goes inward. Call that Mode 3.
I have to apply a sinusoidal force to the Northern-most point on the wineglass (e.g. F = (1 Newton)*Sin(omega*t) ), and see what displacement amplitude each mode gets excited to, due to that force. For instance, Mode 1 and 3 above would be excited by that driving force while Mode 2 would not.
So the output I seek is, say, that Mode 3 gets excited to an amplitude, say, 1.2 times larger than Mode 1 when the same (1 Newton)*Sin(omega*t) is applied, and that Mode 2 is not excited (amplitude is virtually zero because you are driving at a node.
How do I go about this? Thanks
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I figured out how to do this, it's to use the New > 3D > solid > Custom Studies > Frequency domain, and then to add a force boundary condition, and put a constant number into the box for, say, Force in x. This will give a sinusoidal excitation multiplied by that constant which is the amplitude of vibrations.
However I am still unconvinced that the actual deformation you get is what you'd physically expect, or if the deformations are only *relative* from mode to mode.
Meaning, I don't know if this approach takes into account physically-realistic damping at the anchored points, thereby giving the actual deformation amplitude and allowing me to trust the absolute deformation numbers, or if I can only trust the deformation numbers relative to each other -- i.e. trust one mode's deformation relative to another.
Can I trust these deformation numbers as correct, absolute numbers? Thanks
David
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Nagi Elabbasi
Veryst Engineering