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Is there any difference between resonance and flutter. Both of them are related to overlap of external excitation frequency and Natural frequency then increase in amplitude even inifite amplitude theorytically.

What is the difference between them?

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In flutter, the force applied to the structure depends on the deformation and velocity of the structure. Therefore, the force is necessarily at exactly the same frequency as the motion, though it may not be at the correct phase to cause the motion to increase in amplitude.

Often, flutter is caused by the structure having two different vibration modes with similar (but not identical) frequencies, and the flutter occurs at a frequency which is not equal to either of the mode frequencies.

In resonance, in frequency of the force is not necessarily the same as the natural frequency of the structure, and the amplitude of the response depends how close the two frequencies are to each other (and also on the amount of damping the system, of course).

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  • $\begingroup$ It should be noted that the actual vibration frequency is always identical to the excitation frequency in both flutter and normal resonance. Flutter is a fluid dynamics problem. $\endgroup$
    – Phil Sweet
    Apr 3, 2019 at 22:09
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I think flutter is when self excitation happens and there may not be any external vibration. Flutter mostly happens in aviation on airfoils such as wing-tips, ailerons , empennage, and because of self excitation it can grow in intensity to dangerous catastrophic levels even in smooth air.

Airplane manufacturers design their planes in the wind tunnel to eliminate flutter under design operational envelop, even at failure of some sections.

Resonance is when the external excitation frequency is close to that of the natural frequency of the object and causes the vibration amplitude and energy to increase and even become very high if excitation frequency matches that of natural frequency.

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Ok, you have a system of some kind. Normally, (not resonance, and not flutter), you apply a force, you get a displacement. e.g. for sake of argument let's just say application of 10 N force gets 0.1 m displacement.

Now, at resonance, you can apply a small force and get a large displacement. e.g. 10 N force gets 10 m displacment. That's 100X the normal relationship, a big amplification.

Now, at flutter, what happens is that the displacement itself causes the size of the force to change. e.g. there's an intial 0.01m displacement. that displacement, in and of itself, causes a force of 0.1N. The force then causes a larger displacement, 0.1m, which then causes a larger force 1N, when then causes a larger displacement 1m, etc, etc, etc. Both the force and displacement grow exponentially until something happens to stop it.

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In the flutter, actually, we have a new system with different dynamic behavior, namely the frequency, damping, and also the mode shapes are changed. In the flutter, the source of vibration is a disturbance and the system fulls on a self-excitation.
For example in an aeroelastic flutter, the frequency and damping of the wing change as the airplanes speed changes and are different from its structural natural frequencies on the ground.
But in the resonance, the system is not changing and the external effect (force) causes an osmolarity motion or deformation. the source of vibration, in this case, is a harmonic external force with one of the natural frequencies of the system.

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