Professor George Adams - Self Excited Vibrations (Dynamic Instability)

This demonstration illustrates how a fluid-structure interaction can lead to a self-excited oscillation (dynamic instability).

[@aamirnezami4091]

Thanks Sir. This was a very nice explanation.

[@LQDEN]

I'm a little confused, how is this different from forced vibration? In your example, the system is experiencing an external force caused by the fan, shouldn't this be considered forced vibration?

[@georgeadams912]

Thanks for your question. The applied force, in this case from the fan, is constant. Normally a constant force does not create a vibration. In this case the aerodynamics is such that any very small motion (due to ambient vibration) gets amplified. The oscillation is then at the natural frequency of the system and is independent of the fan.

[@LQDEN]

@@georgeadams912 Thank you! So for example, if a worker is hammering on a pipe for example, it would be considered forced vibration because the hammering/force is periodic?
But the same pipe can then experience a self excited vibration due to the flow of water in it? (since the flow/motion of water is constant).

[@georgeadams912]

@@LQDEN Yes and yes.

[@jaeennur9858]

Sir, I have a question. Does it oscillate at its natural frequency or the frequency changes with time?

[@georgeadams912]

It oscillates at its natural frequency. The equation of motion can be found from the damped vibration of a 1 DOF system by using a negative value for the damping constant. In the linear regime that gives vibrations with an exponentially increasing amplitude.

[@jaeennur9858]

Thank you so much