making sense of flow induced vibration results

[Hoomanya]

I have a set of amplitude plots of flow induced vibrations and was wondering if anyone could help me understand a few points. I know that attenuation of amplitudes indicate numerical damping, and increase in the amplitude of vibrations is a sign of instability. But what if:

1) before any of this happens, the code stops and crashes, i.e. going from stable waves with similar amplitudes to the code not converging and crashing? Does anyone know what that indicates?

2) the amplitude peaks increase at some time steps and reduce at other time steps?

3) what do non-smooth waves (in particular at the peaks) indicate? Thanks you in advance!

 

[drawoh]

Hoomanya,

Are your plots coming from measurements of a machine, or are they outputs from FEA software?

--
JHG

 

[IRstuff]

Code nonconvergence is typically a software problem, although that might be associated with a simulation running in a region of instability. Even then, that's more of an indication that the software is ill-equipped to run that particular kind of simulation

TTFN
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[Hoomanya]

thank you. this is result of a numerical code I've written. Any idea about the non-smooth peaks?

 

[drawoh]

Hoomanya,

I have written code modelling vibrations. I have observed non-converging results generally because I coded something wrong. As I recall, I input all the forces from my free body diagram, including force of inertia. When I removed the force of inertia, it all worked. Your mass accelerates due to damping and spring force.

--
JHG

 

[IRstuff]

Typical simulation codes involve state matrix propagations and/or inversions, which can lead to singular matrices.

TTFN
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