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Q factor
Posted 8 mag 2011, 15:07 GMT-4 Version 4.0a 2 Replies
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Hi,
I am using FSI to model an oscillating beam sitting in a fluid. I have my beam and fluid created, now I would like to make it oscillate and extract its Q-factor. Will I be able to do all of this in FSI? How so? Do I introduce a time-varying body/boundary load? What kind of study do I need to run?
Thank you,
Jim
I am using FSI to model an oscillating beam sitting in a fluid. I have my beam and fluid created, now I would like to make it oscillate and extract its Q-factor. Will I be able to do all of this in FSI? How so? Do I introduce a time-varying body/boundary load? What kind of study do I need to run?
Thank you,
Jim
2 Replies Last Post 18 mag 2011, 04:23 GMT-4
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Posted:
1 decade ago
Hi
how do you define your Q factor ? is it the damping of an eigenmode you are looking for ?
in which case I'm not sure how you could extract it via FSI as egenfrequency in a fluid is another story (I believe that is under acoustics, but unfortunately I'm missing that module ;).
You could also try to analyse the step response and observe the decay in transient and extract a Q factor from that
Q factor (in my understanding) are related to the damping: in FSI there is normally only fluid damping (you migh also add some structural damping), that is very dependent on the mode shape, beam shape and the velocity, and the velocity is related to the amplitude which is arbitrary for a structure (normalisation issue in the definition of a eigenfrequency analysis) so the Q factor is a function of other parameters that must be fixed, or defined
--
Good luck
Ivar
how do you define your Q factor ? is it the damping of an eigenmode you are looking for ?
in which case I'm not sure how you could extract it via FSI as egenfrequency in a fluid is another story (I believe that is under acoustics, but unfortunately I'm missing that module ;).
You could also try to analyse the step response and observe the decay in transient and extract a Q factor from that
Q factor (in my understanding) are related to the damping: in FSI there is normally only fluid damping (you migh also add some structural damping), that is very dependent on the mode shape, beam shape and the velocity, and the velocity is related to the amplitude which is arbitrary for a structure (normalisation issue in the definition of a eigenfrequency analysis) so the Q factor is a function of other parameters that must be fixed, or defined
--
Good luck
Ivar
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Posted:
1 decade ago
I define my Q factor as is defined in the mems gyroscope model pdf:
Q = W0 / 2delta = abs(im{lambda}/ 2*re{lambda})
"The relation between eigenvalue ? and a complex frequency f is
where ? is the angular frequency. This expression for the frequency can be inserted directly into the frequency response equations. The eigenvalue solver provides the damped eigenvalue in one step when the system of equations is a quadratic polynomial in the angular frequency, otherwise a few steps of fix-point iteration is usually enough to reach convergence."
Can I do all this in FSI only? I tried to get a couple eigenfrequencies, to no avail.
Any advice on how to proceed from here is appreciated.
--
Thanks!
Jim
Q = W0 / 2delta = abs(im{lambda}/ 2*re{lambda})
"The relation between eigenvalue ? and a complex frequency f is
where ? is the angular frequency. This expression for the frequency can be inserted directly into the frequency response equations. The eigenvalue solver provides the damped eigenvalue in one step when the system of equations is a quadratic polynomial in the angular frequency, otherwise a few steps of fix-point iteration is usually enough to reach convergence."
Can I do all this in FSI only? I tried to get a couple eigenfrequencies, to no avail.
Any advice on how to proceed from here is appreciated.
--
Thanks!
Jim