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Frequency Response in FSI
Posted May 24, 2011, 8:42 p.m. EDT Acoustics & Vibrations Version 4.0a 6 Replies
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Hello,
I would like to obtain the frequency response of a damped cantilever. I can do this in the solid mechanics interface simply by putting an arbitrary, stationary load on the free end of the cantilever, doing a frequency domain study, and plotting "solid.uAmpZ" (oscillations in z direction only) for a point on the free end. When I attempt to do the exact same thing except only using FSI (plotting "fsi.uAmpZ"), I get zero amplitude all around! Is there a reason for this, or have I just made a mistake? Any help is appreciated.
Dara
I would like to obtain the frequency response of a damped cantilever. I can do this in the solid mechanics interface simply by putting an arbitrary, stationary load on the free end of the cantilever, doing a frequency domain study, and plotting "solid.uAmpZ" (oscillations in z direction only) for a point on the free end. When I attempt to do the exact same thing except only using FSI (plotting "fsi.uAmpZ"), I get zero amplitude all around! Is there a reason for this, or have I just made a mistake? Any help is appreciated.
Dara
6 Replies Last Post Jul 11, 2011, 11:17 a.m. EDT
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Posted:
1 decade ago
Hi
i believe you are facing the issue that the harmonic development of FSI physics is not present. I have also this issue for some of my simulations. I suspect that a response could be found in the acoustic module, but I do not have that one ;)
One workaround is to remain in structural SOLID and add the fluid added mass a s a surface load, as well as the added viscosity as a damping load related to he velocity ut,vt,wt
--
Good luck
Ivar
i believe you are facing the issue that the harmonic development of FSI physics is not present. I have also this issue for some of my simulations. I suspect that a response could be found in the acoustic module, but I do not have that one ;)
One workaround is to remain in structural SOLID and add the fluid added mass a s a surface load, as well as the added viscosity as a damping load related to he velocity ut,vt,wt
--
Good luck
Ivar
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Posted:
1 decade ago
Hi Ivar,
Do the newer versions of COMSOL have this functionality that I am looking for? Does FSI in the newer versions support frequency domain studies? From reading other posts on this forum, I got the idea that it does, but as the release notes do not make any mention of it I am not sure.
Thanks for all the help,
Dara
Do the newer versions of COMSOL have this functionality that I am looking for? Does FSI in the newer versions support frequency domain studies? From reading other posts on this forum, I got the idea that it does, but as the release notes do not make any mention of it I am not sure.
Thanks for all the help,
Dara
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Posted:
1 decade ago
Hi
my 4.2 is so recent that I haven't had time to test it correctly yet,
But I see for FSI no eigenfrequency nor frequency domain solver choices, only stationary and time dependent solvers by default.
As I said, I believe structure fluid damping is to be found in the acoustic module, but I haven't managed to justify the purchase of that module yet
--
Good luck
Ivar
my 4.2 is so recent that I haven't had time to test it correctly yet,
But I see for FSI no eigenfrequency nor frequency domain solver choices, only stationary and time dependent solvers by default.
As I said, I believe structure fluid damping is to be found in the acoustic module, but I haven't managed to justify the purchase of that module yet
--
Good luck
Ivar
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Posted:
1 decade ago
Hi Ivar,
Acoustic-Structure Interaction physics gave correct-looking results at first, but after playing with some geometric parameters, the frequency response changed in unexpected ways. For example, the "quality factor" of my cantilever got better in the presence of more damping. This leads to me to believe that the model does not consider fluid damping? I wish I could simply put a body load (fluid pressure) on the structure, but as I am modelling the cantilever in a closed environment, the fluid that is displaced by the vibrations exerts a pressure on the top of my structure which can only be simulated using FSI or the like. Any further suggestions would be greatly appreciated.
Dara
Acoustic-Structure Interaction physics gave correct-looking results at first, but after playing with some geometric parameters, the frequency response changed in unexpected ways. For example, the "quality factor" of my cantilever got better in the presence of more damping. This leads to me to believe that the model does not consider fluid damping? I wish I could simply put a body load (fluid pressure) on the structure, but as I am modelling the cantilever in a closed environment, the fluid that is displaced by the vibrations exerts a pressure on the top of my structure which can only be simulated using FSI or the like. Any further suggestions would be greatly appreciated.
Dara
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Posted:
1 decade ago
Hi
I would say that depends, is your fluid in motion ? (apart from what the structure is pushing"). If it's only a structure vibrating in an "infinitely large" fluid volume then normally you can approximate the fluid presence by a "added mass" and "added viscosity" as structural load cases.
i.e check the book of "Axisa" (try a search on the forum)
In V4.2 COMSOL has added new BC to make this easier to define (but I haven't had time to test them out yet, there are too many new goodies therein, it will take weeks to get around them all ;)
--
Good luck
Ivar
I would say that depends, is your fluid in motion ? (apart from what the structure is pushing"). If it's only a structure vibrating in an "infinitely large" fluid volume then normally you can approximate the fluid presence by a "added mass" and "added viscosity" as structural load cases.
i.e check the book of "Axisa" (try a search on the forum)
In V4.2 COMSOL has added new BC to make this easier to define (but I haven't had time to test them out yet, there are too many new goodies therein, it will take weeks to get around them all ;)
--
Good luck
Ivar
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Posted:
1 decade ago
Hi Dara and Ivar,
I know this thread has been idle for a while but I was wondering if you ever found the solution to your problem. I am having an incredibly difficult time getting my FSI model to converge of a similar situation, and I suspect it's because of the high Reynold's number of the flow in the area of hte oscillating cantilever (and this only gets worse as you increase the frequency of oscillation). I've considered the acoustics module as well and found that the damping terms are insufficient for this purpose. The paper "FSI Analysis of Microcantilevers Vibrating in Fluid Environment" seems to be a good start, but only addresses eigenfrequency analysis (can be found in the Community > Papers & Presentation section of the website)
My model has the cantilever oscillating in proximity to a hard surface (an AFM cantilever approaching contact with a substrate) and this leads me to believe that squeeze-film damping is also a large contributor to the transient characteristics of the cantilever. Did you need to consider this in your model as well?
I know this thread has been idle for a while but I was wondering if you ever found the solution to your problem. I am having an incredibly difficult time getting my FSI model to converge of a similar situation, and I suspect it's because of the high Reynold's number of the flow in the area of hte oscillating cantilever (and this only gets worse as you increase the frequency of oscillation). I've considered the acoustics module as well and found that the damping terms are insufficient for this purpose. The paper "FSI Analysis of Microcantilevers Vibrating in Fluid Environment" seems to be a good start, but only addresses eigenfrequency analysis (can be found in the Community > Papers & Presentation section of the website)
My model has the cantilever oscillating in proximity to a hard surface (an AFM cantilever approaching contact with a substrate) and this leads me to believe that squeeze-film damping is also a large contributor to the transient characteristics of the cantilever. Did you need to consider this in your model as well?