Acoustic-Structural Coupling in Abaqus

[CG1980]

Hello,
i'm trying to reproduce the examples reported in the following article "Eigencharacteristics of fluid filled tanks:..." (

http://task.gda.pl/files/quart/TQ2006/04/TQ410S-E.PDF),

but i'm having some troubles while defining acoustic-structural coupling interaction (my input file is attached to this post). I guess there's something missing since the frequencies don't change between the empty tank case and the water filled tank case, while they should increase their values.
Any help would be really appreciated, thanks!

 

[Mustaine3]

Activate NLGEOM in the static step to enforce that model state changes and the frequency step uses the prestressed model.
And I've also seen in the paper, that at the end of page 7 the youngs modulus of steel misses a zero.

But I'm not sure if that is the reason for your problem.

What about gravity?

 

[CG1980]

Thanks for the hints.
Yes, both the steel Young Modulus and water bulk modulus misses a zero, but they should be ok in my input file.
I'll try with the NLGEOM activated, but my feeling is that there's something missing in the acoustic-structural coupling definition: in fact if i change the fluid properties the frequencies stay the same, while frequencies change when i change the material properties of the tank. Is the *TIE constraint not enough in this case?
About gravity, it looks like the paper doesn't take it into account (pag.8): anyway, gravity can't be applied to acoustic elements if i'm not wrong.

 

[rrg1016]

Try without SIM architecture

 

[CG1980]

@Mustaine3: even if i activate NLGEOM in the static step the frequencies don't change
@rrg1016: if i run the step without SIM architecture in Lanczos Eigensolver then frequencies tend to decrease and modeshapes to change, but that's not what it should happen (frequencies should increase their values)

 

[rrg1016]

From 6.10.1 Acoustic, shock, and coupled acoustic-structural analysis

"Projecting and storing the acoustic coupling matrix during the natural frequency extraction is also available for the Lanczos eigensolver based on the SIM architecture."

"Abaqus by default projects and stores the acoustic coupling matrix during the natural frequency extraction, for later use in coupled forced response analyses. The structural and acoustic regions are not actually coupled during the eigenanalysis."

As I understand you can't see during the natural frequency extraction the acoustic-coupling using SIM architecture.

Moreover if you compare the results of Lanczos with SIM and the empy tank you will see the same eigenfrequencies.

About the change on the frequency values, I guess than the frequencies should decrease because of the water coupling... AKA added mass effect. Maybe this problem is different.

 

[CG1980]

I agree with you, but i guess i'm missing something in order to get increased frequencies of the paper (the tank is clamped at the bottom).
My big question is: in a coupled acoustic-structural analysis like this one is the *TIE constraint between tank and fluid enough to define the coupling or should i introduce some kind of acoustic interface?

 

[rrg1016]

Yes. TIE constraint is enougth.

Don't use SIM architecture to eigenanalysis!!!!!!!!!