Coupled Fluid-Structure Eigen Analysis

[raback]

Hi Kevin,

The HelmholtzStructure cases are hierarchical: structure -> acoustics. The FsiShoebox* cases are strongle coupled: structure <-> acoustics.

-Peter

[kevinarden]

using the ShoboxFsiEigen2d test case with
Density = 2710.0
Youngs Modulus = 70e9
Poisson Ratio = 0.3
The beam frequency should be 87 Hz in air and 55 Hz in water

with

$ AirDensity = 1.0
$ SoundSpeed = 300

ElmerSolver reports
EigenSolveComplex: EIGEN SYSTEM SOLUTION COMPLETE:
EigenSolveComplex:
EigenSolveComplex: Number of converged Ritz values is: 10
EigenSolveComplex: Computed Eigen Values:
EigenSolveComplex: 1 (1.04082768848625157E-004,-1.08456037487612890E-008)
EigenSolveComplex: 2 (-7.9012503238339047,-4.0733589099508940)
EigenSolveComplex: 3 (-7.8696137561585102,4.1349833785406878)

Changing the fluid density results in the imaginary part of the complex eigenvalue 1

results.png (15.77 KiB) Viewed 117 times

Any fluid density over 18 results in a solver error, water would be 1000.0

[eschenk]

I finally got some time to build out a simplified 2D version of the guitar model.

I think the fluid structure modal analysis is working correctly, or at least I get results that are consistent with my expecations, and driving changes either the fluid or the structure causes coupled changes in the joint system reaction at each frequency in a scanning simulation.

For the transient simulation of a coupled structure/fluid with the wave solver for the fluid, I get really strange results. I tweak the structure with a transient load, expecting it to vibrate. It goes through 1/2 cycle of vibration, and settles into a position slightly bent in the direction of the original force with exponential decay in the ongoing motion. The fluid oscilates in response, but also goes to zero quite quickly. Changing the fluid density or sound speed appears to have no effect on the structural response, which is incorrect in any case.

Here is a screenshot of a graph comparing displacement for structural oscilaiton without the fluid coupling vs the joint system, where I also show average pressure. The structure only system oscilates quite a bit as expected with a resonable decay of the oscilation. The coupled system (lower graph) stops moving very quickly.

Screenshot 2024-05-06 193951.png

(74.11 KiB) Not downloaded yet

I've also attempted to do the joint Eigen system analysis in 2D here, but the solver crashes on me rather than coming to a solution.

I've attached a zip file of the geometry and the sif files I've been using.

In the zip there are 9 case studies as follows:

case_air_eigen.sif - Find the eigenmodes of the air volume only. Seems to work as expected
case_air_scan.sif - Scan over frequencies with harmonic analysis to find the eigenmodes of the of the air. Agrees on the first mode. Some variance in later modes. See notes in the files for further thoughts.
case_box_eigen.sif - Find the eigenmodes of the guitar box structure only. Seems to work as expected.
case_box_scan.sif - Scan over frequencies with harmonic analysis to find the eigenmodes of the guitar box structure. Matches the structural Eigenmode solution.
case_box_transient_scan.sif - Transient solve of the structure only in response to an impulse. Top of the graph shown above.
case_joint_eigen.sif - Attempt to do an eigenmode solve for the joint system. Crashes.
case_joint_eigen_test.sif - Frequency space simulation of a single frequency of the joint system. Seems to work.
case_joint_scan.sif - Frequency scan of the modes of the coupled system. This appears to generate correct results as mentioned above.
case_joint_transient_scan.sif - Transient solve the joint system in repsonse to an impulse. Bottom of the graph shown above. The results here are not correct as discussed above.

Taken with Kevin's earlier notes, I think that the conclusion is that while the frequency space harmonic analysis coupling works between Wave Solver and the elastic structural solver, the time coupling does not work and the Eigenmode frequency analsysis does not work either.

[eschenk]

One test I would like to do that I have gotten to is to try frequency scanning on the beam in fluid case Kevin gives here to see if the fluid coupling gives the expected results. I probably won't get to that for a week or so given my free time availability for the next bit.

The idea would be to scan and look for peaks in the curve of the average pressure and/or average displacement (or better yet the integral of the pressure and/or displacement). These should correspond to the frequencies of the eigen modes of the joint system.