Those are static load cases, and don't have any affect on dynamics (unless you have a non-linear system, and you have to reference a static case for non-linear boundary conditions).
Your test/study should be setup as follows:
1) Build your model and define the fluid density on the 1st element.
- run the static analysis
- setup the dynamic model analysis and run
- note the natural frequencies
2) Go back the the model input and delete the fluid density
- run the static analysis
- setup the dynamic model analysis and run
- note the natural frequencies
3) Compare the two sets of frequencies.
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Regards,
Richard Ay - Consultant