Discussion Forum

Hi there,
Comsol
There is a problem I am having when measuring the expected shift in the frequency change on an aluminum shell filled with water subjected to different specified external pressure (psi). The shell is fixed from above (to simulate suspending string w/out gravity), and several probes (3 boundary and 6 point probes) inside and out of the shell surface where mounted in order to monitor vibration changes relative to acceleration (r) response.

The results interpret by the probes via spectrum (acceleration vs. freq.) shows no resonant frequency-shifts (expected at least a few Hz in shifts) on the acceleration,(r) vs. freq. (sweep 16-17 kHz, but reduced to save drive space&time) hint: .The domain step size I used was 5, 2, and 1, even 0.25 for different external pressure, 20, 160, 800, etc. in psi (which is specify in the Solid Mechanics load). The null results remains (no change in res.freq.). I've spent many days tweaking to find the issue and believe it may be an error or missing dependence somewhere in the Comsol program I designed.
Any helpful suggestion regarding this problem will be greatly appreciated?

Sub-Question: I used Non-Geometric Solver instead of linear geometric solver (physics module below menu options). Should this make a difference?
Additional Info:
Recent experimental results had shown their is a shift in frequencies as the external pressure increases in the air filled shell and I want to verify this with liquid fill-shell, and denser liquids. (example a shift of 20Hz) has been observed for a sound source which caused the shell to vibrate.


Problem: How can I get Comsol to respond to changes I have made to the external pressure in psi to generate a shift in the res. frequency? The results always show peak spectrum at same frequency position un-shifted, though at different amplitude amplitudes). A larger power magnitude (Point source of acoustic) or smaller frequency step size does not seem to change the problem.
Note: I only used STUDY1 (pre-stress) and STUDY2 (frequency domain) for compute, ignore STUDY eigen freq. evaluation--if study3 is computed it will alter the spectrum.


(See attached program file attached)

General Default on File:
Acoustic Sound=15watt
External pressure = 160psi
Frequency Step Size=0.25 Hz; (16900, 0.25, 16930) Hz for one of the peaks for air
initially may need re-compute for Study1 & 2, respectively.



--
----------------------------------------------------------------------------------------------------------------
Arts, crafts and sciences uplift the world of being and are conducive to its exaltation.
~Baha'u'llah