1. Acoustic excitation and mechanical analysis of a satellite antenna
Claes Fredö
Ingemansson Technology AB
Magnus Baunge
Saab Ericsson Space AB

2. Why acoustic excitation?
Example spectrum at the payload dB
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3. What does a satellite antenna look like?
These antennas are not the analysis example
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4. Past projects Vibration excitation has been the dominating load.
Acoustic excitation could be replaced by vibration test
Analysis technology was not developed to accurately handle the case of acoustic diffuse field excitation
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5. Today
A higher lifting boost of launchers has increased the acoustic load
Analysis technology can now handle acoustic diffuse field excitation & fully coupled fluid/structure interaction
However, in-depth mechanical analysis tasks are not yet supported by vibro-acoustic analysis software
Stress elements must have a single Gauss point (i.e. Tet4, TRIA3, .. elements), i.e. use elements of poor quality.
Displacement, Acceleration and Stress must be analysed in the global co-ordinate system.
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6. Local/Global coordinates
A local coordinate system
The global coordinate system
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7. In-depth post-processing
Local analysis co-ordinate systems are needed to correctly analyse interface loads.
The analysis of acceleration, displacement and force benefits from the use of local co-ordinate systems as it better follows transducer alignment.
Analysis at centre- and at ply- layers is needed for correct evaluation of composites.
Stress/strain analysis benefit from less conservative estimates on major/minor principal and Von Mises data.
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8. How can such postprocessing be accomplished?
Use global coordinates for the vibro-acoustic analysis (required).
Use local coordinates for the mechanical analysis task.
Extract the structure’s modal response Power Spectral Densities generated by the acoustic load as this provides a complete description of the mechanical response.
Use modal PSD information in a separate analysis run in a mechanical CAE postprocessor that allow more in-depth mechanical analysis.
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9. Three models are at work
BE - acoustic domain
Project data from this mesh onto the simplified mesh
Fluid/Structure interaction
FE structure analysis model (w. local coordinate systems)
FE - structure domain (global coordinate system only)
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10. Some mathematics
Analysis output: Each analysis frequency generates a correlated N by N PSD matrix for the N structure modes
Sum the PSD matrix rows into N modal PSD responses
This yields the modal participation factors for each analysis frequency
Example analysis frequency
Modal PSD for three modes
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11. Contribution from high frequency modes
Fit the RMS value of a SDOF response to the fluid/structure calculated modal responses for the analysed frequency range
Use the excitation sensitivity from the above step to estimate the mode response at higher frequency
Calculate the modal RMS displacement value for this contribution and add this to the response summation.
Modal RMS response
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12. Import to mechanical CAE software
Analysis work flow
Acoustic load
Projection onto simplified mesh
Projection to global coordinates
Fluid/Structure interaction
Structure Modes
Import to mechanical CAE software
Modal summation (SModalParticipFact)
Modal PSD
RMS calculation
Results evaluation
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13. End results
Response PSD and RMS value at any node or element of the FE model and in any coordinate system
Estimation of high frequency mode contribution (not shown)
Acceleration/Displacement
elements/MPCs/SPCs
ply/centre layer
RMS
PSD
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14. Results so far
Analyses have been made for various antennas with different boundary conditions
RMS values for acceleration are of expected and acceptable magnitudes.
Analysis shows that the fluid/structure interaction influences response magnitudes
Analysis covers the frequency range that governs the highest loads
Testing of end product is planned for next year.
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15. Conclusions
Fully coupled fluid/structure analysis with acoustic diffuse field loading can be used to predict mechanical response (displacement, acceleration & stress)
In-depth mechanical analysis can be facilitated by specialised post-processing and allows results to be procured in wider variety of analysis options.
Experimental verification is planned, but results obtained so far are of expected and acceptable magnitudes.
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