calculations can be done with the aid of simple estimation formulae but should be supported by graphical user interfaces to ensure that the work can be done within a reasonable time frame. In the second step the global vibration design is optimised by FEA methods as described above. FEA methods are also used for substructure vibration analysis, e.g. masts and lashing bridges, and components of the propulsion plant, as shafts, gears and pod units, for instance.
Noise prediction The prediction of structure-borne sound propagation in ships is difficult for a number of reasons. The large number of modes participating in any state of high frequency vibration makes it impossible to treat the global sound propagation problem as a vibration problem today. For a typical passenger vessel for a frequency of 1000Hz, an FEA vibration model would lead to several million degrees of freedom. Since predictions for the mean propagation of structure-borne noise are usually required in a particular frequency band, vibration computations would have to be repeated for many frequencies. However, the very fact that information is required averaged only over a frequency band allows an alternative, far more efficient approach based on statistical energy
analysis.The Noise Finite Element Method (NoiseFEM) of Germanischer Lloyd is based on a related approach. NoiseFEM predicts the propagation of noise by analysing the exchange of energy between coupled subsystems, Cabos et al (1999). Validation with measurements on full-scale mock-ups show that the accuracy of NoiseFEM is sufficient for typical structure-borne sound
Typical ‘coarse’ FEA models as used for global vibration analysis of ships have been proven as well suited for NoiseFEM simulations.
predictions for the frequency range between 80Hz and 2000Hz, Wilken et al. (2004). Typical ‘coarse’ FEA models as used for global vibration analysis of ships have been proven as well suited for NoiseFEM simulations. By way of conclusion, it is to be reiterated
that technological progress is rapid, both for hardware and software. Simulations for numerous applications now often aid the decision making process, sometimes ‘just’ for qualitative ranking of solutions, sometimes for quantitative ‘optimisation’ of advanced engineering solutions.
Continuous validation feedback not only improves
the simulation tools themselves, but it also builds confidence in their use. However, advanced simulation software alone is not enough. Engineering is more than ever the art of modelling and finding the delicate balance between level of detail and resources (time, man-power). This modelling often requires intelligence and considerable (collective) experience. The true value offered by advanced engineering service providers lies thus not in software or hardware, but in the symbiosis of highly skilled staff and these resources.
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THE NAVAL ARCHITECT FEBRUARY 2007
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