Simulation of underwater sound radiation from ship structures in full frequency range

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Abstract

To goal was to review and demonstrate methods available for prediction of underwater sound radiation from large ship structures for full frequency (ca. 10...100000 Hz) range.
Background based on associated recommendations and standards was reviewed first. The frequency range dealt with is extensive, even from 1 Hz to 100 kHz.
Physical background and prediction methods available were reviewed next. Although most prediction methods are quite mature today, the sheer size of ships as well as extended frequency range makes the prediction task very demanding. There is a lack of reliable examples of prediction examples and correlation between experiments. Prediction of sound pressure in discrete points or a small area far from a ship is inherently subject to large uncertainty, even is totally isotropic unbounded conditions.
A small structure was the first example. The (ideal) low-frequency range extends up to ca 250 Hz and high-frequency range upwards from 2500 Hz. Point forces were used as excitation. The methods were used over whole possible frequency range. The most drastic finding is the remarkable effect of “Fluid Loading” option of VA One on sound radiation in SEA-models. It was concluded that SEA without “Fluid Loading” overestimates the radiation and a model with it underestimates. However, it is not wise to draw strict conclusions from a single example. This question that should be studied more.
A part of a 164 m long ship was then modelled. It was noticed that a standard “local” type FE model is not feasible to simulations at frequencies over 10 Hz. By modeling only the lower part of the ship with FE and the rest with SEA the upper frequency range could be raised up to ca. 25-30 Hz. To reach 100 Hz or more with a meaningful sized model, the FE part must be smaller. This was tried. A full SEA model was also created as well as a BEM model. The results again scatter quite much. It was also demonstrated how the SEA-prediction at low frequencies could be refined by using input power calculated using FEM.
An analyst must be aware of differences in methods. A SEA model does not account for global vibration fields, only “local” ones. HAJ formulations used between FE-structure and fluid used assume that the structure is baffled. Better approximation is obtained using BEM. SEA and FE-SEA models assume random excitations, whereas correlation effects can be taken into account in full FE. Radiation analysis using BEM is possible in both random and deterministic excitation cases.
This kind of modeling project is instructive. Much of the learned is in the form of tacit knowledge.
Original languageEnglish
PublisherVTT Technical Research Centre of Finland
Number of pages76
Publication statusPublished - 25 Jun 2014
MoE publication typeD4 Published development or research report or study

Publication series

SeriesVTT Research Report
NumberVTT-R-00724-12

Keywords

  • ship
  • underwater noise
  • SEA
  • FEM
  • BEM
  • modeling
  • prediction

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