Simplified Model Of Radiated Noise Sources Of Ship

Noise is produced when any ship moves; this kind of noise transmits all around in water, which forms the radiated noise field of ship. As the size of the ship is huge and a large number of machines and equipments had been mounted on it, the ship will be a complicated volume sound source at near-field; the sound field varies with time and space and has spatial directivity. Usually the radiated noise of the ship is measured only at short range to obtain enough SNR. As a result, it is highly important and necessary to study the modeling of the volume source of ship and develop a method of calculating far-field noise distribution by near-field measurement.The underwater radiated noise of ship is the total effect of noise sources distributed from stem to stern of the ship including part of wake, and the acoustic shielding effect of the hull and wake. The radiated noise of ship can be considered as the noise radiated from some multi-pole sources (monopole source, dipole sources, quadrupole sources, and octupole sources) located near the stem, middle position, and stern of the ship. These multi-pole sources provide a practical model.Proper transmission model is chosen for near and far field numerical computation for corresponding volume source of ship. FFM is used for near-field computation, and the influence of lateral wave had been taken into account. The integral is evaluated by means of FFT. This method is not restricted by source frequency and precise result can be obtained. Whereas the NM (normal mode) theory is more suitable for calculation of low frequency radiated noise in far-field.The validity of multi-pole noise model is verified by FFM for free field and shallow water separately. In the process of numerical simulation, the intensity and location of each multi-pole is adjusted properly, and the received sound field distribution is similar to the horizontal distribution of ship radiated noise publicized in overseas documents. The niche genetic algorithm which is a global self-adaptive probabilistic optimization algorithm had been presented. The intensity coefficient and the location of each multi-pole sources in the three-dimensional space are inverted by using near-field pressure data of ship radiated noise in shallow water. The theoretical pressure calculated using the deduced parameters well matches the corresponding data. The validity of the model of ship radiated noise sources is proved. The precision of the calculated results is analyzed by using different SNR data and the effect of the error of calculated results is also discussed.The method to calculate the spatial directivity in the far-field by using pressure data of ship underwater radiated noise in the near-field is proposed. That is to say, the intensity coefficient and the location of multi-pole sources are inverted by using pressure data in the near-field, can well be used as equivalent radiated noise sources of ship. The corresponding spatial directivity in the far-field is calculated by using algorithm KRAKEN.In Aug.2008, experiment had been carried in the sea area near Island Dawangjia of province Liao Ning, using bottom-sited measurement system. The radiated noise of ship "Fishing Administration 21103" is measured in near-field. The experimental data with the ship at high speed and low speed are processed. The calculated results well prove the validity of the multi-pole model.