Prediction on hull vibration and underwater noise radiation based on modified LMM

  Objectives  Large Mass Method (LMM) is capable to approximately convert excitation acceleration of equipments' seat into force loads, which is a common method for structural vibration response and acoustic radiation simulation analysis. In order to improve the accuracy of LMM applied in ship's noise & vibration response computation, a modified LMM(M-LMM)is presented.

  Methods  Its application principles in hull structure response were analyzed, based on which the appropriate large mass ratio ensuring the application precision of LMM was obtained. Rayleigh damping was then taken into consideration, and LMM was modified with ratio coefficient. The experiment comparing results from original LMM as well as M-LMM with tested ones, together with the computations of underwater noise radiation power and vertical directivity pattern of sound field, were followed.

  Results  For the frequency range of 20 Hz-200 Hz, the analysis results got from M-LMM were closer to measured values compared to that from LMM although their computed vibration response spectrums were roughly similar.

  Conclusions  It is crystal clear that M-LMM can guarantee a simulation of main hull response with higher accuracy(average deviation 2.7%), providing the pre-design stage of vessels with more necessary information.