An Effective And Standardized Prediction Method Of Ship Mechanical Noise In Full Frequencies

As a complex elastic structure in the water,a ship will be vibrated by various equipment loads in the process of operation,and radiation noise will be generated.The violent vibration will affect the normal operation of mechanical equipment and the health of the crews,while the ship underwater radiation noise may destroy the marine ecological environment and degrade the stealth performance of the ship.Ship underwater radiated noise has been confirmed as a major contributor to changes the marine acoustic environment,affecting marine mammals’foraging,breeding,avoidance of natural predators and navigation.Ship underwater radiation noise is also one of the main indicators of ship stealth performance,and advanced sonar technology has put higher requirements on the acoustic stealth of ships.Hydroacoustic identification and deep-sea detection is also a key technology of China’s strategy towards the oceans,in-depth research and control of the ship’s vibration and noise is of great significance to improve the accuracy of deep-sea detection and hydroacoustic identification.Ship underwater radiation noise mainly includes mechanical noise,propeller noise and hydrodynamic noise,and mechanical noise is the major component of ship underwater radiation noise under the low-speed navigation condition.If the ship mechanical noise and its distribution features can be predicted accurately in the design stage,it has great engineering value for the subsequent design and vibration noise control,and also has practical significance for improving the marine acoustic environment and enhancing the accuracy of deep sea acoustic detection.Based on the lack of standardized evaluation process,as well as the efficient and precise methods for ship mechanical noise numerical prediction,the paper thoroughly investigates the influence of three factors on the numerical prediction results of ship underwater radiated noise,such as fluid and structure coupling mode,vibration and acoustic numerical calculation model,and standard form of external excitation load for vibration and acoustic source.According to the completeness of load information,a rapid and standardized calculation method of ship mechanical noise is established in the full frequency,and a three-parameter method is provided to evaluate the existing various calculation methods,covering coupling mode,calculation model,loading method,energy superposition,interval theory and probability model,which solves the problems of lacking standardized,efficient and accurate calculation methods when predicting ship mechanical noise.In the method,The acoustic BEM based on acoustic-structure coupling mode is used in the low frequency domain,and extends the acoustic BEM through the gradient meshing model in the meddle frequency,while the statistical energy method of energy correction per unit bandwidth is adopted in high frequencies,which successively realizes the calculation and evaluation of ship mechanical noise in full frequencies;when the equipment load information is complete,the generalized force is defined as the standard load in the low-mid frequency,and the power flow is defined as the standard load in the high frequency,considering the effect of acoustic cavity excitation.When the load information is incomplete,the interval theory and probability model method are introduced to predict mechanical noise by the upper and lower limit theory and energy averaging method,finally forming an accurate and efficient standardized mechanical noise prediction method.It covers coupling mode,calculation model,loading method,energy superposition,interval theory and probability model to solve the problem of lack of standardized calculation method for ship mechanical noise prediction.And the accuracy and efficiency of the presented method are verified by the three-parameter method quantitatively analyzing the results between the analytical simulation and the hydroacoustic testing of the experimental model.The results reveal that the overall level error is within 3 d B,and the consistency of radiation energy distribution is good,which verifies the reliability of the method.The major research contents of the thesis are as follows.（1）Based on the theories of structural dynamics and acoustics,the calculation method of ship mechanical noise is derived in full frequencies.For low-frequency mechanical noise,the difference and relationship between acoustic-structure coupling and fluid-structure interaction modes are identified,and three methods are proposed.For high frequency,the SEA method is used to predict the ship mechanical noise,and the parameters selection is analyzed.Then,vibration and hydroacoustic tests of the cabin model with 9 working conditions are designed and used to compare the accuracy and efficiency of each calculation method.In addition,the three-parameter method is proposed,which quantitatively describes the difference between the simulation results and the test results by the error of the overall level,the correlation coefficient and the standard deviation of the error of each frequency band.The results of hydroacoustic tests show that the accuracy of the acoustic-structural coupling mode is higher than that of the fluid-structure interaction,and the efficiency of the BEM is higher than that of the FEM,and the acoustic BEM based on the acoustic-structure coupling mode is recommended.Directly using the statistical energy method to predict the mechanical noise,the accuracy is poor,but the energy distribution of radiated noise is better.（2）A gradient meshing model method is proposed to address the problems of the narrow frequency bands and the low efficiency for large-scale ship vibration and acoustic radiation.Based on the wave theory and vibration analysis,the method simulates the structure near the vibration source with a fine meshing,while the structure farther away from the vibration source is simulated with a coarse meshing.Taking into account the accuracy and efficiency,the guidelines of meshing are proposed for structural vibration and mechanical noise,respectively.Based on the gradient meshing model,the calculation of mechanical noise is achieved by acoustic BEM in the middle frequency.The impedance error is about 2d B and the radiated sound power error is within 3d B as verified by numerical model and experimental testing.The gradient meshing model method is simple model,high efficiency,well accuracy and wide application,which is important for the the optimized design,mechanical noise and base impedance of large ships（3）The effect of equipment excitation load on mechanical noise is studied.The results show that vertical acceleration is the main factors affecting mechanical noise,and it reveals the physical mechanism.It is also proposed that the complex mechanical model F_{xy z}M _xyz can be simplified to a load modelF_z M _xy consisting of generalized force components_zF,M_x andM_y.In addition,the sum of sound power which is obtained by the components_zF,M_x andM_y,are close to the results which theF_z,M_x andM_y are simultaneously appled to the structure,with an error being 1.02d B,which indicates that the mechanical noise generated by the various load components basically satisfies the energy superposition.（4）According to the influence of equipment load on mechanical noise,the generalized excitation forces is defined the standard load in low-mid frequencies,and the power flow is the standard load in high frequencies.Other load forms are converted into standard load.Acceleration as the most widely equipment load characterization,the load criteria mainly realize the conversion of acceleration load.In low frequencies,for ideal acceleration,the conversion is realized by the load identification theory,while for bench acceleration,the conversion is realized by the load modelF_z M _xy and the source descriptor invariance.For the high frequency excitation case,the conversion is realized by the equivalent impedance,and its’calculation method is presented.The reliability of the loading method of mechanical equipment is verified by vibration and hydroacoustic tests of the cabin model.In addition,the influence of acoustic excitation on mechanical noise is also discussed,and it reveals that the acoustic cavity resonance effect cannot be ignored in the low frequency.The standardized calculation method of equipment load is summarized when the load information is complete.（5）In view of the load uncertainty caused by the lack of load information,the theory of interval analysis is introduced into calculating mechanical noise of ships,and the upper-lower limit theory of mechanical noise is proposed.The correctness and reliability of the theory is verified by hydroacoustic tests.Based on the principles of structural dynamics and vibroacoustic coupling analysis,the concept of vibroacoustic transfer function of the average acceleration is proposed,and the influence of the excitation load components on the mechanical noise can be directly compared by the vibroacoustic transfer function.On the one hand the vibroacoustic transfer function can determine the major load components affecting mechanical noise in each frequency band,and on the other hand the fluctuation interval of mechanical noise under the average acceleration load can be also determined.（7）In response to the problem of needing a definite value to evaluate whether the design meets the requirements in practical engineering,the probability model is introduced into the calculation of ship mechanical noise,and the energy averaging method of ship vibration and mechanical noise is proposed.Based on the acoustic-vibration coupling theory the form of energy superposition for a given acceleration load is derived,the probability density function is obtained according to a uniformly distributed probability model or the Montr-Carlo method,and the underwater radiated noise under uncertain loads is characterized by averaging.In addition,the energy averaging method is also applied to the vibration response,and the energy averaging equation of vibration response is derived.Finally,the influence of coupling mode,calculation model and equipment load characteristics on mechanical noise is summarized,and the rapid standardized calculation method of ship mechanical noise is proposed in full frequency band,and the reliability of the method is verified by vibration response test and hydroacoustic test.