| In the 21 st century,human beings have entered a period of large-scale development and utilization of oceans.Ocean plays an even more important role in the pattern of national economic development and opening to the outside world,and plays a more prominent role in safeguarding national sovereignty,security,and development interests,in the construction of national ecological civilization,and in the international political,economic,military,and scientific and technological competition.China is a large maritime country with more than 3million square kilometers of ocean area and 18000 kilometers of coastline.Building a strong modern navy,safeguarding maritime rights and interests are essential for development and people’s livelihood.In naval operations,submarines have the advantages of good concealment and long attack range,which not only pose a great threat to surface ships,but also play an irreplaceable role as an important part of strategic deterrence.With the development of antisubmarine technology,the probability of submarines being found,attacked,and extinguished in naval battles also increases.How to effectively reduce the radiated noise of submarines and improve the concealment of submarines has always been the research focus of each military power.There are large and complex fluid-filled pipeline systems in submarine.With the development of vibration reduction technology,noise of main and auxiliary engines,propeller and hydrodynamic noise in submarine have been greatly suppressed.Noise of fluid-filled pipeline system accounts for more and more of the total radiated noise of submarine.The equipment running in the fluid-filled pipeline system will excite the vibration of the wall.It will also cause the vibration of the wall when the fluid in the pipeline passes through valves,elbows,and other pipe fittings.It has the characteristics of low-frequency line spectrum.These low-frequency noises cannot easily be absorbed by sea water and are easily detected during the propagation process,which is disadvantageous to the concealment of submarines.It is of great significance to investigation the low-frequency sound propagation characteristics of the fluid-filled elastic pipeline for suppressing the low-frequency radiated noise and enhancing the concealment of the submarine.Nowadays,most of the investigations on sound propagation of fluid-filled elastic pipeline refer to the research ideas in gas-filled pipeline,regard the wall as an ideal boundary,ignore the role of the wall in sound propagation,and lack of effective noise evaluation method for fluid-filled elastic pipeline.Aiming at the above problems,based on the relationship between normal wave phase velocity and medium wave velocity,this paper reveals the sound propagation mechanism in the elastic fluid-filled pipeline and verifies it by numerical simulation and experimental research.Based on the sound propagation mechanism of elastic fluid-filled pipeline,the causes of low frequency cut-off phenomenon in pipeline and the propagation mode of low frequency sound energy are analyzed.Finally,a method for evaluating the noise of fluid-filled elastic pipeline in non-anechoic pool is proposed.The effect of vibration and noise reduction by replacing steel pipe with composite hose is evaluated and the sound propagation mechanism of elastic fluid-filled pipeline is verified again.The main achievements are as follows.1.Analysis on sound propagation mechanism of elastic fluid-filled pipelineBased on the wave equation,the eigenvalue problem of the sound field in the pipeline is solved by spectral method,and the dispersion curves of the normal wave in the elastic fluidfilled pipeline is obtained.The influence of pipe diameter,wall thickness and wall material parameters on dispersion law of normal wave in fluid-filled pipeline is analyzed.The influence of pipe diameter,wall thickness and wall material parameters on dispersion law of normal wave in liquid filled pipeline is analyzed.The results show that the variation of wall thickness and pipe diameter causes left and right deviation of dispersion curve and changes cut-off frequency of high-order normal waves.The shape of the dispersion curves changes with the change of wall material.The decrease of the velocities of the medium in the wall will cause the dispersion curves to shift downwards and the low frequency mode will disappear.According to the relationship between phase velocities of normal waves and velocities of medium waves,five sound propagation mechanisms in elastic fluid-filled pipeline are found,and the propagation modes and energy distribution of low frequency sound are analyzed in detail.Low-frequency sound energy in elastic fluid-filled pipeline mainly concentrates on the surface of the wall and propagates in the form of surface-B propagation mechanism.Only when the elastic parameters of the wall material satisfy certain conditions,there will be low frequency plane waves in the elastic fluid-filled pipeline.It is difficult for the elastic materials in practical engineering applications to meet these conditions.2.Verification of sound propagation mechanism of elastic fluid-filled pipeline by finite element simulationSteady and transient simulation of sound field in elastic fluid-filled pipeline are carried out by using ACTRAN and COMSOL.The cut-off frequencies of normal waves in pipeline can be obtained from the steady simulation results.The cut-off frequencies simulation results of the two software agree well with the theoretical calculation results,which verifies the accuracy of the numerical simulation and theoretical calculation.The group velocities of the low frequency modes can be obtained from the transient simulation results.The simulation results are also in good agreement with the theoretical calculation results,which verifies the theoretical calculation accuracy of the dispersion curves.The results of transient simulation also reflect the energy distribution laws of different propagation mechanisms,which is consistent with theoretical analysis: at low frequency,the energy of surface-B propagation mechanism is the highest and that of SV-B transmission mechanism is the lowest.The accuracy of sound propagation mechanism of elastic fluid-filled pipeline is verified,and the main propagation mechanism of low-frequency sound energy in elastic fluid-filled pipeline is clarified.3.Verification of sound propagation mechanism of elastic fluid-filled pipeline by experimentAn experimental platform for sound propagation characteristics of elastic fluid-filled pipeline is designed and built.Experimental research on sound propagation characteristics of three fluid-filled pipelines in theoretical research is carried out.The cut-off frequencies,lowfrequency mode group velocities and axial radial energy distribution of normal wave in pipeline are measured in practice.The experimental results agree with theory and simulation.The sound propagation mechanism of elastic fluid-filled pipeline is verified again.The cause of low-frequency cut-off phenomenon in elastic fluid-filled pipeline and the main propagation mode of sound energy are further clarified.It plays an important guiding role in vibration and noise reduction of fluid-filled elastic pipeline and the design of pipe muffler.4.Research on noise control of fluid-filled pipeline based on wall parameter regulationA noise control method for fluid-filled pipeline based on wall parameter regulation is proposed,and a common model of fluid-filled system in engineering is established.Taking the material exchange of wall as an example,the effectiveness of this pipeline noise control method in the actual fluid-filled pipeline system is verified by a pool experiment.The results show that the measurement results of radiated noise at the nozzle of fluid-filled pipeline system are significantly reduced when the wall material is changed from metal to composite material,and the noise line spectrum at a specific frequency decrease by more than 20 d B. |
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