Study On Acoustic Propagation Characteristics Of Fluid-Filled Straight Pipes And Modal Conversion In Bent Pipes

In recent years,the elastic fluid-filled pipeline system has been extensively applied in various engineering fields,including the transportation of fluids in ship pipelines,storage and delivery of petroleum and chemical liquids,urban water supply,and nuclear power plant cooling water pipelines.In the defense sector,while effective suppression of mechanical noise,propeller noise,and flow noise generated by vessels has been achieved,the noise radiated from the internal seawater pipelines remains a detrimental factor to the stealth capability of vessels.In the industrial sector,ultrasound detection technology is widely used for structural health monitoring and non-destructive testing of pipelines.Therefore,it is of great practical significance to study the acoustic propagation characteristics in fluid-filled pipelines,as it can provide theoretical guidance and promote the development of vibration reduction,noise control,and non-destructive testing techniques for fluid-filled pipelines.This paper investigates the acoustic propagation characteristics in fluid-filled pipelines from three aspects: theoretical analysis,numerical simulation,and experimental research.Firstly,the acoustic propagation characteristics of fluid-filled straight pipes are theoretically derived for the absolute soft,liquid simplified wall model,and elastic wall model.The dispersion equation for elastic fluid-filled straight pipes is obtained,and numerical methods are employed to solve the dispersion equation and plot the dispersion curves of phase velocity,group velocity,and the first-order cut-off frequency of non-axisymmetric modes.Based on this,the axial and radial distribution laws of the acoustic field inside elastic fluid-filled pipelines are presented.The influence of pipeline parameters on the acoustic propagation in fluid-filled pipelines is explored,revealing that the resistance to deformation of the pipe wall material and the variation of pipe diameter have an impact on the cut-off frequency.Secondly,models of fluid-filled straight pipes and fluid-filled bent pipes are established,and steady-state and transient acoustic field simulations are performed using the finite element software COMSOL.The cut-off frequency and the energy distribution of the acoustic field in fluid-filled pipelines are analyzed in combination with the theoretical approach.Furthermore,it is observed in the simulation that the non-axisymmetric structure of the bend in fluid-filled pipelines leads to the generation of non-axisymmetric modes during the propagation of sound waves in the pipeline.Based on this "mode conversion" phenomenon,the acoustic propagation characteristics of fluid-filled bent pipes are further analyzed in conjunction with the nonaxisymmetric modes in straight pipes.Finally,PE(polyethylene)material,known for its good elasticity,low cost,and excellent construction convenience,is widely used in various fluid-filled pipeline systems.In this study,PE material is employed as the pipe wall material,and experimental research on the acoustic propagation is conducted for fluid-filled straight pipes and 90° fluid-filled bent pipes with the same cross-sectional dimensions.The experimental results are comprehensively analyzed and compared with the simulation results to validate the effectiveness of the simulation.The acoustic propagation characteristics in fluid-filled straight pipes and bent pipes are summarized accordingly.