Study On Duct Structural-Acoustic Coupling Mechanism And Its Sound Wave Attenuation

As an important carrier of conveying medium,ducts are widely used in various practical engineering occasions,such as modern warships.Specifically reflected in a ventilation and heat exchange air conditioning system or intake and exhaust system of power device,its internal low frequency noise has attracted a lot of research attention.More practically,the duct is usually accompanied with flow medium and thermal field distribution.Therefore,it is necessary to further study sound propagation characteristics and duct noise control in complex environment.Vibro-acoustic coupling phenomenon between the duct and flexible structure is a typical acoustic characteristic and it is also an effective suppression technique to control duct low frequency noise.Then comprehensive study on the vibro-acoustic coupling mechanism in a fluid or thermal duct field is helpful to deeply understand the wave propagation characteristics,and also is an important premise and foundation for the structural-acoustic design.Motivated by this,the following work are carried out:Based on the energy principle,a two-dimensional fully coupled model for a duct with flexible structure is first established.Structural elastic boundary is simulated by a series of constrained springs,and the dynamic response is solved by the Rayleigh Ritz method.Radiated sound field in the duct is determined via a zonal vibration velocity integration across the structure surface.Compared with the traditional finite element or Galerkin modelling methods,the current proposed analytical Rayleigh-Ritz subsystem approach can effectively introduce the complex substructures without reformulating the system governing equation,and would easily consider various complicated effects such as elastic boundary conditions.The coupling mechanism between the flexible structure and duct is explored in the simulation part,and the boundary constraint effect of vibro-acoustic coupling silencing system is clarified for the first time.A fully coupling dynamic model for the complex sub-structures and three-dimensional duct acoustic field is established,for which the non-uniform constrained boundary and irregular shape are considered for the structure and backed cavity,respectively.Standard Fourier series supplemented with boundary smoothed auxiliary functions are used to construct the structural vibration displacement,and its boundary stiffness distribution is uniformly expanded into Fourier series.Based on the energy principle and Rayleigh Ritz method,in conjunction with the spatial coordinate transformation regularization of irregular acoustic cavity,the predicted response matrix equation of the complex coupling system can be determined.Numerical simulation results show that the system coupling strength can be effectively changed by the non-uniform elastic restraining boundary and the angle inclination of acoustical cavity walls.Meanwhile,experimental test about the duct coupled with flexible structure is also performed,which verifies the correctness of current theoretical model and prediction analysis.Energy transmission mechanism of the duct-structure coupling system is comprehensively explored via the study of internal acoustic radiation characteristics,including the radiated sound power,modal radiation efficiency as well as radiation mode.The transmission sound intensity energy flow and its divergence distribution information of the whole system are solved from the sound pressure and vibration velocity responses.The coupling mapping relationship between the modal radiation efficiency/radiation modes and duct acoustic field is revealed.Numerical results indicate that the internal radiation modes for the structure in a duct are strongly affected by the duct cut-off frequency and its multiples,and the spatial distributions of radiation modes are dominated by the duct length direction.Moreover,energy backflow phenomenon is remarkable around the resonant frequencies of the vibro-acoustic coupling system,and the spatial energy sources and sinks are mainly concentrated near the flexible structure.Duct phononic crystal structure is obtained in this thesis via a periodic extension for the vibro-acoustic coupling locally resonant element.Bandgap formation mechanisms and characteristics for the locally resonant and Bragg reflection are analyzed.The acoustic response of a resonant element is first obtained on the basis of the previous energy principle,then a sound pressure transfer matrix equation can be established through the continuity of sound pressure between elements.Combing the Bloch wave vector theory,the dispersion relation and acoustic bandgap characteristics for the periodic structure can be further determined.Flexible membrane and membrane-mass are used as the resonant cell,respectively.Numerical results show that the added mass can significantly alleviate the membrane tension dependence and enhance the sound attenuation performance,which will further effectively adjust the acoustic bandgap position and also the coupling strength.The optimal bandgap can be obtained by adjusting the system parameters such as membrane tension,lattice constant,mass position and its weight.The dynamic characteristic analysis model for the duct vibro-acoustic coupling system in the presence of mean flow is established.The framework of energy formulation combined with Rayleigh Ritz subsystem modeling approach is also adopted.For such a method,the advantage is that the total kinetic and potential energies of the structure and acoustic cavity as well as their coupling work term are independent from the mean flow.The internal radiated sound field by the vibrating structure is solved via the rederived surface velocity integral based on the generalized Lighthill formulation,with the consideration of particle velocity discontinuity around the structure surface.The effect of mean flow on structural vibration behavior and sound pressure distribution are predicted in numerical simulation,meanwhile,the coupling strength among structural modes is revealed under different flow Mach numbers.Finally,the mean flow effect on periodic structure acoustic characteristics are studied,including the effective dynamic parameters,dispersion curve and bandgaps.Acoustic characteristics,thermoacoustic oscillation and its stability suppression of the duct vibro-acoustic coupling system are systematically studied.The duct acoustic behaviors in a thermal environment are divided into two cases: one is the acoustic propagation/structural-acoustic coupling problem with temperature field distribution;the other is the thermoacoustic coupling problem with heat source and corresponding stability suppression strategy.For an arbitrary temperature gradient distribution,a linear discrete processing method is proposed to quickly predict the acoustic characteristics,when considering a heat source,a transfer function of sound wave passing the heat source is determined by continuity conditions.In numerical simulation,the change law of vibro-acoustic coupling characteristics under different temperature fields is obtained,with a conclusion that the deteriorating effect of temperature filed on sound attenuation can be repaired by designing appropriate lattice constant.Furthermore,the underlying control mechanism and controllability of acoustic and intrinsic modes are revealed.Investigation carried out in this thesis provides a comprehensive theoretical model for the duct vibro-acoustic coupling system.The influence of complex substructure and practical factors on the vibro-acoustic coupling characteristics is discussed and analyzed.Based on the coupling behavior of flexible structure in a duct,a series of low-frequency broadband acoustic wave suppression studies are performed,and a perfect optimization design scheme is proposed.Corresponding numerical results can provide a theoretical reference for the subsequent study of model optimization and the corresponding duct noise control.