Analysis Of Acoustic Modal Characteristic Of Underwater Elastic Cavity

One of the main noise sources of underwater vehicle is hydrodynamic noise,and the main structure of the low-frequency peak hydrodynamic noise is the various kinds of openings in the hull of the vehicle.When the high-speed flow passes through the upstream edge of the opening,the abrupt transition of the structure will cause the separation of boundary layer and unstable flow,thus generating unsteady pressure,which is called the boundary layer shear oscillation.In the field of ships and water conservancy,the fluid Mach number is generally low and the acoustic radiation of the cavity shear layer oscillation is weak.However,when the sound wave propagates in elastic cavity which is filled with water,the elastic wall of the cavity will reduce the acoustic modal frequency of the cavity,which increase the probability of coupling resonance between the acoustic mode of the cavity and the hydrodynamic oscillation of the cavity,thus producing strong line-spectrum noise.Based on this background,this paper studies the effect of the elasticity of the cavity structure on the coupled resonance and acoustic radiation in flowing water.Taking the underwater Helmholtz cavity as the research object,this paper uses theoretical analysis and numerical simulation to conduct a detailed study on the mechanism of shear selfsustained oscillation and resonance sounding of the underwater cavity.In this paper,the acoustic finite element theory considering the influence of elastic cavity wall on its acoustic modes,the large eddy simulation theory for flow noise in cavity and the acoustic analogy theory are introduced in detail.The acoustic modal characteristics of Helmholtz resonator are studied when the cavity structure is elastic and the acoustic propagation medium is static water.The lumped parameter model of underwater cavity is established based on acoustic-electrical analogy theory,and the empirical formula of the first order acoustic-solid coupled resonance frequency is obtained.Acoustic finite element software is used to calculate the acoustic-solid coupled resonance modes of cavity structures.The analytical results of empirical formula are in good agreement with the simulation results,and the correctness of the calculation method is verified by each other.Furthermore,the influence of the elastic wall positon and elasticity on acoustic mode of cavity is studied.Then the flow noise characteristics of underwater Helmholtz resonator with rigid wall is studied.Based on the mixed CFD numerical method,the cavity flow-induced noise is obtained,and the mechanism of cavity shear self-sustaining oscillation is analyzed through the vorticity cloud contour.The cause of peak frequencies is analyzed by the sound pressure spectrum curve at each monitoring point.Finally,considering that the cavity is an elastic structure and the medium is the flowing water,the corresponding curve of sound pressure at each data point in the cavity is calculated by introducing the flow field in the acoustic-solid coupling model as the sound source.Compared with rigid resonators,the causes of peak frequencies in the curve and the effects of elastic cavity walls and flowing water on the noise of peak frequencies are analyzed.The effect of cavity wall thickness on resonance frequency is analyzed,and some methods of controlling elastic resonance in the underwater cavity are obtained.