Study On The Recovery Of Acoustic Radiation Characteristics Of Underwater Targets In Free Field Based On Boundary Element Method

Accurately acquiring the acoustic radiation characteristics of a target forms the basis of target recognition and control.In order to obtain the target's intrinsic acoustic characteristics that are not affected by the environment,the acoustic characteristics of target must be measured in the free field.Although conventional sound field facilities such as large lakes,reservoirs and anechoic tanks approximately meet the free-field conditions under experimental conditions,there are many limitations and deficiencies in these sound field facilities and the sound source cannot produce an ideal free sound field in these sound field facilities.For example,large lakes and reservoirs are easily disturbed by weather and bio-acoustic background noise:anechoic pools are difficult to meet the free-field conditions at low frequencies.Therefore,to measure the free field acoustic characteristics of the target,various methods and techniques must be used to make up for the deficiency that the ideal free field cannot be realized.Aiming at the problem that the free sound field are difficult to produce due to the influence of the boundaries of sound field and disturbing sources when measuring the acoustic radiation characteristics of underwater targets,the sound field recovery method based on the boundary element method(BEM)is used to recover the free-field acoustic characteristics of the underwater target in a bounded noisy environment in this thesis.The main contents of this thesis are as follows:(1)The sound field recovery method based on the BEM is derived.First,the sound pressure and the normal velocity on the measurement surface which is at the middle position of the two surfaces are obtained based on the sound pressures measured on the two surfaces.Second,the BEM is used to separate the outgoing sound pressure which is the sound pressure away from the sound source and the ingoing sound pressure which is the sound pressure in the direction of the sound source on the measurement surface.Then the BEM is used to calculate the scattered sound pressure caused by the ingoing sound field incident on the surface of the target with rigid body assumption.Finally,the scattered sound pressure is subtracted from the outgoing sound pressure to obtain the free-field sound pressure of the underwater target.The calculation results show that the scattered sound field can be ignored because the wavelength is larger than the size of the underwater target.And the outgoing sound field and the recovered sound field agree well with the free sound field in a bounded environment at low frequencies.But as frequency increases,the scattered sound field cannot be ignored because the wavelength is smaller than the size of the underwater target.And the outgoing sound field is obviously different from the free sound field.However,the recovered sound field is the same as the free sound field.Even if the frequency is very low,the scattered sound field cannot be ignored in a bounded noisy environment because of the strong ingoing sound field.But the recovered sound field and the free sound field are always the same.Therefore,the sound field recovery method based on the BEM can eliminate the interference of the boundary and the disturbing source on the free sound field radiated by the target,and recover the free-field acoustic radiation characteristics of the target in a complex environment.(2)The sound field recovery method based on the BEM and evanescent wave filtering method are combined to solve the problem of obtaining the free-field acoustic radiation characteristics of the target and far-field radiation hot-spots in a near-field bounded noisy underwater environment.The sound field recovery method based on the BEM is used to recover the free-field acoustic radiation characteristics radiated by the target which is vibrating in current behavior based on the information on the measurement.Then the supersonic sound intensity method and the surface contribution method are used to filter out the evanescent waves on the plane measurement surface and the measurement surface with complex geometric shapes,respectively.It is demonstrated that the method combining the sound field recovery method based on the BEM and evanescent wave filtering method can not only eliminate the interference of the boundary and the disturbing source on the free sound field radiated by the target but also filter out the evanescent wave and identify the far-field radiation hot-spots in the near-field environment.(3)Aiming at the problem that the conventional sound field recovery method based on the BEM is not applicable due to the inability to wrap the sound source with a closed measurement surface when the target is sitting on the boundary,an improved sound field recovery method based on the BEM combined with the principle of mirroring is established.The improved sound field recovery method based on the BEM can be used to solve the problem of acquiring the half-space acoustic characteristics of the target sitting on a rigid boundary or a soft boundary in a complex environment.Numerical simulation and experimental results show that the improved sound field recovery method based on the BEM can eliminate the influence of the boundaries and disturbing sources when the target is sitting on a rigid or soft boundary,and recover the half-space acoustic field characteristics of the target.(4)To solve the problem that the conventional sound field recovery method based on the BEM can only be used to recover the acoustic radiation characteristics radiated by the underwater structure which is vibrating in current behavior,but cannot recover the acoustic radiation characteristics radiated by the underwater structure in the free field because the fluid-structure interaction is not considered,a sound field recovery method based on the BEM considering the fluid-structure interaction is established.Due to the strong fluid-solid coupling in water,the scattering of the ingoing sound field incident on the target surface cannot be simplified as a rigid body scattered problem,but an elastic body scattered problem.That means that the vibrational behavior of the underwater structure in the free field is different from in the bounded noisy environment.The elastic body scattered sound pressure is the superposition of the geometric scattered sound pressure(rigid body scattered sound pressure)and elastically radiated sound pressure.First,the outgoing sound field and ingoing sound field are separated by BEM.Second,the rigid body scattered sound pressure is calculated by BEM.The vibro-acoustic coupling method can be used to calculate the vibrational velocity on the surface of target caused by the ingoing sound field incident on the target.And the obtained vibrational velocity on the surface of target is used to compute the elastically radiated sound pressure.Further,the elastic body scattered sound pressure can be obtained by adding the geometric scattered sound pressure and the elastically radiated sound pressure.Finally,the free sound pressure radiated by the underwater structure in the free field is recovered by subtracting the elastic body scattered sound pressure from outgoing sound field.Numerical simulation results show that the conventional sound field recovery method based on the BEM can be used to recover the free-field acoustic radiation characteristics radiated by the structure which is vibrating in current behaviors.But the sound field recovery method based on the BEM considering the fluid-structure interaction can be used to eliminate the influence of boundaries and disturbing sources in the bounded noisy underwater environment and recover the acoustic characteristics radiated by the target in the free field.