| Low-frequency and high-power underwater acoustic transmitting technology for long-range underwater acoustic communication and stealth target detection is an important research direction in underwater acoustic engineering in recent years.The researcher’s demand for transmitted sound power exceeds hundreds of kilowatts,and the demand for low frequency has reached tens of hertz or even lower.How to meet the above two requirements separately or simultaneously has become an important scientific research for underwater acoustic transducer researchers.To meet the current needs,in this paper,low-frequency,high-power,high-efficiency and small-size acoustic emission technology has been studied by utilizing new Giant Magnetostrictive Materials(GMM)and innovative composite structural shells.The GMM has gradually developed and matured from the 1980 s to the beginning of this century.It has the characteristics of large magnetostrictive strain,high energy density and low sound velocity.It is a comprehensive performance improvement and technology progress for low frequency and high power underwater acoustic transducers.The progress provides a huge space for development,and also leads to a research upsurge in the application of GMM for underwater acoustic transducers.In this paper,aiming at the key scientific problems in the application of giant magnetostrictive materials and the key technical difficulties in exerting the excellent properties of new materials,the influence and application range of the working points of magnetic bias and prestress on the strain of magnetostrictive materials are studied,and two types of drivers of rare earth materials single-rod driver(the auxiliary closed magnetic circuit)and multi-group closed driver(the closed magnetic circuit)are designed and realized.The magnetic circuit structure and the static/dynamic magnetic fields distribution of the single-rod driver were optimized by using Finite Element(FE)software,and the dynamic magnetic field range of rare-earth rods for high-power operation was clarified.The basic types of “口” structure and “回” structure are technically decomposed,with the static and dynamic magnetic field distribution of the three-dimensional model implemented.Additionally,the arrangement and spacing of the type “回” structures were analyzed and the linear output dynamic range was simulated when driven under the high power condition(driving current 10Arms).In order to further exert the technical advantages of new materials and fully improve the comprehensive performance of low-frequency high-power underwater acoustic transducers,this paper selects the basic types of flextensional transducers,and proposes two new designs of flextensional transducers with composite shell structures respectively.Operating characteristics of low frequency large volume displacement and ultra-low resonant frequency:(1)The fish-mouth flextensional shell with nested double-shell is excited by the developed multi-group closed structure driver.The design idea and the structure of the fish-mouth flextensional transducer with nested double-shell are proposed,with their structural evolution process expounded and structural characteristics analyzed.Several sets of structural static parameters of the double-shell transducer were studied.Taking the capability of withstanding the hydrostatic pressure of 150 meters as the design target,the deformation and stress variation laws of the double shells under static pressure and dynamic pressure of full driving power were obtained.In this paper,the driving force of the driver and the stiffness matching of the double-shell are studied.The frequency characteristics and acoustic radiation characteristics of the double-shell transducer in water are calculated with different diameters of rare earth rods.The better stiffness matching range is obtained with TPR as the optimization index.The performances of double-shell transducer and the single-shell transducer are analyzed and compared,and the TPR and the quality factors of the two transducers are simulated and calculated.The calculation results have shown that the performances of double-shell transducer are superior to that of the single-shell transducer in terms of frequency characteristics and sound radiation characteristics,and has the advantages of lower frequency,higher power and smaller size.A fish-mouth flextensional transducer prototype with nested double-shell was developed and its performance was tested.The resonant frequency of the prototype in water was 715 Hz,the sound source level was 204.5d B,and the measured electro-acoustic efficiency was 39.3%,showing excellent performances of the low frequency,high power,high efficiency and small size in the prototype.(2)The ideas of the ultra-low frequency flextensional transducer with the superimposed structure are proposed with each flextensional shell of multiple elliptical shells mechanically serially connected.Each substructure of shell is excited by a single-rod rare-earth driver.In this paper,the frequency characteristics of the superimposed shells are analyzed.It is proved that the multi-shell superposition structure greatly reduces the structural rigidity with high effectiveness and feasibility to decrease resonant frequency.At the same time,the equivalent circuit of the superimposed structure transducer is deduced,and the mathematical expressions of the impedance and the frequency function of the transducer are obtained by using the branch impedance method.The superimposed transducer is optimized by the FE method.By analyzing of the displacement and phases of multiple key points on the superimposed shell,and calculating of the amplitude amplification ratio of each shell,it is quantitatively proved that the superimposed shell can realize the accumulative amplification of the multi-level displacements.The contribution of different positions of the shell to the overall vibration displacement is also analyzed in detail.It is concluded that the farther the shell is deviated from the structural symmetry center,the smaller the displacement contribution of the transducer is.Then,the changes of the performance parameters of the transducers with different numbers of shells superimposed were compared and analyzed in detail.It was proved that the more the number of shells is,the faster the resonant frequency of the transducer drops,whereas the more the number of shells is,the lower the TPR of the transducer is.An ultra-low frequency flextensional transducer prototype with a superimposed shell was developed and the maximum external dimension of the transducer wasφ230×630mm.It was tested on a lake experiment.The resonance frequency in water was130 Hz and the measured sound source level was 180.4d B,realizing ultra-low frequency,small size and medium transmission power performance.Focused on the requirements of low frequency and high power,this paper has desgined new structure,has applied new material and combined the characteristics of new materials and new structures to model and analyze physical processes such as magnetic field drive,electromechanical coupling,and acoustic radiation output.Based on the optimized configuration of each sub-structure,new application technologies have been utilized to develop high-power and high-efficiency driving vibrators and two new types of underwater acoustic transducers.The technology to realize low-frequency-high-power and ultra-low frequency acoustic emission has been explored.In this paper,these methods and technologies have made a good theoretical and practical foundation for the innovation of flextensional transducers. |
