| In recent years,the application of ultrasonic technology in military,security,medical and other fields makes nonlinear acoustics rapidly develop into a hot spot in the basic research and application of modern acoustics.The propagation of sound waves is a fundamental and permanent subject in acoustics.Sound waves can propagate in any medium,and the propagation characteristics of sound waves are closely related to the properties of the medium and its surrounding environment.No matter in theory or in practice,as long as it involves vibration or sound wave application technology is related to sound propagation.In this paper,the nonlinear propagation of ultrasonic wave in liquid medium,excitation of acoustic parameters and nonlinear effects are studied.The main research work and innovations are as follows:1.The nonlinear acoustic propagation in ideal liquid and viscous liquid is studied.Based on the continuum hypothesis,the nonlinear propagation equation of sound in an ideal liquid medium is derived by using the Taylor series expansion of the equation of state under adiabatic conditions,combined with the motion equation and continuity equation containing nonlinear terms.The wave equation is solved and the nonlinear wave solution under second order approximation is obtained.Considering the viscosity and heat conduction of liquid,the equation of state of liquid is modified.The second order nonlinear wave equation of viscous hot liquid is derived by using the above method.The wave equation is solved by numerical simulation,and the attenuation of sound energy and the distribution of sound pressure in viscous liquid are analyzed.2.The harmonic wave generated by ultrasonic propagation in liquid and its propagation law are studied.In this paper,a three dimensional nonlinear wave equation considering viscosity and heat conduction is obtained by introducing the modified equation of state from the basic fluid motion equation.The second and third harmonic expressions are obtained by successive approximation method.The higher order harmonic expression can be obtained by using the equation of state with higher order terms.In this paper,the harmonic wave and its propagation in single frequency radiation field are numerically analyzed,and the sound pressure distribution curves of fundamental wave and harmonics are obtained.It can be seen from the analysis that the fundamental wave has the largest sound pressure and the harmonic wave has the smallest sound pressure.The fundamental sound pressure decreases with the propagation distance increasing,while the harmonic sound pressure increases within the critical distance.In all harmonics,the second harmonic has the largest content.The generation and propagation of the second harmonic are further studied in this paper.The relation between the second harmonic and the driving sound pressure and frequency is obtained.When the fundamental frequency is constant,the sound pressure of the second harmonic increases with the increase of the driving sound pressure.But when the driving sound pressure is constant,the sound pressure of the second harmonic decreases with the increase of the driving frequency.3.The interaction between ultrasonic waves and excitation of acoustic parameters are studied in this paper.Based on the approximate method of solving the single frequency acoustic wave equation,the nonlinear acoustic wave equation is established when different frequency acoustic waves interact in the propagation process.The equations are solved by the method of step by step approximation,and the distribution curves of the sound pressure of the sum frequency wave and the difference frequency wave and the distribution law of the sound energy in the sound field are obtained by numerical analysis.It is found that with the increase of propagation distance,the content of sum difference frequency wave increases first and then decreases.Among them,the sum wave mainly concentrates near the sound source and decays rapidly,while the difference wave accumulates and decays slowly and spreads over a large distance.It can also be seen from the curve of sound pressure distribution that when the composition of incident wave is different,the sound pressure distribution of sum and difference frequency wave is also different.In this paper,the variation and distribution of fundamental wave sound energy in single and dual frequency radiation sound field are compared.It is found that when the total input sound energy is the same,the fundamental wave energy in the single frequency radiation field decays faster,while the sound energy in the far field is small and the main component is the fundamental wave energy.In the dual-frequency radiation field,the attenuation of sound energy is slow,the distribution of sound field is uniform,and the sound frequency in the far field is rich.4.The cavitation effect of ultrasonic wave in liquid and the influence of cavitation on sound propagation are studied.The propagation characteristics of ultrasonic cavitation field are closely related to the content,size and nonlinear vibration of cavitation bubbles.Therefore,the dynamic behavior of the cavitation bubble is firstly studied,and the relationship between the vibration of the cavitation bubble and the driving sound pressure,frequency and liquid viscosity force is obtained.The propagation model of sound wave in cavitation fluid was established by using helmholtz equation and the distribution of sound pressure in ultrasonic field was simulated numerically.The relationship between the size and content of cavitation and the sound pressure,frequency and the action time of sound wave is analyzed.It is found that the attenuation of sound pressure is obviously accelerated in the cavitation sound field.When the driving sound pressure increases,the sound pressure in the sound field decreases faster.Therefore,the abnormal attenuation of sound energy is more obvious in the high intensity sound field with cavitation. 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