Wave Propagation Characteristics In Complex Media

In this thesis,I studied several selected topics on the elastic wave propagation characteristic in complex media,which mainly consists of three aspects: theoretical study of the reflection and refraction problem of finite amplitude acoustic waves on fluid-fluid interface,numerical study of elastic wave propagation characteristic in media with vertical gradient,and numerical simulation design and experimental research of equivalent gradient acoustic lens.The main work and conclusions are summarized as follows:1.Theoretical study for the reflection and refraction problem of finite amplitude acoustic waves on fluid-fluid interface is introduced.1)The fundamental and harmonic wave equations of one-dimensional problem are derived.With the help of source conditions and interface continuity conditions,harmonic solutions of reflection and refraction problems on the interface are derived in detail.2)The fundamental and harmonic wave equations of two-dimensional problem are derived.Using the method of separation of variables and the method of variation parameters and under source conditions and interface continuity conditions,harmonic wave equations are solved.The expressions of harmonic solutions for interaction region of incident and reflection in fluid I and for transmission region in fluid II are finally obtained.3)The difference on the scope of application between the harmonic solutions in precious studies and the harmonic solutions herein for two-dimensional problem is discussed.2.Elastic wave propagation characteristics in media with vertical gradient are studied numerically.1)Wave equations in vertical gradient medium expressed by different parameters for time domain,frequency domain problems and their dimensionless forms are deduced.Governing equations and boundary conditions for numerical simulation in finite element software are also derived.2)The feasibility of beaming focusing in medium with vertical gradient is proved by numerical simulation in frequency domain,and the influence of wave velocity gradient on beam focusing is analyzed.3)Using the time domain wave equation,singular sinusoidal pulse propagation in both homogeneous medium and vertical gradient medium with hyperbolic secant wave velocity distrution is simulated,and the propagation characteristics in both media are studied.The influence of wave velocity gradient on singular sinusoidal pulse focusing is discussed.4)Through numerical simulation of wave propagation in vertical gradient medium with limited size along direction of wave propagation,the influence of the size of vertical gradient medium on focal length and focusing intensity is analyzed.3.Simulation design and experimental research of equivalent gradient acoustic lens are carried out.1)One kind of planar gradient acoustic lens is designed.Both numerical simulation and experimental research are made.The downside of this design approach in practical engineering application is discussed,espacually in determinating the working frequency of acoustic lens.2)According to the results of numerical and experimental study,Rayleigh-Bloch wave mode theory is supposed to be an important physical mechanism of acoustic beam focusing in muti-layer planar gradient acoustic lens.3)There is an optimum working frequency for beam focusing of this kind of planar acoustic lens according to RB wave mode theory.The ratio of corresponding wavelength to lattice size is 3.8.The design procedure of planar gradient acoustic lens is improved to be more applicable on the basis of RB wave mode theory.According to the new design procedure,acoustic lens with designed working frequency are numerical simulated and experimentally realized.The research tests and verifies the availability of the improved design method.4)A radial gradient acoustic lens is designed and produced to reduce the intense acoustic scattering in planar gradient acoustic lens.Simulation results indicate that the design is feasible for attenuating acoustic scattering while improving focusing effect.Experimental measurements show that the focusing gain reached high to 14 dB.The conclusion is that radial gradient acoustic lens can improve focusing effect and increase beam focusing gain.