海底爆破辐射声场的理论及数值研究

对固体基底浅海中声波传播的色散特性作了数值模拟与分析. 利用液体及固体中声传播理论导出了固体基底内爆破源在浅海中所激发声场的解析解, 在此基础上, 通过数值计算, 得出了浅海中不同空间点处的瞬态声压时域波形, 并在瞬态声压时域波形的平滑伪Wigner-Ville分布基础上, 对固体基底内爆破源在浅海中所激发瞬态声场的特性作了详细分析. 关键词： 海底爆破 声场 色散 瞬态声压     

管道中激光激发瞬态导波的理论研究

利用本征函数展开法详细讨论了有限长管道中导波的激光激发，给出了当四个相同能量的激光脉冲轴对称打在管外壁上时，管壁上各点振动的瞬态解，通过数值计算，给出了主要模式的瞬态波形，且从色散关系出发对瞬态波形的特征进行了定性解释。     

北极冰-水耦合系统中声波传播特性

利用固体和流体介质中波传播理论,导出了冰-水两层复合结构中导波频散方程。进一步,利用二分法对频散方程进行了数值求解,得到了ω-k频散曲线(ω与k分别为圆频率和波数),以及相速度和群速度频散曲线。结果表明:冰-水两层复合结构中导波由具有相同厚度水层和冰层中导波耦合而成,但与水层和冰层中导波频散曲线相比,复合结构中导波频散曲线除第1阶模式外,其余高阶模式均发生了很大变化。从原水层第1阶模式的截止频率开始,复合结构第2阶模式的相速度曲线被压低,各高阶(大于2阶)模式的相速度曲线出现一个跃变点,群速度曲线出现一个极大和一个极小值。水层越厚,复合结构各高阶模式的截止频率越低,相同频带内导波模式越丰富。水层厚度保持不变时,复合结构各阶模式的相速度和群速度曲线均随冰层厚度的增加而向低频方向移动。另外,还进一步分析了冰-水复合结构的导波波结构,发现第1阶导波模式的能量主要集中在冰层内和海表面附近,而2阶以上高阶导波模式的振动位移幅度随深度方向呈现周期性特征,并且模式阶数越高,振动越复杂。      

Mechanism of the excitation of single pure mode L（0, 2） and its interaction with the defect in a hollow cylinder

Guided elastic waves have a great potential in pipe inspection as an efficient and low-cost nondestructive evaluation （NDE） technique, among which the wave of mode L（0, 2） receives a lot of attention because this mode is the fastest mode in a weakly dispersive region of frequency to minimize dispersion effects over a long distance and sensitive to the defects distributed circumferentially. Though many experimental and numerical researches have already been carried out about the excitation of L（0, 2） and its interaction with the defect in a hollow cylinder, its excitation mechanism has not been clarified yet. In this paper based on the transient response solution of the hollow cylinder, derived by the method of eigenfunction expansion, the theory about the exciting mechanism of mode L（0, 2） is advanced and the effects of the spatial distribution, vibration frequency and direction of the external force on the excitation are discussed. And the pure mode L（0, 2） is excited successfully under the parameters obtained through theoretical analysis. Furthermore, its interactions with some kinds of defects in hollow cylinders are simulated with the method of finite element analysis （FEA） and the results agree well with those obtained by other researchers.     

Analysis and improvement of sound radiation performance of spherical cap radiator

A spherical cap radiator is one of the important parts of an underwater wide-beam imaging system. The back radiation of a traditional spherical cap radiator, which is composed of a vibrating cap and a rigid baffle, is strong and its far-field directivity function may fluctuate in big amplitude in the vicinity of the polar axis. These shortcomings complicate the processing of the reflective waves received for imaging the targets. In this study, the back radiation is weakened by adding an acoustic soft material belt between the vibrating cap and the rigid baffle. And the fluctuation mentioned above is lowered remarkably by dividing the spherical cap radiator into many annuluses and a relatively smaller spherical cap, and by controlling the phase retardations of all elements appropriately. Furthermore, the numerical experiments are carried out by the finite element method (FEM) to prove the validity of the above methods.     

无限液体介质内管道轴对称纵向导波激发与传播特性研究

利用声-结构耦合有限元法,分别对轴对称分布径的向及轴向外力作用在无限液体介质内未充液及充液管道内壁所激发导波进行了模拟,并进一步利用短时傅里叶变换技术对瞬态波形作时频分析,在此基础上,结合色散及衰减曲线,探讨了外力分布特征对导波激发的影响。研究表明,无限液体介质内未充液管中L(0,2)模式及充液管中L(0,3)~L(0,4)模式皆具有高群速度、弱色散且弱衰减频带,适宜用于缺陷探测,而通过控制轴对称分布外力的频率,并令其沿轴向作用于管内壁或外壁,可实现上述导波模式的高效激发。