The Vibration and Sound Radiation Characteristics of an Underwater Non-circular Shell Covered with a Free Damping Layer
|Keywords||non-circular shell damping layer sound pressure level sound power level radiation efficiency level average vibration velocity level|
Underwater vehicle includes manned and unmanned vehicle. They can implement underwater exploration, detection and even military attack and defense. In the present, ocean development is increasingly important. Underwater vehicle increasingly got the attention of all countries, for it has played an important role in the civilian and military affairs. With the development of modern social science and technology, high demand and challenge at the hidden feature of the underwater vehicle are undoubtedly put forward. Because radiation is an important indicator of submarine’s overall performance and the direct impact of hidden factors, it attracts much attention from internal and external research institutions. Currently, submarine is often simplified to simply supported cylindrical shells by scholars, but non-circular shell has not been extensively investigated. For the mechanism design of submarine is more and more complex and diversity, it can’t meet the actual and engineering needs to simplify submarines as cylindrical shells. It is imperative to study the vibratory and acoustic characteristics of non-circular shells.Based on the above reasons, this paper used radiation sound pressure level、sound power levels、radiation efficiency and surface vibration average velocity magnitude as model evaluation indexes, to explore the acoustic radiation characteristics of a cylindrical shell under unit harmonic force in infinite ever-flowing field, considering the effect of free damping layer.Ignoring the mass of free damping layer and turning it into simplified massless spring, the vibration energy was consumed by tension and compression deformation. Based on classical shell theory, this paper deduces underwater vibration control equations of the non-circular shell covered by a free damping layer. By using Fourier integral transform, the structure vibration control equation is simplified to one-order variable coefficient differential equations. Through the application of the precise integration method, the surface vibration displacements expressed by the shell surface displacementμ,v,w are obtained. To radiation pressure, synthesizing Helmholtz’s equation、Neumann’s boundary conditions and the Sommerfeld’s far field radiation conditions, with virtual boundary integral method and the virtual boundary element method, the acoustic pressure expressed by damping layer surface displacement is obtained. Finally, the coupling vibration equation of shell in flow field is solved. In numerical solution, this paper discusses the influence of complex stiffness of the damping layer, non-circular shell ovality and thickness on the non-circular shell vibrational radiation characteristics. Certain references for low noise design in underwater non-circular shell are provided.The results show that the sound pressure on the non-circular shell surface has greatly been reduced by covering free damping layer and its value increases when the complex stiffness increases. The radiation sound pressure level and the mean vibration velocity level reduced Significantly for being covered the damping layer. The radiation sound pressure level, the mean vibration velocity level, radiation efficiency reduce with reducing of the complex stiffness. Sound radiation reduces greatly with complex stiffness Z decreasing. The relationships of the sound radiation with the ellipticity and the thickness are complex.