VIBRATION OF 2 CONCENTRIC SUBMERGED CYLINDRICAL-SHELLS COUPLED BY THEENTRAINED FLUID

The vibrational characteristics of a point-driven ''double shell'' (tw o concentric submerged cylindrical shells coupled by the entrained flu id) are investigated theoretically and experimentally. Of particular i nterest are the shielding effects, if any, of the outer shell upon the inner shell. The theory on the double shell is based on Flugge's infi nite-shell equations, the Helmholtz wave equation, and boundary condit ions at the fluid-structure interfaces. This theory is used to model a finite double-shell structure in wave number-frequency space. Experim ents are carried out in which generalized near-field acoustical hologr aphy (GENAH) is employed to provide the experimental vibration charact eristics in wave number-frequency space of the finite double shell. It is confirmed theoretically and experimentally that the outer shell of the double shell exhibits two separate dispersion curves: A higher-fr equency dispersion curve exhibits in-phase vibrations with respect to the inner shell, and a second lower-frequency curve, out-of-phase vibr ation. The higher-frequency dispersion curve of the double shell is ve ry similar to the dispersion curve of the single shell (the inner shel l without the outer shell), and thus is identified as a forced wave nu mber response. The lower-frequency curve seems to be dependent on the free wave number response of the outer shell alone of the double shell . A double-shell structure can usually reduce its vibrational amplitud es by splitting the response of single-shell's forced vibration into t he responses of inner-shell's forced vibration and outer-shell's induc ed vibration. However, it radiates low-frequency underwater sounds ine vitably according to the lower-frequency dispersion curve. Furthermore , the appearance of the inner shell's dispersion curve on the outer sh ell seems to indicate that the shielding influence of the outer shell is not completely effective.