Fluid-structure coupling by the entrained fluid in submerged concentric double-shell vibration

Vibrational characteristics of a point-driven “double shell” (two concentric submerged cylindrical shells coupled by the entrained fluid) are investigated. Strong-and weak-coupling theories are proposed to model the interaction of how the inner and outer shells are coupled by the entrained fluid. Each theory is based on Flugge's infinite-shell equations, the Helmholtz wave equation, and boundary conditions at the fluid-struc tureinterfaces which depend on the coupling situation. Strong coupling forms a stand ingwavefield in the entrained fluid; weak coupling, a free wavefield. Experiments are carried out where generalized near-field acoustical holography (GENAH) is employed to provide the experimental vibration characteristics in wavenumber-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-frequency curve bears in-phase vibrations with respect to the inner shell and suggests a forced response of the inner shell; the other lower-frequency curve, out-of-phase vibrations, and an induced response of the outer shell. Fluid-structure coupling condition de finitelyaffects the lower-frequency curve, but barely the higher-frequency one. Com parisonbetween theoretical and experimental results demonstrates the validity of the strong-coupling model.