Structural and acoustic responses of a submerged hull

Abstract

Underwater radiated sound from marine vessels is a significant problem for research, fishing and military vessels, and is a major source of pollution in the marine environment. Vibrational modes of a marine vessel are excited by forces from the propeller that are transmitted via the propeller-shafting system, resulting in subsequent sound radiation. Structural forces arising from on-board machinery also excites the vessel and contributes to structure-borne sound. The aim of this thesis is to characterize the vibro-acoustic responses of a submerged pressure hull under external harmonic force excitation from the propeller-shafting system and internal excitation due to on-board machinery. The pressure hull is simplified as a fluid-loaded cylindrical shell. Both deterministic and statistical models are developed. At low frequencies, the structure is modelled using finite elements while the surrounding water is modelled using boundary elements. At higher frequencies, a hybrid finite element/statistical energy analysis (FE/SEA) method is implemented, in which a block mass to simulate on-board machinery is modelled as a deterministic subsystem with low modal density, while the cylindrical shell and an internal plate on which the rigid mass is mounted are modelled as statistical subsystems with high modal density. A hybrid FE/SEA model of nested cylindrical shells is also developed. Annular ribs are used to link the inner and outer shells. To examine the effects of mass distribution along the length of the submerged pressure hull, a semi-analytical model of a fluid-loaded cylindrical shell comprising a linear array of evenly distributed dipole sources is presented. The radiated sound of a neutrally buoyant cylinder with uniform and non-uniform stepwise mass distribution in the longitudinal direction of the cylinder is then investigated. The effect of non-uniform mass distribution on peak sound power of the first few cylinder bending modes is described. Finally, experiments have been conducted to measure the natural frequencies and mode shapes of a scaled coupled cylinder/plate structure in air. A freely-suspended open cylindrical shell with an internal rectangular plate longitudinally connected to the shell via brackets is considered.