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Modelling the vibroacoustic behaviour of an underwater vehicle is important to predict the impact on the marine environment, to ensure that the noise and vibration levels do not pose discomfort for crew, and to assist in identifying mitigation strategies to reduce underwater radiated noise. The motivation of this paper is to examine the vibroacoustic responses of a simplified physical model of an underwater vehicle corresponding to a finite cylindrical shell submerged in a heavy fluid of infinite extent, representing a classic fluid-structure interaction problem. In this work, the fluid-loaded cylindrical shell with different internal structures corresponding to a uniform shell without ring-stiffeners, a shell with ring-stiffeners, and a shell with ring-stiffeners and an internal plate are examined. The cylindrical shell is excited by either a ring force or a point force in the radial direction to excite the lowest order shell circumferential modes. The structural and acoustic responses of the submerged hull from low to medium frequencies are predicted. To achieve this, a range of techniques to cover and bridge the frequency ranges are implemented, as well as to observe the advantages and limitations of each technique. Due to structural complexity, theoretical models are prohibitively challenging and numerical simulations are preferred. Combining the finite element method (FEM) and the boundary element method (BEM) provides a powerful numerical tool at low frequencies. However, such a fully coupled FEM/BEM approach becomes limited at higher frequencies and subsequently requires a large number of elements to capture the dynamic behaviour of the structure as well as significant computational cost. Alternative predictive methods that use a reduced number of degrees of freedom typically include wave-based approaches and energy-based methods such as Statistical Energy Analysis (SEA). In an SEA model, the structure is modelled as an assembly of subsystems and considers the input power to each subsystem due to external excitation, the power dissipated via damping in each subsystem, and the power transmitted between subsystems. Results from SEA models are usually limited to high frequencies because of the underlying assumptions of high modal density and weak coupling between structural subsystems. In this work, a hybrid FEM/SEA method will be implemented to predict the vibroacoustic responses of the submerged cylindrical shell in the mid-frequency range. For comparison with the numerical approaches implemented here, a semi-analytical sub-structuring method is also used. The sub-structuring method can take into account geometric complexity by combining an analytical model of the global cylindrical shell and FEM models for the sub-structures corresponding to the ring stiffeners and internal plate. The semi-analytical method is based on the admittances at the junctions between the sub-structures as well as continuity conditions. Compared to the fully coupled FEM/BEM approach, the semi-analytical technique can be extended to higher frequencies and as such, can be used to bridge the frequency gap between the low frequency deterministic FEM/BEM approach and the mid-frequency hybrid FEM/SEA approach. The physical mechanisms into structure-borne radiated sound for the three cylindrical shell cases examined here are discussed.
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