Numerical prediction of the signature of maritime platforms

Prediction of noise emitted by maritime platforms is an important topic in naval research. Underwater noise radiated from marine vessels is a significant source of pollution in the marine environment and a major source is the propeller. This paper outlines the work by the Vibration and Acoustics Group at the University of New South Wales on the development of predictive models of the vibro-acoustic responses of a submerged hull due to propeller excitation. Early work saw the development of analytical models of a simplified idealisation of a submerged hull, in which the submerged body was represented as a fluid-loaded ring-stiffened cylindrical shell with internal bulkheads and end closures. Only the structure-borne transmission path from the propeller through the shafting system to excite the hull was considered. These initial analytical models only considered the zeroth circumferential shell modes resulting in breathing hull motion, although later models showed that the bending hull modes also play a significant role on the structure-borne radiated sound. The availability of greater computing power gave rise to the development of fully coupled 3D finite element/boundary element (FE/BE) models. The vibrating structure is modelled using quadratic shell elements while the infinitely extended fluid domain is represented by linear boundary elements. The two-way fluid-structure interaction between the hull and surrounding fluid medium using a mortar coupling scheme has been implemented. In this FE/BE approach, both structure-borne and fluid-borne transmission of the propeller noise sources are included. More recent work has investigated the hydrodynamic noise sources generated by a marine propeller. Sound waves are generated by fluctuations in the flow around the hull and its propeller. Accurate resolution of these fluctuations requires detailed computational fluid dynamics (CFD) models with an enormous computational cost and data storage requirement. A hybrid CFD/BEM technique will be presented in which acoustic sources based on Lighthill's acoustic analogy are extracted from transient CFD simulations to predict both the direct and scattered transmission paths from the hydrodynamic sources to a receiver.