Minimisation of the sound power radiated by a submarine through optimisation of its resonance changer

An important cause of sound radiation from a submarine in the low frequency range is fluctuating forces at the propeller. The forces are transmitted to the hull via the shaft and the fluid. Sound radiation occurs due to hull and propeller vibrations as well as dipole sound radiation caused by the operation of the propeller in a non-uniform wake. In order to minimise sound radiation caused by propeller forces, a hydraulic vibration attenuation device known as a resonance changer can be implemented in the propeller/shafting system. In this work, cost functions that represent the overall radiated sound power are investigated, where the virtual stiffness, damping and mass of the resonance changer were chosen as design parameters. The minima of the cost functions are found by applying gradient based optimisation techniques. The finite element and boundary element methods are used to model the structure and the fluid, respectively. The adjoint operator is employed to calculate the sensitivity of the cost function to the design parameters. The influence of sound radiation due to propeller vibration on the optimisation of the resonance changer as well as the influence of the reduction in amplitude for higher harmonics of the blade-passing frequency on the control performance is investigated.