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Abstract
An important cause of sound radiation from a submarine in the low frequency range is fluctuating forces at the propeller. The forces arise from the operation of the propeller in a non-uniform wake and are transmitted from the propeller to the submarine hull through both the shaft and the fluid. The sound radiated from the submarine is due to the combination of sound radiation caused by hull and propeller vibrations as well as dipole sound radiation from the propeller. To improve the stealth of a submarine, the radiated sound power can be reduced using passive and active noise and vibration control mechanisms. In this thesis, dynamic models of a submarine hull and propeller/shafting system are developed. To reduce the radiated sound fields, passive control is introduced using a hydraulic vibration attenuation device known as a resonance changer, which is implemented in the propeller/shafting system. Active control techniques are implemented using either tuned actuators or a control moment.
To initially obtain the structural and acoustic responses of a submarine hull, an analytical model and fully coupled finite element/boundary element model are developed, for a simplified physical model of the hull. The submerged body under axial excitation is modelled as a ring-stiffened cylindrical shell with finite rigid end closures and separated by bulkheads into a number of compartments. Lumped masses are located at each end to maintain a condition of neutral buoyancy. In the low frequency range, only the axial hull modes in accordion motion and axial vibration of the propeller/shafting system are examined, which gives rise to an axisymmetric case. The frequency responses, axial and radial responses of the cylinder and the radiated sound pressure from both the analytical and computational models are compared. A dynamic model of the propeller/shafting system developed computationally and including the resonance changer is then coupled to the FE/BE model of the hull which is subject to both structural excitation from the propeller/shafting system and acoustic excitation from the propeller. The influence of tailcone properties on the structural and acoustic responses of the submarine are investigated.
Passive control is implemented to attenuate the hull responses using a resonance changer. It is demonstrated that the performance of the resonance changer is negatively influenced at frequencies above the fundamental axial resonance of the hull by the effect of forces transmitted through the fluid. When the resonance changer is optimised to minimise excitation of the hull via the propeller shaft, the increased axial movement of the propeller results in an additional sound field that excites the submarine hull in a similar manner to the fluid forces that arise directly from the hydrodynamic mechanism. Cost functions that represent the submarine radiated sound power are developed, 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 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 is investigated. The influence of the reduction in amplitude for higher harmonics of the blade passing frequency on the control performance is also examined.
Different active control strategies are investigated, in which active control is applied to the propeller/shafting system and/or to the submarine hull. Active vibration control and discrete structural acoustic sensing based on the far field radiated sound power were considered in the development of the cost functions. In addition, the performance of a of a combined passive and active control system is investigated. Significant reduction of radiated sound power is achieved when an active control system using tuned actuators is combined with a resonance changer.
The structural responses of a model scale, free flooded submarine tailcone are investigated computationally and experimentally. The tailcone is represented by a thin-walled conical shell attached to a stiff plate. The stiff plate represents the pressure hull end plate of the submarine and is subject to axial excitation correlated to propeller forces. Good agreement between the computational and experimental results are found for the tailcone in air as well as for the submerged tailcone.