Prediction of Propeller-Induced Hull-Pressure Fluctuations
The cavitating propeller often forms the primary source of noise and vibration on board ships. The propeller induces hydroacoustic pressure fluctuations due to the passing blades and, more importantly, the dynamic activity of cavities in the propeller’s immediate vicinity. The accurate prediction of the resulting vibratory hull-excitation forces is indispensable in the ship design process, but is not always warranted. From this follows the main objective of the thesis, which is the development of improved prediction capabilities for propeller-induced hull-excitation forces based on experimental and computational procedures. On the basis of experience and a literature study several topics have been selected that are considered most in need of improvement. On the experimental side, the model scale effect on the effective ship wake has been studied. An improved model testing procedure has been developed, which is based on the use of a geometrically non-similar model hull form designed by means of a RANS method. It is shown how the closer resemblance of the model’s wake field with that of the real ship improves the similarity of the propeller cavitation dynamics and thereby the prediction of the resulting first blade rate order hull-pressure fluctuations. On the basis of the boundary element method, a computational method has been developed for the computation of the scattering effect of the hull on incident pressures caused by propeller noise sources. The method has been validated with model scale experiments on propellers with and without cavitation. The same boundary element method has been used to correct for the influence of model hull vibrations on the assessment of hull-excitation forces. Guidelines have been derived for the execution of model scale experiments so as to minimize vibration-induced hull pressures. Inverse scattering techniques have been applied to the determination of the propeller source strength from measured or computed hull pressures. On the basis of the source strength, the pressure distribution on the hull may be derived from which effective vibratory excitation forces follow. It is proposed to use propeller noise source strengths or hull-excitation forces instead of local pressure amplitudes in contract specifications. It is strongly recommended that for the correct prediction of pressure fluctuations at higher orders of the blade passage frequency, tip and leading edge vortex dynamics are studied as well as the effect of gas content on the dynamics of the cavitating vortex.