Abstract
A pump-jet without tip clearance is proposed to propel a scaled SUBOFF model. The hydrodynamic performance of the pump-jet behind the SUBOFF, modal characteristics and vibro-acoustic responses of the SUBOFF – pump-jet under unsteady excitations are reported and compared with those by the pump-jet with a 2 mm tip clearance. The hydrodynamics are obtained by CFD (Computational Fluid Dynamics) and vibro-acoustic responses are predicted by coupled FEM (Finite Element Method) and direct BEM (Boundary Element Method). The CFD model of the pump-jet without tip clearance is validated by experimental results in a cavitation tunnel through steady hydrodynamics. Fluctuation pressure, standard deviation of fluctuation pressure and spectra of the new pump-jet behind SUBOFF are demonstrated. It is shown that fluctuation pressure on a single blade and fluctuation force of the rotor are suppressed remarkably, as well the amplitudes at BPF (blade passing frequency) and its multiples is greatly attenuated for the monitoring points on the rotor and duct. Since a shroud is added for rotor blades and stiffness is presented between the shroud and the duct due to hydrodynamic lubrication, a new mode of coupled rotor and duct deformation is borne and modal frequency at longitudinal mode of shaft is increased due to change of the cantilever supporing state of rotor blades. New force transmission paths for the rotor via the duct to excite the hull, and that for the duct via the shafting to excite the hull are presented, which will transmit part of vibration energy when applying excitation forces on the rotor or duct. The sound radiation power level is greatly suppressed after eliminating the tip clearance. Especially at the characteristic structural modes due to combined effect of alternating the structural modes to be far away from the characteristic frequencies of excitation forces and suppressing fluctuation pressures on the rotor and duct.
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