Research On Characteristics And Optimization Of Flow-Induced Noise In A Shaftless Pump-Jet Thruster

Among ship propulsion devices,the shaftless pump-jet thruster is one of the most cutting-edge and promising thrusters,especially in the military field.The low noise performance of shaftless pump-jet thruster has become a focus and hot field of current research.By combining experimental testing and numerical simulation methods,the energy performance,flow field characteristics and sound field characteristics of a shaftless pump-jet thruster are studied,and the effects of three structural parameters,namely,blade number,blade hub-diameter ratio and blade deflection angle on the flowinduced noise of the thruster are explored.Eventually the flow-induced noise optimization scheme is finally formed.The main work and research results of this paper are as follows:1.The development and evolution of ship propulsion technology are introduced in detail,while the design theory and method of pump-jet thruster and the performance research results and current situation are summarized.In addition,the shortcomings and development trends of the research on shaftless pump-jet thruster are pointed out.2.The energy performance and vibration characteristics of the shaftless pump-jet thruster under different flow rates are studied through the performance test.The test results show that:(1)With the increase of flow rate,the head and power of the thruster gradually decrease,and the efficiency first increases and then decreases,reaching the highest value under the design flow rate;(2)The main vibration frequency of the thruster is APF,while there are harmonic frequencies of 12,24 and 36 times APF at the exit of thruster.3.Based on the RANS flow field numerical simulation method combined with RNG k-? turbulence model,the energy performance,pressure distribution,velocity distribution,vortex structure and pressure pulsation under different flow rates of the shaftless pumpjet thruster are researched.The numerical simulation results show that:(1)With the increase of flow rate,the thrust of the thruster gradually decrease;(2)The area of the high pressure zone on the blade surface gradually decreases with the increase of the flow rate,and the overall pressure on the suction surface of the blade is lower,which is more likely to cause cavitation;(3)As the flow rate increases,the tip vortex group at the outlet section of the impeller gradually expands and strengthens,ultimately covering the radial center,while the wake vortex and cylindrical vortex in the outlet section gradually separate;(4)The main frequency of pressure pulsation is APF,and the amplitude at the main frequency near the wall decreases gradually along the flow direction,while it in the radial direction rises first and then decreases from the wall to the axis due to the influence of the vortex structure.4.Based on Lighthill’s acoustic analogy theory,the sound field in shaftless pump-jet thruster is numerically simulated,and the SPL frequency domain characteristics,OSPL characteristics and SPL distribution of the shaftless pump-jet thruster are studied.The numerical simulation results show that:(1)The SPL main frequency of the flow-induced noise of the thruster is APF,and there is a high peak value at 2APF;(2)With the increase of flow rate,the SPL at the main frequency near the wall area and the axis area increases gradually;(3)With the increase of flow rate,the OSPL in the thruster increases gradually,and it decreases gradually along the flow direction in the near-wall area.5.The effects of blade number,blade hub-diameter ratio and blade deflection angle on thruster flow-induced noise are studied based on the numerical simulation method,and the flow-induced noise optimization scheme is proposed on this basis.The research results show that:(1)As the blade number decreases from 7 to 5,the blade hub-diameter ratio increases from 0.2 to 0.25 and the blade deflection angle increases from-2° to +2°,the flow-induced noise of the thruster gradually decreases,while the maximum reduction of SPL amplitude at the main frequency of each scheme are 5.70 d B,1.66 d B and 1.82 d B,respectively and he maximum reduction of OSPL are 3.99 d B,1.06 d B and 1.69 d B,respectively;(2)The optimized scheme can make the maximum reduction of the SPL amplitude at the main frequency and the OSPL reach about 4.86 d B and 4.93 d B respectively while basically maintaining the energy performance of the thruster.