Study On Pressure Fluctuation Of Propeller And Induced Hull Vibration And Cabin Noise

Nowadays,with the accelerating pace of ship developing towards the trend of large-scale,high-speed and lightweight structures,larger engine power is required,thus resulting in increasingly serious vibration and noise problems.According to the statistics,80%cases of harmful vibration on ships are caused by propeller excitation forces,among which pressure fluctuation is the dominant factor.The traditional research methods for solving pressure fluctuation have their respective shortcomings.With the rapid development of technology of computer hardware and software,significant improvement on accuracy and stability of calculation can be achieved by adopting CFD numerical method solving complex flow field problems,which provides a new approach for carrying out research on pressure fluctuation.In this paper,CFD numerical method is employed for the investigation on predicting and controlling pressure fluctuation induced by propeller.To improve the accuracy,numerical results of pressure fluctuation are applied to the prediction of hull vibration and cabin noise.The main work is as follows:1)Based on CFD method,numerical simulations are performed to predict pressure fluctuation induced by the propeller of a coastal passenger ship.A ship-propeller model,based on sliding mesh technology,is established for transient calculating.Some MATLAB programs are written to achieve coordinate importing,pressure monitoring,and data processing for batch of monitoring points.The reliability of the model is validated through comparing the calculated values with test values of the ship resistance at different speeds.The distribution of pressure fluctuation on the D×D area of hull bottom is detailedly analyzed,and the variation laws of pressure fluctuation are respectively discussed under series of tip clearance ratios,rotational velocities and ship speeds.2)Based on CFD method,the research on controlling pressure fluctuation induced by propeller is carried out through proposed two solutions,including changing blades number and introducing air curtain.The results show that both methods have significant efficiency.When the number of blades respectively increases from 4 to 5 and 5 to 6,the attenuation of pressure fluctuation peak can reach23%and 51%respectively.When the air flow is set between 0.012~0.024m~3/min,the attenuation of pressure fluctuation peak rises from 40%to 54%.From the angle of structural response,the induced structure-borne sound will drop by 6dB with 50%reduction of excitation.3)Based on FEA,numerical simulations are performed for global vibration analysis,local vibration analysis and forced vibration analysis for the coastal passenger ship.Basic principle of the vibration analysis for hull structure is introduced,and the finite element model of the whole ship is established.Global vibration for the whole ship and local vibration for decks are analyzed respectively,and the natural frequencies of both are assessed according to corresponding criterion.The calculation accuracy of hull vibration response respectively induced by three propeller excitations scheme are analyzed and compared,and result shows that prediction accuracy can be observably improved when the pressure fluctuation calculated by CFD method is applied to hull vibration analysis.4)Based on SEA,cabin noise is predicted and evaluated for the coastal passenger ship.The SEA acoustic model for whole ship is established,and calculation methods of acoustic parameters such as internal loss factor,coupling loss factor and input excitation are determined.The prediction accuracies of different frequency truncation models are discussed.The prediction accuracies of cabin noise respectively induced by two propeller excitations scheme are compared and analyzed,and results demonstrate that precision can be significantly improved when the structure-borne noise calculated through CFD-FEA approach is applied to cabin noise prediction.