Active/Passive Noise Suppression Method And Mechanism Researches On Aeroacoustic Of Rotor(Tail-rotor)

At present,among the helicopter aeroacoustics,the rotor blade-vortex interaction noise is an important research subject in the field of helicopter aerodynamics.In this paper,the active and passive noise reduction methods and the noise reduction mechanism are investigated aiming at the rotor(tail-rotor)aerodynamic noise especially the BVI noise in forward flight.Based upon the acoustic analogy method and fluid-structure interaction method,a CFD/CSD/FW-Hpds integrated method is established to calculate the rotor(tail-rotor)aerodynamic noise.On this basis,the noise reduction mechanisms and acoustic characteristics of the BVI noise reduction by passive control method(mainly with different anhedral angles)in oblique descent flight are analyzed.The noise reduction mechanisms and parameter influences of the BVI noise reduction by IBC active control method,and the noise reduction characteristics of scissor tail-rotor are analyzed.The different noise reduction mechanism of the BVI noise by the active/passive noise reduction methods are revealed.Further,combined with the active/passive control methods,the rotor aerodynamic noise under different active/passive integrated control is analyzed and investigated,and the scissor tail-rotor noise under the interaction of main rotor wake is analyzed,too.Combining with the actual work environment of rotor and tail-rotor and the rule of noise radiation,a comprehensive noise reduction method of rotor(tail-rotor)aerodynamic noise is explored.The major investigation of this paper is as follows:As premise and background of the work,the purpose and significance of the present work are described in the first chapter.The helicopter rotor flowfield and aerodynamic characteristics calculation method,rotor aeroacoustics calculation method and rotor(tail-rotor)noise reduction methods are briefly reviewed.Also,the problems in present researches and corresponding methods which will be used in this paper are briefly introduced.Considering the complex motion characteristics of the rotor blade and the deformation characteristics of the elastic blade,a moving-embedded structure grid system including the body-fitted grids around blade and background grids are generated for the numerical simulation of the rotor unsteady flowfield in the second chapter.On this basis,an accurate CFD/CSD coupling method is developed for the calculation of the flowfield and the aerodynamic characteristics of the elastic helicopter rotor in forward flight.The compressible Reynolds averaged Navier-Stokes equations are adopted to simulate rotor flowfield for rotor noise computation.To take into account both computational accuracy and efficiency of present numerical methods,Roe-MUSCL scheme,implicit LU-SGS scheme and S-A turbulence model are employed.The CSD module is developed based on Hamilton's variational principles and the medium deformation beam theory.The corresponding information transfer strategy of the aerodynaimic/structural interface are given.By numerical example validations,it is demonstrated that the present CFD/CSD coupling methods satisfy high accuracy requirements on simulating the flowfield and aerodynamic characteristics of the rotor,which provides favorable foundations for futher investigations on rotor aeroacoustic prediction and noise reduction methods.Combined with the established flowfield solution method,a highly robust rotor aeroacoustic calculation method based on acoustic analog method are establised in the third chapter,the sound source integral surface is penetrable integral surface.On this basis,an aeroacoustic calculation model suitable for the rotor in forward flight are developed.By numerical example validations of UH-1H model rotor and AH-1G model rotor,it is demonstrated that the present rotor noise prediction method satisfies the high accuracy requirements on calculating the aerodynamic noise especially the BVI noise,which provides the basis for the investigation of noise reduction method.At the same time,a preliminary experimental research on the rotor BVI noise of is carried out in the anechoic chamber.The influence factors of the BVI noise are experimentally analyzed.The experimental results are compared with the calculated results,which proves the effectiveness of present method.The experimental results provide noise reduction strategies for BVI noise reduction method.Based on the flowfield and aeroacoustic calculation methods,the passive noise reduction method of the rotor aerodynamic noise mainly BVI noise has been invetigated in the fourth chapter.The acoustic characteristics of the rotor with different anhedral angles in oblique descent flight are analyzed,which demonstrates that the new blade-tip with reasonable anhedral angles can reduce the rotor noise effectively,especially the BVI noise in oblique descent flight.On this basis,a new low-noise blade is designed for a helicopter rotor.The aerodynamic noise comparisons between the designed rotor and the prototype rotor are studied,which shows that the low-noise blade can effectively reduce the rotor BVI noise.At the same time,the acoustic characteristics of the scissor tail-rotor with different blade shape parameters under different flight conditions are calculated and compared.Based on this,the noise of the scissor tail-rotor under the main rotor wake is also analyzed,and some new conclusions are obtained.The active noise reduction method of rotor BVI noise is researched in the fifth chapter.The aeroacoustic characteristics of the UH-60 A rotor in oblique descending flight are calculated with and without IBC control.Comparisons of the effects of different control parameters such as phase angle,amplitude and frequency on rotor aeroacoustic characteristics show that the negative pressure peak of the blade surface(especially the blade-tip surface)in the advancing side decreases with IBC active control,resulting in decrease of the blade airloads where the BVI phenomenon occurs.In addition,with IBC control,the blade-tip vortex concentration decreases,and the distance between the blade and the blade-tip vortex increases,resulting in significant reduction of the blade-vortex interaction noise.The BVI noise can be reduced about 7d B with reasonable parameter combination of phase angle,amplitude and frequency.On this basis,the noise reduction effects with IBC control under different flight conditions such as different flight velocities and different descent trajectories are given.The control effects show that BVI noise SPL can be effectively reduced by IBC control method under different flight conditions.In the sixth chapter,a comprehensive noise reduction method including different active and passive noise reduction methods are investigated.The rotor aeroacoustic characteristics under different control methods are stuied,and the noise suppression laws of different active and passive noise reduction methods are found out preliminarily,which provides an effective noise reduction method for BVI noise suppression.It can be seen that IBC active control method plays a major role in the process of comprehensive noise reduction.There are some differences in the control laws of IBC control.With IBC control,the propagation direction of noise is changed,which is benifit for noise reduction.Specifically,for the reference rotor,the IBC control has the best noise reduction effect at the advancing side,while for the low-noise blade rotor,the IBC control has the best noise reduction effect at the retreating side;after the IBC control,the propagation direction of the noise has also changed,which is conducive to noise reduction.Finally,the whole researches are summarized in the seventh chapter,the main innovations in the paper are listed and the prospects for the future work are described.