Blade-Vortex Interaction Noise Suppression Method Researches Based On Harmonic Control

With the wide application of helicopters,not only have the advantages of helicopters been discovered,but also the influence of helicopter noise has been paid attention to.Serious rotor blade-vortex interaction(BVI)phenomenon occurs when helicopter operates in low speed oblique descent flight.As an important part of rotor aerodynamic noise,the reduction of blade-vortex interaction noise is always a challenging subject.Researchers are committed to using active control to suppress BVI noise.In this thesis,on one hand,the methods of computing rotor aerodynamic loading and noise under balde-vortex interactional state are investigated.On the other hand,the noise reduction mechanism and parameter influence of harmonic control method are analyzed in order to find an effective control scheme.The major contributions of the author's research work are as follows:As the background of present work,the objectives of the research are firstly described.The research and development in the field of the methods about rotor wake analysis,rotor aerodynamic noise calculation and BVI noise control are briefly introduced.The difficulties and deficiencies in the current research are also pointed out.In addition,the solutions used in the present research as well as the direction of the work are briefly brought forward.A method based on free-wake model for computation of the rotor aerodynamic loading is established in the second chapter.The method is used to compute the loading distribution on blade surface,free-wake geometry and blade tip vortex trajectories.In the third chapter,a method based on Farassat Acoustic Formula 1A in time domain for rotor BVI noise prediction is developed.The method is used to investigate the distribution and radiation characteristics of BVI noise.In the fourth chapter,the mechanism of BVI noise is analyzed and the influence of three factors on BVI noise are discussed: the distance between the blade and tip vortex,the blade-vortex angle and the intensity of tip vortex.The best scheme of harmonic control to reduce BVI noise is selected according to the sweep of phase angle,amplitude and frequency.The reasons of noise reduction effect changes caused by different control parameters are analyzed.In the fifth chapter,the multi-frequencies harmonic control method is preliminarily investigated and the BVI noise variation under different flight state parameters is discussed.Finally,the whole research work in this paper is summarized,the innovation of this paper and deficiencies in the research process are briefly pointed out,and the prospects for the future work are described.