Theoretical Study On The Visualization Of Dipole Source On Rotating Blades

A major aeroacoustic source in helicopter and wind turbine is the dipole source on rotating blades.Acoustic invisibility of a helicopter is determined by the nosie level of rotating blades,and the level will cause safety and reliability problems.Also,noise radiated by a wind turbine will cause impacts on the nearby residents.Thus,it is an important topic on identification of sources and reduction of noise level on rotating blades.Beamforming is an typical sound visualization technique that has been widely used in wind tunnel tests,and is a hot topic in the aeroacoustic research areas.The propagation of dipole source is added to the conventional beamforming method in this study,which aims to solve shortcomings and limitations of the existing acoustic imaging methods.Investigations were performed for static and rotating dipole sources,including wind tunnel tests for rotating dipole sources.The proposed dipole based beamforming method is capable of locating both dipole and monopole sources in a same scheme.Besides,a extracting method was proposed in order to separate the source and background noise containment in the cross-spectral matrix.The extraction method is designed for cases of complex background noise and low SNR,which could usually be found in wind tunnel tests.This work was supported by the NSFC of China(NO.11574212)and the U21 Graduate Collaborative Research Award“Asia-Pacific Collaboration in Flow-Induced Noise and Vibration”.The main contents of the dissertation are summarized as follows:(1)The theory of the static and rotating beamforming are derived,which is based on the assumption of monopole sources.Results of time domain and frequency domain beamforming are simulated,and the shortcomings and limitations are listed.(2)A dipole based beamforming method is proposed in this study,which aims to locate monopole and dipole sources without any prior knowledge.Orientations of dipole sources are no longer needed,and the source type between monopole and dipole is not necessary in the proposed method.Besides,source type of a dipole and a monopole source could be found by the method at some cases as illustrated in discussion part of the method.(3)The orientations of rotating dipole sources in rotating blades are analysised,which is illustrated as a function of azimuth and pithch angle of blades.A dipole based beamforming method is proposed based on the knowledge of rotating dipole,and the method is capable of locating rotating monopole and dipole sources at an arbitrary array position.(4)A separation method is proposed in this study which aims to separate source and background noise in CSM.The two parts in CSM is separated based on low-rank and sparsity of matrix,which correspond to source and noise containment in the CSM.The method could exhibit better performance in suppressing background noise especially for strong and complicated background noise.(5)The proposed methods have been verified by a series of simulations and experiments,which includes measurement of a small UAV rotating blades in the semi-anechoic chamber room,a wind turbine model with three NACA0012 blades,and a scale model of helicopter rotor in the wind tunnel.Satisfactory results were presented in locating dipole sources for different measuring environments and setups.