Experimental And Numerical Study On Mechanism And Suppression Method Of Turbo-machinery Broadband Noise

Aerodynamic noises generated by turbomachinery dominate the spectrums of aircraft engine in most cases.There're two main components:discrete tones due to rotor-stator interaction and turbulent broadband noises produced when airflows pass the blade rows.The importance of turbulent broadband noise is increasing gradually with the development of noise reduction technology for discrete tones.Normally,the broadband noise generated by turbulence-blade interaction is the main source of turbulent broadband noise,while trailing edge noise?TE noise?defines the minimum achievable noise of a turbomachinery.However,there're evidences to say that TE noise may be crucial in some cases,especially when tones generated by laminar boundary layer?LBL?or vortex shedding from blunt trailing edge exist.Focusing on the characteristics,generation mechanism and control technology of turbulent broadband noise,the present paper carried out theoretical and numerical studies,following aspects of the subject are addressed in this dissertation:?1?The source identification method based on microphone array in free field is expanded to deal with the problems generated by acoustic measurement in normal test bed.?2?The source identification method in normal test bed is used to investigate three questions:the relative importance of leading edge?LE?and trailing edge noise for slightly curved blade,the characteristics of trailing edge noises radiated from slightly curved blade and cascade with large turning angle,and the noise reduction potentials of serrated trailing edges which are set on slightly curved blade and cascade with large turning angle.?3?Mixing method of flow field and sound field is used to investigate two questions:the generation mechanisms of trailing edge noise generated by slightly curved blade and cascade with large turning angle,and reduction mechanisms of trailing edge noise by use of serrated trailing edge.?4?Improved empirical methods for fan and low pressure turbine noise prediction are developed.The characteristics of broadband nose generated by fan are investigated.The main conclusions as follows are obtained through above researches:?1?Clear sound results from a target sound source can be obtained by use of well designed microphone array and inverse methods.The influences of reverberation,standing wave,background noises and shear layer correction in a normal test bed should be considered firstly.?2?The trailing edge noise of a slightly curved blade is as important as the leading edge noise under the given experiment conditions.It is found that overall sound pressure level?OASPL?varies approximately as U^6.5 and U^5.1?U is the mean flow velocity?for the leading edge noise and trailing edge noise respectively when flow attack angle equals zero degree.A U^6.1 scaling law for the trailing edge noise of cascade with large turning angle is also found.When serrated trailing edges are installed,the scaling law of trailing edge noise turns into U^6.6and U^5.7 for slightly curved blade and cascade respectively.At the same time,the overall sound pressure level is reduced up to 9.2 dB and 5.9 dB for the corresponding object.?3?In low turbulence and moderate Reynolds number condition,the contributions of flow fluctuations generated during the transition process of laminar boundary layer?LBL?are very important for TE noises radiated from slightly curved blade and cascade with large turning angle.The generation mechanisms of trailing edge noise for cascade include two aspects:the characteristics of flow fluctuations in upstream boundary layer and downstream viscous wake.?4?In low turbulence and moderate Reynolds number condition,the value of noise reduction by use of serrated trailing edge is up to 14 dB?OASPL?for both of slightly curved blade and cascade with large turning angle.There're two main reasons for the noise reduction:the amplitude of pressure fluctuations within LBL on the surface is reduced remarkably,and the span wise correlations near trailing edges decrease while stream wise vortices increase with the application of serrated TE.Moreover,vortex shedding from blunt TE of cascade is also eliminated by these broken-type saw teeth.?5?Contribution of fan broadband noise to fan total noise decreases with the increase of engine power.The broadband noise can account for up to 90%or at least 5%of the total noise under different polar angle.Generally speaking,higher contribution of fan broadband noise to fan total noise can be obtained with greater bypass ratio.