Numerical And Experimental Study On Turbo-machinery Ducted Mode Identification

With the advent of high bypass ratio turbofan engines,turbo-machinery noise has become a prior source of modern commercial aircraft propulsion noise.At present,the importance of turbo-machinery turbulent broadband noise is increasing gradually with the development of noise reduction technology for discrete tones due to rotor-stator interaction.In order to study the mechanisms of noise generation,or to design a new type of high efficiency noise absorption instrument,it is necessary to analyze the process of noise generation and propagation,identify modal distribution structure in flow duct of fan and also realize the spectral characteristics of turbulent broadband noise.In this paper,a systematic study of methodology and experimental application is carried out for mode decomposition and measurement of turbomachinery broadband noise.The main work and achievements are as follows:(1)Broadband noise mode coherence and identification method research.In this paper,an acoustic modal identification experiment research test platform was built,experimental measurement was conducted on ducted broadband noise generated by a single loudspeaker and a single-stage axial fan to obtain the mode coherence characteristics.Based on this thesis,this paper built an broadband mode determination method and solved the broadband mode identification problem.With the application of one-pulse-per-revolution method,a new rotating mode test system was designed.In the semi-anechoic chamber,noise performance of an axial fan was measured in detail with this new test system.The experimental results show that the internal modes within the flow duct of turbomachinery are statistically incoherent.The sound power level of incident sound waves is higher than that of the reflected waves,and the maximum disparity can be 10 dB.Modes with azimuthal order from-6 to +6 are more prominent below 1500 Hz in the incident sound waves,and the co-rotating modes carried more energy.(2)Theoretical and experimental research on fan turbulent broadband noise radiation.Based on Wiener-Holf method,numerical and experimental studies are conducted for multimode radiation.Each mode can be expressed as a function of mode cut-on ratio,the radiation function is related to the amplitudes and direction function of individual cut-on mode and the results increase with higher Mach number.It is verified experimentally for the first time in this paper that ducted modes meet the rule of equal energy per mode at high frequencies.A two-microphone method for mode identification is then established and developed.The sound power results and radiation characteristics of turbomachinery ducted noise are calculated successfully with the sound pressure spectrum acquired with two microphone.(3)New methods for mode identification.Based on the further study of broadband noise statistical mode coherence characteristics,two new methods for mode identification are established in this paper,named combined sensor array method and rotating array method.Compared to other methods,the required numer of microphones is dramatically reduced in these new techniques,this advantage is especially evident in the measurement of mid-high frequency duct noise and this method is very suitable for engineering applications.With the application of a ring and axial line sensors or rotating arrays,fan inlet modes can be determined accurately.The maximum deviation in the resolved incident sound waves is less than 1 dB,while the reflected mode waves calculated by combined sensor array method is consistent with that of rotating array mode decomposition method.(4)Then the thesis established an array design method for mode identification based on error theory.By using the technique of conditional number analysis in linear algebra,a preliminary study on the error transfer characteristic in mode identification procedure was conducted,the deviation propagation trends in circumferential and radial modes was analyzed numerically based on the established error evaluation method.The effect of different microphone arrays on modal identification is obtained by superimposing tiny perturbations into the artificial sound fields.The design guidelines of microphone array was further established.The results show that the radial rake has large deviation in mid-low frequency noise measurement,and is more suitable than the axial arrays for mode identification in high frequency range.However,the condition number of axial arrays is less than 10 in mid-low frequency,while the errors increase dramatically at high frequencies.m=0 mode is dominant in controlling the accuracy and mode decomposition frequency range,which is mainly consulted in the array design.The accuracy of axial arrays is sensitive to the microphone spacing.