Study On The Flow And Acoustic Mechanism Of Noise Reduction Of Turbomachinery Blade With Bionic Configuration

With the development of large aircraft industry and the implementation of major special projects for large-scale aircraft and aero-engines in our country,aircraft and aero-engine noise problems are currently receiving more and more attention.Aero-engine is the main noise source of aircraft.With the increasing of the bypass ratio of aero-engine and the development of tone noise reduction technology,aero-engine broadband noise becomes more and more important.The reduction of engine blade leading edge and trailing edge broadband noise is one of the difficulties and hotspots of the current research.This paper carries out systematic experimental and numerical investigation on broadband noise reduction of blade/turbomachinery using trailing edge serrations and wavy leading edges inspired by the silently-fly owl and humpback whales,respectively.The main research work and contributions are as follows:(1)In order to accurately identify and investigate the noise characteristics of blade leading and trailing edges,conventional beamforming algorithm(CB algorithm)and CLEANSC deconvolution algorithm for the microphone array data processing are developed and improved.The CB algorithm and CLEAN-SC algorithm are verified using the international standard test cases.The results of identification of sound sources with different degrees of complexity indicate that the CLEAN-SC algorithm has obvious advantages over the CB algorithm and can significantly suppress the side lobes.The CLEAN-SC algorithm developed in this paper shows good accuracy and robustness.The identification error of the complex standard sound source is within 0.2 dB across a wide frequency range.The algorithm can provide a reliable experimental method for the follow-up research.(2)Based on large eddy simulation(LES)and acoustic analogy theory,a hybrid flow/acoustic field calculation method for blade broadband noise prediction in free-field environments is successfully developed and validated using the canonical rod-blade interaction experiment.In addition,based on the unsteady Reynolds-averaged Navier-Stokes equations(URANS)and Goldstein's duct acoustic equations,a hybrid flow/acoustic field prediction model for ducted axial fan noise prediction is developed and validated using axial fan noise experiment carried out in a semi-anechoic chamber.The validation results show that the hybrid flow/acoustic field prediction model can predict the sound field within an error of 2 dB and,therefore,can provide a reliable numerical simulation tool for subsequent research.(3)Taking the NACA65(12)-10 blade as research object,the noise reduction effect of trailing edge serrations and wavy leading edges on blade traing edge and leading edge noise is experimentally investigated using the mircrophone array.The effects of trailing edge serration/wavy leading edge amplitude,wave length,the incoming velocity,the incoming attack angles and different incoming turbulence on the noise reduction are studied.The key parameters affecting the noise reduction effect of trailing edge serrations are identified as well as the basic rules of noise reduction.A critical length of trailing edge serrations and the best parameter range of wave length of trailing edge serrations are found to obtain significant noise reduction.In addition,a quantitative correlation between the noise reduction and the trailing edge serration/wavy leading edge amplitude is obtained.The trailing edge serrations can reduce overall sound pressure level of trailing edge noise as much as 8 dB in the experiment while the wavy leading edges can reduce overall sound pressure level of leading edge noise as much as 17.7 dB for rod turbulence-blade interaction and about 10 dB for grid turbulence-blade interaction.(4)The noise reduction mechanism of trailing edge serrations and wavy leading edges is investigated using the hybrid LES/acoustic field prediction technique and the predicted noise reduction is in good agreement with the experiment.Results show that trailing edge serrations can generate strong streamwise vortices at the edges of the sawtooth roots,suppressing largescale spanwise vortex shedding at the trailing edge of the straight trailing edge blade.Blade lift and drag fluctuations are greatly reduced.In addition,trailing edge serrations can significantly reduce the pressure fluctuation amplitude around the transition zone on the blade pressure side surface.The spanwise correlation and coherence of pressure fluctuation are also reduced while the variation of phase of the pressure fluctuation along blade span is increased,leading to the noise reduction.Wavy leading edge can significantly reduce the pressure fluctuation around the blade leading edge and blade unsteady load.In addition,the correlation and coherence of pressure fluctuation along span is also reduced,companied with a reduced spanwise coherence length.The phase of the pressure fluctuation along blade leading edge span is greatly changed,leading to destructive phase interference and noise reduction.(5)The noise reduction effect of wavy leading edges on real axial fan tone and broadband noise is numerically investigated.The numerically predicted noise reduction amplitude and rules agree well with experiment results obtained in the isolated blade case.It is pointed out and proved for the first time that under similar conditions,the noise reduction law obtained on the isolated blade is also applicable to real axial fans,which is of great significance for the wavy leading edge design for real fans in the future.LES is also carried out to investigate the fan noise reduction mechanism of wavy leading edge vanes.It is found that the noise reduction mechanism of wavy leading edges on an axial fan is similar to that on an isolated blade.Moreover,it is pointed out that the reduced pressure fluctuation amplitude is largely due to the oblique interaction between the incoming turbulence and the wavy leading edges,which can successfully explain why the bigger the ratio of the amplitude and wave length of wavy leading edge(A/W),i.e.the larger the angle of inclination,the more noise reduction.The research work of this thesis lays a solid theoretical foundation and develops scientific research methods for noise reduction of fans,and gains important technical reserves for aero-engine noise reduction.