Study On Flow And Noise Characteristics Of Jet Impingement

Jet impact noise widely exists in industrial production.The research on jet impingement noise mainly focuses on the high-speed jet,ignoring the aerodynamic noise problem generated by low-speed jet(v<0.4 Ma).In this paper,noise prediction research and noise mechanism are carried on around the jet impingement of two obstacles——inclined flat plate and long cylinder.Combined the large eddy simulation with Lighthill's acoustic analogy,this paper considers the factors which affect the accuracy of numerical calculation.such as grid distribution,mathematical control equations and acoustic integration surfaceThe numerical calculation results of the jet impingement on the inclined plate are compared with the experimental values,the results show that the noise prediction accuracy is verified.Continued researching on this model at different plate angles,different jet velocity and different jet impingement distance.The results show that the low-frequency noise characteristics of jet impingement are similar to free jet,which mean that the low-frequency noise is mainly radiated by the shear vortex shedding from the nozzle outlet.The jet impingement plate has obviously broadband peak noise than the free jet,and the flow impingement and the secondary jet on the surface of the plate contribute significantly to the mid-band noise.Velocity is the decisive factor for the total sound pressure level.The angle of the inclined plate will change the turbulent energy distribution,and thus influent the noise characteristics.The acoustic analogy method decouples sound from flow calculation solution without obtaining where the sound really coming from.It is impossible to describe the sound source distribution of complex flow.Therefore,based on the model processing and numerical method of jet impingement on plate,the mean square value of the pressure coefficient on the flow field is introduced to describe the pressure pulsation characteristics of the flow field.The energy spectral density of pressure coefficient is obtained by fast Fourier transform(FFT).The numerically calculated flow signal is correlated with the noise characteristics of the far-field experimental data.The results show that the characteristic parameters from the flow field monitoring points can reflect the characteristics of the flow noise.The spectral analysis of the characteristic parameters can be used to explain the frequency characteristics of the far-field noise,which can be the basis and research analysis tools of jet impingement aerodynamic noise prediction.Reasonably we can explain the distribution of the sound source through the strength of the sound dipole.The results show that the strength of the dipole and quadrupole acoustic source are both magnitude of 10~8,Which means that the noise is radiated by the two kinds of sound sources together.the dipole acoustic source mainly distributed on the impingement surface.the quadrupole acoustic source is distributed on the nozzle outlet and the impingement surface,and the intensity on the impingement surface is smaller than the dipole acoustic source.