Noise Analysis And Large Eddy Simulation Of A Round Jet From A Nozzle With Porous Inner Wall

Jet noise is one of the main noise sources of civil aircrafts.Their sound pressure level may reach 150?160dB,which is greatly harmful to the environment.So far,attenuation of jet noise is mainly achieved by reducing the large-scale vortical structures in the turbulent jet flows.Among them,passive controlling methodologies have been paid more attention,because they have no need of additional mass or momentum,and these methods are relatively easy to be implemented.Porous treatments are a kind of effective passive controlling methods for flow and noise.However,the application of porous nozzle wall for jet noise controlling has not been tried by researchers,as unveiled in the literature search.In this paper,the passive controlling method of changing the nozzle into porous inner wall is numerically investigated so as to find its effects on the jet flow and noise.The main contents are two parts are as follows.Firstly,the compressible round jet is investigated by large eddy simulation.The jet Mach number is 0.75 and the Reynolds number is 50000.Numerical results are validated by available experimental data.Two major stages for the shear layer instability development are observed and analyzed.After this,two-point space-time correlation of velocity fluctuation and pressure fluctuation in the shear layer are investigated.Their distribution and evolution in time and space are presented and discussed.The migration characteristics of vortices are analyzed.The study provides a base for further investigation and controlling of jet noise.In the second part,the inner wall of a subsonic nozzle is changed to be porous by adding blind holes.Large eddy simulation combined with the FW-H method are employed to study the unsteady flow field and far-field noise radiation at Mach number M=0.6.The results show that the porous wall of the nozzle has some effects on reducing the turbulence intensity.Comparisons of the turbulence correlation coefficients reveal that both the temporal and spatial correlations of the turbulent fluctuations are also suppressed by the porous inner wall.Large scale turbulence structures are weakened.The generation,development and evolution of vortices in the shear layer are studied by visualization and analysis of the vorticity,the dilatation and the Q-criterion.The results show that the pairing of vortices is suppressed and the flow mixing is enhanced by the porous wall,and as a result,the development of the shear layer is accelerated.Finally,through the comparison and analysis of the spectral characteristics,the feasibility of noise reduction by 1mm-hole structure is confirmed.