Numerical Simulation And Spatial Stability Analysis Of Chevron Jet

Jet mixing noise,one of the main contributor of noise emission from aircraft,can be further reduced by machining serrations,or chevrons,into the nozzle lip,resulting in a serrated jet flow.Noise reduction mechanism has been investigated in this work by numerical simulation and spatial stability analysis.Main aspects of present study are summarized as follow:(i)Large eddy simulations are performed based on the base flows,that are result from the Reynolds-averaged Navier-Stokes(RANS)simulations,to investigate serrated jet.The influences of nozzle configuration is examined,and the relationship between noise reduction and large coherent structures is discussed in detail.The results show:1)chevron jet accelerate turbulence mixing,resulting in a faster decrease of the poten-tial core.Chevron jet decrease the turbulent fluctuation at the potential core region,and also decrease the flow intermittency.2)The overall sound pressure levels of chevron jets decrease as high as 3dB at the shallow angles compared with the round jet.Noise at each fourier azimuthal mode is also reduced in the best noise reduction case.3)By performing proper orthogonal decomposition(POD),the most energetic coherent structures at specified coupled azimuthal wavenumbers are extracted and the chevron jet show smaller coherence than round jet.(ii)The linear spatial stability characteristics of means flows produced by round and six chevron nozzles are investigated.The influence of frequency and streamwise location of base flow is examined.Linearised Euler Equations are used in calculating the downstream wavepacket pattern and in investigating the noise emission efficiency from the instability wave.1)We find three instability modes exist around outlet of serrated jet,which are associated with radial shear,azimuthal shear and both radial and azimuthal shear respectively.2)Besides,the frequency of the most unstable eigenvalue related to the radial shear is higher than that related to the azimuthal shear.3)Compared to the round jet,the chevron jet reduces the growth rate and phase speed of the most unstable eigenmodes.And the efficiency of sound emission is also reduced from ana-lyzation of Linearized Euler equations.We conclude that the variation of the instability wavepackets can reflect the variation of sound emission efficiency.