Noise Generation By Instabilities In Axisymmetric Supersonic Jet

The aerodynamic noise in the jet flow field is mainly composed of the small-scale turbulence structure and the large-scale coherent structure.In the supersonic jet,the large-scale coherent structure dominates,and this large-scale structure is essentially a kind of unstable wave.Unstable waves in supersonic jet are the main cause of jet instability.When the velocity of unsteady wave propagates downstream with supersonic velocity relative to the incoming stream,the unstable wave radiated noise.In this paper,the aeroacoustic fields of low Reynolds number(Re=3950)and moderate Reynolds number(Re=70000)supersonic(Ma=2.1)axisymmetric jet are studied.The parabolic stability equation(LPSE)based on the linear NS equations in three-dimensional cylindrical coordinate system and the Wu X.integral method are used to study the evolution and acoustic radiation characteristics of the axisymmetric and helical modes unstable waves.The conclusions are as follows:1.In the moderate Reynolds number case,the unstable wave grows rapidly in a short range,reaches the peak value and then decays rapidly.This rapid change process is in the core region of the jet;in the low Reynolds number,the unstable wave grows and decays in a long space.The rapid change process takes place near the core of the jet.As the Strouhal number(St)increases,the peak position of the unsteady wave pressure disturbance are moving upstream.The helical mode dominates when St is small,and the helical mode and the axisymmetric mode play a major role when St is large.The frequency range of the unstable waves in different Reynolds numbers is different.Under the condition of moderate Reynolds number,the frequency range of the unstable wave is wide;under the low Reynolds number,the frequency range of the unstable wave is narrow.2.Under the condition of low and moderate Reynolds number,the growth characteristics of the unstable wave and the peak of the disturbance pressure amplitude from the core region determines the far field acoustic radiation characteristics.When the Reynolds number is the same,the axisymmetric mode has the same acoustic radiation characteristics as the helical mode.When Reynolds number is low,the unstable wave frequency band is relatively narrow.Since the unstable region of the disturbance wave is long along stream wise direction,the acoustic radiation region is large;the sound source is decentralized;and radiation angle is hard to be determined.For moderately large Reynolds number,the unstable wave has a large unstable frequency wave band;the short unstable region results in a narrow radiation region,concentrating sound source and strong radiation directivity;and the direction angle increases with the increase of Strouhal number.3.The normal velocity has negligible effect on the acoustic field for low Reynolds number,while it has obviously effect and needs to be considered when Reynolds number is large.