Sound Generation By Sideband Instability In Supersonic Mixing Layer

The problem of aerodynamic noise seriously restricts the rapid development of the aviation industry.Further understands the mechanism of aerodynamic noise,and thus prevents or mitigates the harm caused by noise,has aroused widespread concern in academia and industry.As a typical flow of free shear,supersonic mixing layer has an important engineering practical significance and is widely used in application scenarios such as supersonic combustion ramjet and aeroengine.In this paper,the supersonic mixing layer was chosen as object.In order to study the nonlinear evolution of a pair of instability modes with close frequency,the parabolic stability equation in the affine coordinate system was developed.The near-field dynamics was analyzed in detail,such as the spectrum broadening mechanism,the evolution characteristics of difference mode and its low-frequency modes.The Wu’s integral theory was used to study the far-field Mach wave radiation.The affecting factors on difference modes and Mach wave radiation were taken into consideration.The main conclusions were summarized:(1)In the supersonic mixing layer,the difference mode and its low-frequency disturbances have strong growth ability.The nonlinear evolution of disturbances and exciting of difference mode result in spectrum broadening.(2)The frequency of the difference mode and growth rate of principle instability wave are critical factors on the evolution of difference mode and its low-frequency ones.For fast mode,smaller the frequency is,faster grow of the difference modes are.Thus,spectrum broadening becomes more and more obviously.Otherwise,for slow mode,the frequency has a little influence on difference modes.The growth ability and spectrum broadening have changed little when the difference frequency decreased.Both for fast mode and slow mode,difference mode and its low-frequency modes grow rapidly while the principle wave has a larger growth rate.(3)Acoustic radiation is directional and dependent on the dynamical properties of pressure disturbances in near-field.The exciting of difference mode and its low-frequency ones enhance the Mach wave radiation and broaden the range of radiation.For fast mode,Mach wave radiation is stronger and wider while the difference frequency is small.For slow mode,the frequency has a pretty small impact on Mach wave radiation in far-field.