Shear layer characteristics of supersonic free and impinging jets

Abstract

Theinitialshearlayercharacteristicsofajetplay an important role in the initiation and development of insta- bilities and hence radiated noise. Particle image velocimetry has been utilized to study the initial shear layer develop- ment of supersonic free and impinging jets. Microjet control employedtoreduceflowunsteadinessandjetnoiseappearsto affectthedevelopmentoftheshearlayer,particularlynearthe nozzle exit. Velocity field measurements near the nozzle exit show that the initially thin, uncontrolled shear layer devel- ops at a constant rate while microjet control is characterized byarapidnonlinearthickeningthatasymptotesdownstream. The shear layer linear growth rate with microjet control, in both the free and the impinging jet, is diminished. In addi- tion, the thickened shear layer with control leads to a reduc- tion in azimuthal vorticity for both free and impinging jets. Linear stability theory is used to compute unstable growth rates and convection velocities of the resultant velocity pro- files. The results show that while the convection velocity is largely unaffected, the unstable growth rates are signifi- cantly reduced over all frequencies with microjet injection. For the case of the impinging jet, microjet control leads to near elimination of the impingement tones and an apprecia- ble reduction in broadband levels. Similarly, for the free jet, significant reduction in overall sound pressure levels in the peak radiation direction is observed.