The effect of upstream disturbance on the angle of sound emission in a supersonic round jet

抄録

The three-dimensional time-dependent compressible Navier-Stokes equations are numerically solved to study acoustic emission mechanisms in a supersonic round jet at high convective Mach numbers. A 5th-order compact upwind algorithm developed by Deng et al. (1996) is used for spatial derivatives and a 4^th-order Runge-Kutta scheme for time advancement. The Navier-Stokes characteristic boundary conditions are used in the streamwise and radial directions and periodic boundary conditions in the azimuthal direction. Numerical results for the convective Mach number M_c = 0.97 are presented (M_c is defined by Eq.(14) in Section2). Two different cases were investigated. The first case is the jet forced by the linear unstable modes. The second case is the jet flow forced randomly. The numerical results provide new physical features of the Mach wave generated in supersonic round jets, which lead to extinction of the Mach wave by introducing disturbance condition. Upstream disturbance conditions play an important role for the emission of the Mach wave in supersonic jets. The pressure fluctuations generated by the growth of the opposite helical mode are shown to be linearly superimposed into the jet near sound field. The numerical results indicate that the jet forced with a pair of first helical modes can indicate the elimination of Mach waves at restricted emission azimuthal angles due to the interference of these modes. The partial elimination of the Mach wave also appears in a turbulent jet at the frequency of the artificially forced an optimal combination of the helical modes at the inlet region. This forcing technique will be extended to the Mach wave reduction in the distinct azimuthal direction.