Effects of unstable oblique modes on the acoustic filed in a supersonic plane turbulent jet

The three dimensional time dependent compressible Navier Stokes equations are numerically solved to study acoustic emission mechanism in a supersonic plane jet at high convective Mach numbers using high-order compact upwind schemes. High order compact schemes of 5th order developed by Deng and Maekawa (1996, 1997)⁽¹⁾⁽²⁾ are used for spatial derivatives and a 4th order Runge-Kutta scheme is employed for time advancement. Navier-Stokes characteristic boundary conditions are used in the streamwise and vertical directions and periodic boundary conditions in the spanwise direction. Two cases for convective Mach number Mc=1.17 are presented. The first case is the jet flow forced randomly. The second case is the jet forced by random disturbances and a pair of linear unstable oblique modes with a. subsonic phase speed. The numerical results provide new physical insights into three-dimensional structures and acoustic wave generation mechanisms in a plane turbulent jet. Intense Mach waves radiating at low-frequencies are attributable to the growth of two-dimensional radiating mode with a supersonic phase speed in the jet shear layers, Upstream disturbance conditions play an important role for the evolution of the jet shear layers. The growth of a pair of oblique modes is responsible for the Λ structure in the jet shear layers, which yields sooner decay of the centerline velocity due to the shear layer rapid evolution. Therefore, the intense sound radiation observed in the randomly forced jet can be reduced by forcing with a pair of oblique modes.