Direct Computation of Mach Wave Radiation in an Axisymmetric Supersonic Jet

The intense Mach waves radiated by the growth and decay of linear instability waves in the shear layer of a perfectly expanded, axisymmetric jet with an initial centerline Mach number of Mj = 2.0 are directly computed by solution of the compressible Navier-Stokes equations on a computational domain that includes both the near and far fields. The directly computed far-field sound is compared to predictions obtained using an analysis based on linear stability theory, Lighthill's equation, and the Kirchhoff surface method. All of the predictions are in good agreement with the direct computations. Using Lighthill's equation, we demonstrate that it is essential to properly address the acoustical noncompactness of the sources. It is also shown that linear stability theory can be used to specify the source terms in Lighthill's equation; the resulting predictions are also in good agreement with the computations.