Numerical Simulations of Guided Jet Waves

The acoustic waves that are mainly trapped into the potential core of jets, recently referred to as guided-jet waves (GJW), are investigated by performing numerical simulations of wave propagation. For that purpose, a single-frequency monopolar sound source is imposed on the axis of an isothermal round jet, modeled as a cylindrical shear layer. The waves radiated by the source are computed by solving the Euler equations. The shear-layer velocity profile is a hyperbolic-tangent profile, with a momentum thickness equal to 20 per cent of the jet half-width to avoid the rapid growth of Kelvin-Helmholtz instability waves. Three jet Mach numbers, namely M=0.75, 0.95 and 1.10, and source Strouhal numbers ranging up to 2, 1.2, and 0.8, respectively, are considered. In the three cases, GJW belonging to the first two radial GJW axisymmetric modes are obtained. Depending on the frequency, they propagate in the upstream or in the downstream directions, and are entirely or partially contained in the flow. Their levels inside and outside the jet are represented in the frequency-wavenumber space. They are also quantified upstream and downstream of the source. They vary smoothly with the frequency for M=0.75, but in a sharp and saw-tooth manner for M=0.95 and 1.10. Outside the jet, their variations are consistent with the footprints left by the GJW in the sound spectra near the nozzle of jets.