Discrete Noise Spectrum Generated by Acoustically Excited Jet

The shear layer of an axisymmetric air jet (Re - 50,000) was perturbed using a symmetrical acoustic excitation chamber surrounding the nozzle at a frequency fe =fs, where fs is the shear-layer instability frequency. The excitation organized the large-scale structures in the shear layer into a sequence of three successive vortex-pairing stages at fixed streamwise locations, producing the subharmonic frequencies/e/2,/^/4, and/,/8. The pressure spectra in the near field and the acoustic spectra in the far field contained peaks at the frequencies corresponding to the rate of formation of large-scale turbulent structures through vortex pairing. The broadband noise of the excited jet was reduced by as much as 10 dB compared with the non-excited jet.