Tones in the Acoustic Far Field of Jets in the Upstream Direction

The presence of tones in the acoustic far field of isothermal round jets for angles phi & GE;120 & DEG; relative to the flow direction is investigated for Mach numbers between M=0.60 and 2 based on data from both experiments and large-eddy simulations. For all Mach numbers, as observed in the jet near-nozzle region in previous studies, peaks are found in the far-field pressure spectra regardless of the nozzle-exit boundary-layer properties. They emerge near the cutoff frequencies of the free-stream upstream-propagating guided jet modes predicted by a vortex sheet model, sharply for M & GE;0.90 but more weakly for lower Mach numbers. The variations of their characteristics with the radiation angle are shown. As the angle increases, the peaks are more prominent, and the most apparent ones are related to lower azimuthal modes. Thus, peaks associated with the axisymmetric mode strongly predominate in the upstream direction.

Automated summary

The study investigated the presence of tones in the acoustic far field of isothermal round jets for different angles, revealing peaks in the far-field pressure spectra for all Mach numbers. The characteristics of these peaks varied with the radiation angle, with more prominent peaks associated with lower azimuthal modes as the angle increased.