Tones Due to ‘Guided Jet Modes’ in the Forward Arc of High-Speed Jets

An experimental study is conducted investigating the characteristics of ‘guided jet wave’ resonance tones in the forward arc of jet noise radiation fields. These spectral peaks disappear in the far acoustic field in the aft and sideline directions, which is why they went unnoticed in decades of jet noise measurements. However, it is clearly shown in the present study that they radiate in the forward arc (shallow upstream polar locations in the approximate range of theta<45°). Jet noise spectra in the forward arc, data on which had been lacking in the literature, are not smooth but are characterized by these peaks. This is found for high subsonic to supersonic jets up to the highest jet Mach number covered in the experiment (MJ =1.9), for round, rectangular as well as convergent-divergent nozzles. In heated jets, these spectral peaks appear to get weakened especially around transonic conditions whereas they clearly persist in supersonic conditions.