The effect of low lobe count chevron nozzles on supersonic jet screech

An experimental study was conducted into the fluid and aeroacoustic properties of jets issuing from nozzles with two or four chevrons. Flow visualisation in the form of schlieren imaging was utilised to investigate the general change in jet structure. Schlieren imaging revealed a highly asymmetric flow field produced by the azimuthal asymmetry at the nozzle lip. The shock structures within the flow were significantly altered in both symmetry and spacing by the introduction of chevrons. A spatial two-point correlation was used to classify the dominant azimuthal instability mode produced by each of the respective nozzles. It was found that the two-lobe nozzle favoured a lateral flapping mode towards the chevron sides of the nozzle. The four-lobe nozzle, when coherent structures were visible, produced a mode somewhat more difficult to classify, being either a helical mode or a multi-axial flapping mode. A complex relationship was observed between the range of pressure ratios over which strong screech tones are evident, and the number of lobes. The two-lobe nozzle provided screech suppression below a nozzle pressure ratio of 2.7, but directional screech amplification at higher pressures. The four-lobe nozzle provided less screech amelioration at low pressures, but eliminated the screech tone entirely for most other pressures studied. The flapping oscillation of the two-lobe nozzle produced a highly asymmetric acoustic field. The four-lobe nozzle produced a relatively symmetric acoustic field.