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

摘要

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.