Experiment of the effect of nozzle spacing on twinjet screech tones

The near field of the supersonic twinjet noise was measured experimentally with fully expanded Mach number ranging from 110 to 160 and nozzle spacing from 16-32 times of nozzle outlet diameter. The phase difference of the near field screech tones radiated from the two jets was analyzed in detail, and the effect of nozzle spacing on the coupling of the twinjet was explored. The experimental results showed that there were two different patterns for the A mode of twinjet screech tone, ie:symmetric and antisymmetric along the central plane of the two nozzles. In the antisymmetric mode, the intensity of the screech tone in the internozzle region decreased, which was slightly larger than the amplitude of the single jet in the symmetric mode. For the twinjet operating in B mode, the screech tone generated by the two jets were in phase in the internozzle region, which reinforced the oscillation of the twinjet, and the amplitude of the screech tone in the internozzle region can reach to 160dB. The phase difference of the C mode needs further study. Comparing the results with different nozzle spacing, it was found that the coupling was suppressed for the B mode when the nozzle spacing was larger or smaller, while both the amplitude and the dominant Mach number range of the C mode increased with the nozzle spacing.