Abstract: Acoustic distribution in the external flow field of a multi-cycle pulsed detonation tube is quite different from that in a single-cycle detonation flow. The detonation jet has an important influence on the acoustic characteristics in a subsequent cycle. The analysis of unique directivity and features in both time and frequency domains are presented in this paper. An acoustic experiment is performed on the detonation system for both single-cycle and multi-cycle using a 80mm detonation tube in free space. Result shows that the high frequency component in the detonation sound within the angle of 45° would be suppressed due to turbulence in the jet region. Severe attenuation of sound pressure peak in this region results in the formation of heart-shaped directivity patterns with a highest SPL at 45°. The relationship between jet diameter, angle and cut-off frequency is established, revealing a cut-off frequency of about 500 Hz within 30°. The detonation jet causes a gourd-shaped projection in the shockwave front near the axis, significantly reducing propagation time. The acoustic efficiency of the multi-cycle detonation is about 12%, greatly higher than that of a conventional jet.

Key words: multi-cycledetonation, impulsesoundwave, directivity, timeandfrequencydomains, gasjet