Control of the Radiation of Screech Tone by Cavity Tone Generated in the Supersonic Jets

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In recent years, a supersonic passenger airplane of next generation is developping. The supersonic jets will be used as the thrust of this airplane. However, it is well known that very strong sound waves called screech tone is radiated. This screech tone is generated by the feedback loop consisting of downstream-convecting coherent vortical structures around the jet and upstream-propagating sound waves in the ambient. The radiation of such very intense noise from the supersonic jets will cause the serious noise pollution near the airports as well as the fatigue failure of the airplanes. Therefore, this experimental study was performed to clarify the possibilty of the noise reduction radiated from supersonic jets. The acoustical characteristics of the cavity tone and the screech tone radiated by the jet issuing from a rectangular nozzle with cavity were investigated in the last year. In this year, the acoustical characteristics of the tones radiated by the supersonic jet discharged from … More the circular nozzle with cavity were investigated. From the experimental results, the following things were found : (1) The screech tone whose frequency is higher than that of cavity tone is effectively reduced. (2) The sound pressure level (SPL) of the screech tone radiated from the jets exhausting from the nozzle with cavity becomes lower than that of the screech tone radiated from the free jet discharging from the nozzle without cavity by about 30dB at most. The SPL of the cavity tone is lower than that of the screech tone radiated from the free jet issuing from the nozzle without cavity by about 20dB at most. (3) When the length of the cavity is 1.2 times the nozzle diameter, one of two resonance frequencies, 11.5kHz and 14.0kHz, is appeared. When the frequency of the cavity tone of 11.5kHz is appeared in the pressure ratio range higher than 3.5, the frequency is changed into 14.0kHz abruptly during decreasing of the pressure ratio. The reverse process is not observed. When the cavity tone is produced in the pressure ratio range lower than 3.5, it is observed that the frequency of the cavity tone firstly appeared is not changed during decreasing of the pressure ratio. Less