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

近年来，下一代超音速客机正在研制中。超音速喷气式飞机将用作这架飞机的推力。然而，众所周知，会辐射出非常强的声波，称为尖叫声。这种尖叫声是由反馈回路产生的，该反馈回路由喷射器周围的下游对流相干涡流结构和环境中的上游传播声波组成。超音速喷气式飞机辐射出的如此强烈的噪声，将造成机场附近严重的噪声污染以及飞机的疲劳失效。因此，进行这项实验研究是为了阐明降低超音速喷气机辐射噪声的可能性。去年对带空腔的矩形喷嘴喷出的空腔音和刺耳音的声学特性进行了研究。今年，研究人员对从带空腔的圆形喷嘴喷出的超音速射流辐射的声学特性进行了研究。实验结果表明： (1)有效降低了频率高于空腔音的刺耳音。 (2)从有空腔的喷嘴排出的射流辐射的刺耳声的声压级(SPL)比从无空腔的喷嘴排出的自由射流辐射的刺耳声的声压级(SPL)最多低约30dB。空腔音的声压级比无空腔喷嘴自由射流辐射的刺耳音的声压级最多低20dB左右。 (3)当腔体长度为喷嘴直径的1.2倍时，出现11.5kHz和14.0kHz两个共振频率之一。当11.5kHz的空腔音频率出现在高于3.5的压力比范围内时，频率在压力比降低过程中突然变为14.0kHz。没有观察到相反的过程。当空腔音在低于3.5的压力比范围内产生时，观察到首次出现的空腔音的频率在压力比降低期间没有改变。较少的

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