连续超声速射流对撞诱导激波聚焦的机理研究

Investigation on Mechanism of Shock Focus Induced by Continuous Supersonic Jet Collision

关键词:  超声速射流  啸声  激波  压力震荡

Key words:  Supersonic jet  Screech tone  Shock  Pressure oscillation

摘要:

为研究连续超声速射流对撞诱导激波聚焦的机理，在喷口间距为70mm，不同入射压力的条件下进行了自由射流对撞、二维凹面腔内的射流对撞及三维凹面腔内的射流对撞诱导激波聚焦实验。通过对比自由射流对撞辐射啸声频谱和凹面腔内射流对撞中凹面腔底部的动态压力频谱，结合纹影照片对连续超声速射流对撞诱导激波聚焦的机理进行了研究。结果表明，沿射流对撞面两侧的椭球形激波阵面的高频拉锯式振荡是连续射流对撞诱导激波聚焦的本质。当两侧的波阵面同时到达凹面腔底部发生碰撞时，激波聚焦得以完成；在一定的入射压力范围内，由于射流对撞面不稳定，两侧椭球形激波阵面不能同时到达两喷口中间的对称面，因此在自由射流对撞的辐射噪声频谱上共存两种模式的啸声。在凹面腔内的射流对撞中，凹面腔底部的压力脉动频谱上也同时存在两个频率十分接近的峰值；随着入射压力的进一步提高，两侧的椭球形激波阵面经过一段时间的协调同步后同时到达喷口中间的对称面，辐射噪声和压力脉动频谱中双频率值共存的现象消失，并统一为单一值。

Abstract:

In order to investigate the mechanism of shock focus induced by continuous supersonic jet collision，the experiments including free jet collision，jet collision inside 2D cavity and jet collision inside 3D cavity were carried out. During experiments，the interval between two opposite nozzles were 70mm and the incident pressure were decreased monotonously. Then with the help of schlieren graphs，the spectrum of the screech tone radiated by free jet collision and that of the pressure oscillation at the apex of the cavity were compared and analysed. The results show that the essence of shock focus induced by continueous supersonic jet collision is the high frequency seesaw between both sides of the ellipsoidal shock front along the collision surface. When both sides of the ellipsoidal shock front arrived and impacted at the apex of cavity simultaneously，the shock focus completed. Moreover，when the incident pressure arranged in certain extent，under which the collision surface is not stable，each side of the ellipsoidal shock front couldn't arrive the symmetrical surface of two opposite nozzles synchronously. For this reason，two kinds of screech tone were present on the frequency spectrum of the noise in free jet collision. When jet collision happened in cavity，the frequency spectrum of the dynamic pressure at the cavity apex also showed two peaks with its values close to each other. If the incident pressure increases，the collision surface will be stabilized and the two sides of the ellipsoidal shock front would arrive at the symmetrical surface of two opposite nozzles simultaneously after some correction and coordination. At this time，these two peaks in spectrum of the screech tone are radiated by jet collision and that of the pressure oscillation disappears and be unified.