关键词: 超声速射流；啸声；激波；压力震荡

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.

Key words: Supersonic jet；Screech tone；Shock； Pressure oscillation