Visualization of Self-Sustained Oscillation in Impinging Sonic Jets by LIF

Feedback mechanism of self-sustained oscillation in impinging jets with M = 1.0 was studied experimentally by using a laser-induced fluorescence (LIF) technique. Phase velocities of disturbances in the flow field were obtained from a time-dependent cross-correlation function between a reference pressuresignal on the wall and the LIF signal in the flow field. A feature of disturbance propagation can be visualized by drawing maps for the cross-correlation coefficients on a plane at successive delay-times and making an animation. There are two feedback loops : one is closed by a neutral wave propagating upstream inside the jet and another is achieved by sound wave propagating outside the jet. Both the axisymmetic and helical modes of resonance were clearly observed by use of the present method.