Self-Sustained Oscillation in an Axisymmetric Impinging Shear Layer

Self-sustained oscillation of a shear layer has been experimentally investigated in an axisymmetric jet impinging on a sharp-edge ring. As many as five modal stages are identified with discrete changes of the dominant frequency as the distance between the jet-nozzle exit and the ring is changed. The dominant frequency observed in each stage can be correlated by a simple empirical equation when nondimensionalized with the initial momentum thickness of the shear layer. The frequency change between two successive stages is also shown to obey a general rule of modal transition. An analysis of the phase-lag of velocity signals at various streamwise locations reveals a downstream travelling wave, while the associated vortex formation is confirmed by smoke-wire flow-visualization.