Self-excited oscillations of a high-speed impinging planar jet

This paper examines aeroacoustic tone generation of a high-speed planar gas jet impinging normally on a flat, rigid surface. Experiments are performed over the complete range of subsonic jet velocities for which tones are generated, and over the complete range of impingement distance for which tones occur for a single nozzle thickness of h = 3 mm. The behavior of the planar impinging jet case is compared to that of the axisymmetric case and found to be significantly different, with tones being excited at much larger impingement ratios and lower flow velocities. The acoustic tones have been found to be generated by both symmetric and anti-symmetric jet instabilities, coupled with resonant acoustic modes occurring between the nozzle and plate surface. The nature of the flow instabilities has been investigated using phase-averaged particle image velocimetry measurements. The frequency behavior of the resulting tones is predicted using a simple feedback model, which allows the identification of the various shear layer modes of the instabilities driving tone generation. Finally, a thorough dimensionless analysis is performed in order to quantify the system behavior in terms of the appropriate length and velocity scales.