The Effect of Fluid-Resonant Coupling in High-Speed Impinging Planar Jet Flows

Abstract

This study investigates the effect of fluid-resonant coupling, i.e. the coupling between unstable modes of an impinging jet with resonant acoustic modes occurring between the nozzle and the impingement surface, on the self-excited oscillations of high-speed impinging planar jet. In order to investigate this phenomenon, a series of experiments have been performed using a high-speed impinging planar jet with varying nozzle thickness (h) and impingement distance (xo), for a single Mach number in the compressible flow regime. The test results reveal that the jet oscillation is controlled by a fluid-dynamic mechanism for small impingement distances, where the unstable mode of the jet is controlled by the impingement ratio. At larger impingement distances, the response is dominated by a fluid-resonant mechanism, in which the various hydrodynamic modes of the jet couple with different resonant acoustic modes occurring between the nozzle and the impingement surface. Within the fluid-resonant regime the system produces acoustic tones that are excited predominantly as a function of the impingement distance, with the nozzle thickness and impingement ratio having only minor effects on the tone frequency. Flow visualization images show that the same unstable mode is excited for multiple nozzle thicknesses at a constant impingement distance, despite the wide variations in associated impingement ratio.