Control of High-Speed Impinging Jet Resonance

The intense pressure fluctuations generated by high-speed impinging jets dramatically increase in amplitude and exhibit peaks at discrete frequencies under certain flow conditions. It is generally accepted that a feedback loop consisting of downstream-convecting organized motions and upstream-propagating acoustic disturbances is responsible for the maintenance of this flow resonance. This paper describes an effort to control impinging-jet resonance through the addition of an annular stream. Fluctuating wall-pressure and near-field acoustic measurements were made in a high-speed (Mj = 0.8-1.6) impinging cold jet issuing from coaxial round converging nozzles. Coherent motions critical to the maintenance of the feedback loop were examined via real-time conditional acquisition of schlieren images. The results of the fluid dynamic control were dramatic. The discrete impingement tone was eliminated and the broadband noise was decreased by an order of magnitude when the Mach number of the annular jet was properly selected. It is hypothesized that the addition of the annular stream alters the character of the downstream-convecting instabilities, thereby interrupting the fluid-dynamic feedback.