CONTROL OF HIGH-SPEED IMPINGING-JET RESONANCE

The intense pressure fluctuations generated by high-speed impinging je ts dramatically increase in amplitude and exhibit peaks at discrete fr equencies 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 now resonance. This paper describes an effort to control impinging-jet resonance through the addition of an annular str eam. Fluctuating wall-pressure and near-field acoustic measurements we re made in a high-speed (M(j) = 0.8-1.6) impinging cold jet issuing fr om coaxial round converging nozzles. Coherent motions critical to the maintenance of the feedback loop were examined via real-time condition al acquisition of schlieren images. The results of the fluid dynamic c ontrol were dramatic. The discrete impingement tone was eliminated and the broadband noise was decreased by an order of magnitude when the M ach number of the annular jet was properly selected. It is hypothesize d that the addition or the annular stream alters the character of the downstream-convecting instabilities, thereby interrupting the fluid-dy namic feedback.