Howling in a Model-Scale Internally Mixed Confluent Nozzle Related to Excited Core-Jet Instability

Abstract

In this experimental jet-noise study, the subsonic flow of air through a model-scale, dualstream, internally mixed nozzle is shown to produce acoustic tones as high as 30 dB above the broadband jet noise which also result in a broadband amplification of jet noise known to occur in acoustically excited jets. The tones are the result of a feedback phenomenon that requires the impingement of the core jet upon the final-nozzle lip and the excitation of instability waves in this jet. It is shown that suitably thickening the boundary layer at the core-nozzle exit (where the shear layer between the core jet and bypass stream begins its development) eliminates these tones and associated broadband amplification. The effects of the length of the mixing duct and heating the core stream on the onset of the howling are discussed. Jet flow data acquired using both particle-image velocimetry (PIV) and high-speed schlieren imaging support these claims. A proper orthogonal decomposition of PIV measurements is used to educe periodic oscillations present in the flow via phase averaging. Simple calculations are used to argue that an acoustic mode inside the mixing duct sets the frequency of the resonance.