Self-Sustained Oscillations of a Ducted Jet Grazing a Sharp Lip

A subsonic, axisymmetric, internally mixed jet exhibited powerful, periodic self-sustained oscillations and tonal acoustic emissions of the same frequency when the bypass stream was operated at a lower pressure ratio than the core stream. Experimental acoustic and flow measurements with unheated and heated flow were collected for two mixing-duct lengths. It was found that, in all cases, the oscillations occurred only when the core jet impinged on the final-nozzle lip. As the mixing duct was lengthened and the core-stream total temperature increased, this phenomenon occurred more readily (a smaller pressure differential between the two streams led to core-jet impingement and associated oscillations). The oscillations were extremely orderly, containing a well-defined signature of the core jet’s instability waves. Moreover, a jump in the oscillation frequency was attended by a switch in the azimuthal order of the excited core-jet instability waves. This phenomenon was suppressed entirely by adding a suitable boundary-layer trip to the core nozzle. All evidence indicated the oscillations and tones were due to an aeroacoustic feedback phenomenon inside the nozzle.