Tones Encountered with a Coannular Nozzle and a Method for Their Suppression

Abstract

With multistream coannular exhaust flows, sometimes tones occur that may fail the nozzle hardware meeting noise regulation standards. A two-stream nozzle was studied experimentally and numerically in an attempt to identify the sources of such tones and explore remedies. Sharp tones occurred in a range of low jet Mach numbers $M_j$ . The source was investigated by parametric variations that included inner nozzle lip thickness, lip-to-lip distance between the inner and outer nozzles, and the length and geometry of the inner nozzle duct. Ultimately, it was inferred that the tones occurred due to a coupling between vortex shedding from the struts, which held the nozzles and the centerbody together, and various duct acoustic modes. Thus, with a thicker strut geometry, the tones occurred in a higher $M_j$ range (up to 0.85) because a given shedding frequency occurred at a higher $M_j$ , and vice versa. A leading-edge treatment of the struts was shown to eliminate the tones via disruption of the vortex shedding.