Reduction of noise from supersonic jet flows
Acoustic characteristics and flow behavior of interacting coaxial supersonic jets have been investigated experimentally. A number of arrangements of the inner and outer coaxial nozzles were used. Far field acoustic measurements were made for each nozzle arrangement. Preliminary near field measurements were also made for one typical nozzle arrangement. Substantial reductions in both peak sound pressure and total radiated power were obtained over a range of operating conditions of the coaxial jet flows. Spark-shadowgraphs recorded at these operating conditions revealed the consistent appearance of a characteristic pattern of shock structure which effectively eliminates the usual repetitive shock cells associated with super sonic jet flows. The corresponding velocity profiles deduced from pitot-pressure surveys of coaxial jet flows indicate that the large mean shear at the inner boundary of the outer annular jet as well as the mixing are favorably modified owing to the interaction of inner and outer jet flows. The dependence of the measured noise reduction on the observed flow changes is discussed.