An experimental investigation of the aeroacoustic properties of high-speed, helium/air mixture axisymmetric jets

The acoustic and aerodynamic properties of high-speed axisymmetric jets are investigated experimentally in a recently refurbished high-speed jet noise facility. Mach numbers from 0.5 to 1.5 are tested with an emphasis on Mach 0.9 and 1.5 jets. Reynolds numbers for the current study range from approximately 200,000 to 600,000. The properties of heated jets which are important in aeroacoustic studies, namely low density and high velocity, are simulated in the current study by adding helium to the jet flow. In addition, an optical deflectometry system is used to provide unique two-point space-time correlation measurements within the jet shear layer.; A combination of acoustic, mean flow, and optical deflectometry measurements are made for both pure air and helium/air mixture jets at various helium concentrations. Far-field acoustic measurements indicate very reasonable agreement between previously measured heated jet directivity patterns and those at simulated temperature ratios using helium/air mixture jets. The addition of helium also shows strong similarities to the addition of heat in the mean velocity profile measurements. A shortening of the potential core and a slight decrease in jet spreading rate are observed with helium addition—the same trends observed for heated jets. Optical deflectometry measurements near the end of the potential core along the jet lip line exhibit distinct cross-correlation curves for the pure air jet cases. However, helium/air mixture jets display much lower levels of correlation and little evidence of large-scale structure in the measured spectra. It is believed the strong visual density gradients throughout the shear layer effectively mask the large-scale structure, thus reflecting a limitation of the optical deflectometer. Finally, a decrease in normalized convection velocity with simulated heating (helium addition) is observed.