Measurements of supersonic helium/air mixture jets

An enhanced method of using helium/air mixture jets to simulate the aeroaco ustic properties of hot jets is presented. By using helium to reduce the je t density and to increase the jet acoustic speed, unheated nominal Mach 1.5 jets are tested that have jet-to-ambient density and acoustic speed ratios that approximately match those from a hot jet with a jet to-ambient static temperature ratio of 1.2. The jets are operated at a reduced Reynolds numb er (approximately 2.7 x 10(4)), which allows the use of diagnostic measurem ent tools such as hot-wire anemometry and active control via glow discharge excitation. Mean and fluctuating flowfield and acoustic measurements from a near perfectly expanded Mach 1.5 elliptic and round jet are presented. Di rect comparisons of the cold and simulated heated jets are made. Compared t o the pure air jets, the helium/air mixture jets showed increased instabili ty wave phase speeds near or exceeding the ambient acoustic speed, increase d noise levers, and increased coupling between the flowfield fluctuations a nd the radiated acoustic field. These features are consistent with the theo ry of Mach wave radiation, the dominant noise source in high-speed jets. Th e data presented show that the helium/air simulation is able to capture the dominant noise characteristics of actual heated jets. The use of this grou p of diagnostic measurement techniques is an added benefit of the simulatio n that is not available in conventional heated jet experiments.