Measurements of Supersonic Helium/Air Mixture Jets

An enhanced method of using helium/air mixture jets to simulate the aeroacoustic properties of hot jets is presented. By using helium to reduce the jet density and increase the jet acoustic speed, unheated nominal Mach 1.5 jets are tested which have jet-to-ambient density and acoustic speed ratios which 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 numbers (approximately 27,000) which allows the use of diagnostic measurement 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. Direct comparisons of the cold and simulated heated jets are made. Compared to the pure air jets, the helium/air mixture jets showed increased instability wave phase speeds near or exceeding the ambient acoustic speed, increased noise levels, and increased coupling between the flowfield fluctuations and the radiated acoustic field. These features are consistent with the theory of Mach wave radiation, the dominant noise source in high speed jets. The data presented show that the helium/air simulation is able to capture the dominant noise characteristics of actual heated jets. The use of this group of diagnostic measurement techniques is an added benefit of the simulation which is not available in conventional heated jet experiments.