Abstract The mixing characteristics of a Mach 1.9 jet at three levels of overexpansion, corresponding to nozzle pressure ratio (NPR) 3, 4 and 5, in the presence of a sonic co-flow (secondary flow), which was submerged in a subsonic co-flow (tertiary flow) was studied experimentally. For these NPRs the secondary co-flow is sonic with underexpanded levels and the tertiary flow Mach number was found to be 0.41, 0.71 and 0.85, respectively. The centerline decay results of the primary jet show that the jet mixing is abated by the co-flow, at all levels of expansion. However, in spite of the reduced mixing encountered by the supersonic primary jet, the waves in the jet core are found to be weaker in the presence of co-flows. This may be regarded as an advantage from the shock associated noise point of view, in accordance with Tam’s theory; which states weaker the waves in the core, the lesser is the shock associated noise. The results show that the reduced mixing environment caused by the sonic co-flow alone leads to the jet core elongation of about 20%, 23% and 49%, at NPRs 3, 4 and 5, respectively. The core length of the jet is found to increase by 29%, 46% and 62%, respectively, at NPRs 3, 4 and 5, when both sonic and subsonic co-flow streams are present.

An experimental investigation has been carried out to study the characteristics of underexpanded three-stream sonic jets delivered from three coaxial convergent nozzles at Nozzle Pressure Ratio (NPR) 2, 3, 4 and 5. Annular gaps of 0.5 mm and 1.5 mm were given for the secondary and tertiary nozzles, respectively. Centerline pressure measurement and shadowgraph visualization were used to analyze the characteristics of jet with and without coflow. In the absence of coflows, the core length of core sonic jet was found to increase with increasing NPR. With sonic secondary coflow, the core length of the jet gets elongated at each NPR. The jet core is further elongated in the presence of both sonic secondary and tertiary coflows. Though the core of the central sonic jet gets elongated, in the presence of coflow, the elongated core possesses weaker waves than the core without coflow. The weakening of waves by two coflows was better than one coflow. The weakening of waves by coflows can be deemed as an advantage from aeroacoustics point of view. The maximum elongation in core length due to sonic secondary and tertiary coflows of 40% and 55%, respectively, at NPR 3.

An experimental investigation was done to understand the effect of sonic coflow jet on the core length of Mach 2 jet at different levels of expansion. The sonic jet surrounding the primary Mach 2 jet was produced using a sonic nozzle, whereas a convergent-divergent nozzle was used for Mach 2 jet. The annular gap between the sonic and supersonic nozzles was 1mm. For a fixed main jet NPR, the NPR of the sonic jet was varied from 4 to 6 (in steps of 1), which correspond to underexpanded levels of the sonic jet. The NPR of Mach 2 jet was varied from 4 to 6, which correspond to overexpanded states of Mach 2 jet. Without sonic coflow, the core length of Mach 2 jet was found to increase with the increase of NPR which was quantified using shadowgraph visualization. For all the main jet NPRs, the increase in coflow NPR resulted in the increase of core length. For each NPR of Mach 2 jet, the core length increases with the increase of coflow NPR. The maximum core length for NPRs 4 to 6 was at coflow NPR of 6 and minimum was at coflow NPR 4. The sonic coflow was found to retard the mixing of Mach 2 jet at all the pressure ratios, but it resulted in weakening the strength of waves present in the core of Mach 2 jet.