Characteristics of Sonic Three-Stream Coflow Jets

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.