Characteristic variation of Mach number in subsonic co-flowing jets by varying lip thickness

Jet noise reduction is achieved by jet mixing enhancement which is attributed to potential core length reduction of the primary jet. Varying lip thickness can be considered as a passive control method to reduce the potential core, to enhance mixing. This article focuses on certain novel attributes of subsonic co-flowing jet Mach number decay for the wake dominated flow by varying the lip thickness, which is defined as the gap between the core nozzle and surrounding duct along with varying bypass ratio. Three different lip thicknesses namely of 2 mm, 10 mm, and 15 mm at jet exit Mach number of 0.6 was experimentally studied for the bypass ratios ranging from 0.7 to 6.4. To study the flow characteristics, Pitot static measurements was conducted experimentally in both axial and radial directions of the jet. To visualize the jet flow development for the tested cases, numerical simulations with experimental validation, were also carried out. For exit Mach number of 0.6, the single free jet has a potential core length up to X/D_p = 3.8, whereas for 10 mm lip co-flowing jet with bypass ratio 0.7, 1.4 and 6.4 the potential core length extends up to X/D_p = 2.8, 2.7 and 2.1 and their corresponding percentage reduction is 26, 29, 45% respectively. Similarly for 15 mm lip for co-flowing jet with bypass ratio 1, 1.7 and 6, the potential core length extends up to X/D_p = 2.7, 2.2 and 1.6 and their corresponding reduction percentage is 29, 42, 58% respectively. Concluding that for the thick lip co-flowing jet, the presence of recirculation zone at the near field of the nozzle wall, the centreline Mach number increases initially due to the wake dominant sub-atmospheric pressure, exists in the flow regime and also when the bypass ratio increases, the potential core length decreases.