Lip Thickness Effects on Supersonic Co-axial Jet Mixing

The effects of lip thickness (which is defined as the distance separating the primary nozzle and the secondary nozzle) on co-axial supersonic jet (M _P = 2.0) decay at different expansion levels have been studied numerically and experimentally. Co-axial supersonic jets from lip thicknesses of 1.7mm (thin lip), 5mm (mid lip), 10mm (thick lip), and 15mm (ultra-thick lip) at nozzle pressure ratios of 5 (over-expanded level), 7.824 (correctly expanded level) and 10 (under-expanded level) have also been studied. In this work, lip thickness variation is considered as a passive control, while the Mach number ratio variation is considered an active control parameter in determining the flow field characteristics. Furthermore, a grid independence study and turbulence comparison analysis were conducted with a thin lip of 1.7mm. Jet centreline pitot pressure decay, and shock waves present in the jet core were analysed. The results show that the mixing of the thick lip (10mm) and ultra-thick lip (15mm) is superior to the thin lip (1.7mm) and mid lip (5mm) at all the nozzle pressure ratios of the present study. Additionally, the thick lips (10mm & 15mm) of the co-axial supersonic jets have a strong influence on the jet mixing. This is because of the vigorous interaction that occurs between the primary and secondary jets in the intermediate region, decreasing the supersonic core length of primary jet, thereby increasing the mixing. Coaxial jets with lip thicknesses of 10mm, and 15mm exhibit significantly enhanced mixing compared to those with a 1.7mm and 5mm lip thickness, up to 60 (PMN) [Percentage Mach Number (PMN) is defined as the percentage of surrounding jet exit velocity with respect to the primary nozzle exit velocity] of the secondary jet.This study gives an exhaustive numerical study of the impact of lip thickness as a passive control and Mach number ratio as an active control on the mixing and decay behavior of co-axial supersonic jets. By systematically quantifying the effect of lip thickness on jet core length and mixing efficiency at different expansion levels, this study provides valuable insight for optimizing nozzle design in aerospace propulsion and noise reduction applications, thereby addressing a critical gap in understanding interaction between nozzle geometry and supersonic jet dynamics.