Effect of primary nozzle lip thickness on elliptic subsonic co-flow jet characteristics

Purpose This study aims to investigate the influence of primary nozzle lip thickness on the mixing and spreading characteristics of an elliptic co-flowing jet, with emphasis on the pot ential core, recirculation zone and axis-switching. Design/methodology/approach Numerical simulations were performed on an elliptic (AR 2) primary jet surrounded by a modified elliptic co-flow jet with three lip thicknesses (3, 6 and 9 mm). Both the jets were considered at Mach 0.8 by maintaining an annular gap of 5 mm, and mass-flow ratios of 0.7, 0.94 and 1.18 were examined. The analysis was carried out in ANSYS CFX using a SolidWorks generated nozzle model and a structured mesh created in ICEM CFD, with the shear stress transport turbulence model adopted for its proven accuracy in prediction of jet characteristics. The jet development was assessed through Mach decay plots and corresponding Mach contours in the XY, XZ and YZ planes; and mass-entrainment plots. Findings The results show that an increase in nozzle lip thickness enlarges the asymmetric recirculation zone, which enhances jet mixing by shortening the potential core of the primary jet and promotes earlier axis-switching. Bifurcation of the primary jet at the end of the potential core was observed due to the influence of the co-flow. Furthermore, the co-flowing jets displayed relatively slower decay in the far field, attributed to reduced mass entrainment. Practical implications The findings offer practical value for aerospace systems where controlled jet mixing is essential. The ability to tune potential core length, entrainment and axis-switching through lip-thickness variation and co-flow momentum provides a passive means of improving exhaust mixing, reducing noise and lowering infra-red signature in turbofan nozzles. The results are also relevant to gas-turbine combustors, where enhanced air–fuel mixing and stable flame development are critical for efficient operation. Originality/value This work provides a novel contribution by numerically examining the role of primary nozzle lip thickness in elliptic co-flow jets, a configuration that has received limited attention in prior studies.