Entrainment mechanism in a turbulent offset jet

In this study, a turbulent offset jet with an offset ratio of 5 and a Reynolds number of 7500 is numerically investigated using Large Eddy Simulation (LES), aiming to explore the entrainment mechanisms based on the characteristics of the turbulent/non-turbulent interface (TNTI). First, the distance from the mean position of the TNTI to the jet centerline increases significantly faster in the pre-attachment zone than in the wall jet zone, suggesting a stronger entrainment process in the former region. Second, the conditionally averaged profiles show larger jumps in the velocity and vorticity across the TNTI in the pre-attachment zone, thereby promoting the entrainment of fluid from the non-turbulent region into the turbulent region. Finally, the entrainment is further analyzed through the mechanisms of the nibbling and engulfment. The results show that the contribution of engulfment to the entrainment is less than 2 %, indicating that engulfment is not a significant mechanism in this flow. This indirectly suggests that entrainment is primarily driven by the nibbling process. By quantifying the nibbling flux, it is found that the nibbling flux in the pre-attachment zone is approximately double that observed in the wall jet zone, further explaining higher entrainment in the former region. • The entrainment mechanism in an offset jet is analyzed using the TNTI. • Entrainment in the pre-attachment zone is higher than in the wall jet zone. • Nibbling is the main entrainment mechanism on the interfacial layer of offset jets. • Average nibbling flux in the pre-attachment zone is double that in the wall jet zone.