Offset dense jets in co-and-counter flow: Experimental and numerical study

This study investigates the mixing behavior of turbulent offset dense jets discharged into co- and counter-flowing ambient currents under varying confinement ratios. A comprehensive experimental campaign using planar laser-induced fluorescence (PLIF) was conducted for two densimetric Froude numbers, supported by Reynolds-averaged numerical simulations. The experiments reveal that co-flow produces smoother centerline decay trends and more asymptotic behavior, whereas counter-flow often triggers low-frequency oscillations, enhanced lateral spreading, and delayed decay. Confinement plays a central role: at small confinement ratios, the counter-flow jet forms persistent recirculation zones, suppressing flapping motions and reducing dilution; at larger confinement ratios, counter-flow promotes stronger unsteadiness and faster decay at sufficiently high current Froude number. Proper orthogonal decomposition (POD) was employed to examine the large-scale dynamics and revealed that counter-flow scenarios exhibit pronounced low-frequency flapping behavior, whose strength and spatial extent depend strongly on confinement. Numerical predictions captured the general trajectory and recirculation patterns but consistently over-predicted dilution, especially at higher current Froude number values. The combined results provide new insight into offset jets released into non-stagnant coastal environments.