Primary atomization at the turbulent/non-turbulent interface of a liquid round jet

Primary atomization in turbulent liquid jets arises from interactions between turbulence and the liquid–gas interface, yet the role of the turbulent/non-turbulent interface (TNTI) in droplet formation remains unclear. Using large-eddy simulation with a volume-of-fluid method, we examine the relationship between TNTI dynamics and primary atomization in a liquid round jet at Re=6520, We=7740, Oh=1.35×10−2, and ρl/ρg=13.76. We identify TNTI in streamwise–radial planes and observe two characteristic entrainment mechanisms: Taylor-microscale nibbling and jet-half-width-scale engulfment, similar to those observed in single-phase turbulent jets. Spatial filtering and box-counting analyses indicate fractal-like scaling of the extracted TNTI, with a planar box-counting dimension of approximately 1.3. The interface length and mean entrainment velocity follow power-law trends with filter size, yielding an approximately scale-independent mass flux. Near-TNTI droplets have a mean cross-sectional area approximately 0.76 times that of far-TNTI droplets, while the corresponding mean aspect ratios are approximately 2.2 and 2.4, respectively. Jet-half-width-scale engulfment is associated with a large number of smaller droplets, whereas Taylor-microscale nibbling is associated with more elongated droplets. Among the near-TNTI droplets, the count is approximately 23% higher near concave than convex interface regions. Illustrative instantaneous sequences show that droplets near concave TNTI regions are accompanied by opposite-signed, λ-scale azimuthal-vorticity concentrations and progressively elongate in the intervening stagnation-point-like flow. Consistent with these observations, the conditionally averaged field exhibits a similar organized opposite-signed vorticity pattern around droplets near concave TNTI regions. Together, these results suggest that the characteristic scale and geometry of TNTI entrainment could be associated with droplet size and elongation.