High-pressure gas atomization (HPGA) is a widely used method for producing metal powders using high-velocity gas jets, offering high efficiency for large-scale production. Achieving small and spherical powders is critical for this process, which requires a comprehensive understanding of the primary breakup of liquid metals. However, the highly turbulent nature of gas jets complicates the breakup process, making it difficult to control. Here, we explore the influence of vortical structures on the primary breakup during atomization using large-eddy simulations for an annular-slit, close-coupled gas atomizer with molten aluminum and nitrogen gas. We extract individual droplets from the instantaneous flow field and classify them as fibers, ligaments, or spheroids based on their sphericity and aspect ratio. In the near field (z/D < 4), smaller and more spherical droplets are produced compared to the far field (z/D > 4). To analyze the effects of turbulence on the droplet breakup process, we track individual droplets to investigate how strong adjacent vortical structures influence droplet breakup, focusing on the near field. Approximately 70% of the droplets that evolve into spheroids detach far from the nozzle inlet (r/D > 1.5) and experience frequent breakups, averaging more than four times during their lifetime. The droplets undergoing breakup interact with strong vortical structures over 10 times more frequently than those that remain intact. Conditionally averaged flow fields further show that the droplets continuously interact with strong vortical structures before the breakup, generating opposing rotational forces. After the breakup, the maximum magnitude of the surface normal vorticity, which represents the rotational force acting on the droplet interface, decreases by nearly 35%. A comparison of the Weber number (We) for droplets interacting with strong and weak vortical structures indicates that droplets overlapping with strong vortical structures maintain higher We values (35 < We < 80). This range corresponds to the multimode breakup, ultimately leading to droplet breakup. Our findings provide valuable insights into improving nozzle designs from the perspective of recirculation zones and vortical structures, contributing to the production of high-quality spherical powders in HPGA.

Backflow (BF) events, distinguished by negative wall-shear stress (τx), are rare phenomena occurring in the near-wall region of fully developed wall turbulence. Although these events manifest as small-scale patches of viscous scales, they originate from collisions between large-scale structures (LSSs). Hence, we explore the formation of BF, focusing particularly on interactions with the surrounding LSSs to elucidate the associated inner–outer interactions. We perform direct numerical simulations of turbulent channel flows at Reτ = 180 and 550, including a narrow box simulation at Reτ = 550 to restrict the LSSs. We observe the presence of wide BFs, which are absent at the lower Reynolds number and in the narrow box simulation. These wide BFs have widths significantly larger than the mean size of typical BF regions. Temporal tracking of the BFs with surrounding LSSs and vortical structures reveals that wide BFs result from symmetric collisions between streamwise-aligned high- and low-speed LSSs, whereas narrow BFs stem from asymmetric collisions. In the symmetric collisions, the upstream high-speed structure overrides the downstream low-speed structure, forming a wide shear layer and a significant velocity jump at the interface. This induces a strong prograde vortex near the wall, which elongates laterally and descends owing to the downwash motion of the high-speed structure, ultimately inducing wide BF regions. Conversely, the narrow BF regions develop from the asymmetric collisions occurring at the sides of the spanwise-aligned LSSs, forming narrow, laterally tilted shear layers. The large-scale collisions also induce extreme positive-τx events, particularly noticeable over broad streamwise extents during symmetric collisions. These insights into BF dynamics can inform the development of novel drag reduction strategies by manipulating LSS collisions.