Evolution of coherent structures in parallel jet impingement of air knife

Coherent structures dominate energy transport, momentum exchange, and mixing in air jets and therefore govern impingement and shear behavior on the solid surface. In this study, the evolution of coherent structures in impinging jets of air knives was investigated numerically using large eddy simulations. Three cases with a height to width ratio (H/d) of 9.1, 7.1, and 5.0 in a hot-dip galvanizing process were studied. The dominant modes of the velocity fluctuation and pressure fields were extracted using the proper orthogonal decomposition method, and the power spectral density of these modes was analyzed. The results show that pairs of vortices in the jets form and evolve downstream of the air knife exit, and they subsequently undergo mixing and breakup diversely near the steel strip wall. Based on the energy decay rate, the first and second modes of the velocity fluctuation and pressure fluctuation were identified as their respective dominant modes. The phase angle difference between the transverse velocity and pressure fluctuations governs the intensity of vortex mixing. At H/d = 9.1, the near-wall flow comprised large-scale, low-frequency structures. At H/d = 7.1, the jet comprised complex multi-scale structures, including large-scale vortices and independent secondary structures. At H/d = 5.0, the large-scale vortical structures broke up, and the flow energy was redirected to high-frequency, small-scale structures. These flow regimes were possibly relevant to the quality of the coating in the galvanization.