Influence of Vortex Evolution on Pressure Fluctuation of Air- Knife Jet Impingement Coating

Fluctuations in pressure on the impingement wall of an air knife jet adversely affect the uniformity of the coating thickness. Vortices in the jet cause uneven local pressure and flow instability by enhancing the jet's mixing and diffusion. To investigate the influence of vortex on pressure fluctuation during the production of high-aluminum Zn-Al-Mg steel strips, the unsteady flows of the air knife jet under three working conditions at H/d=9.1, 7.1, and 5.0 in production were numerically simulated by using the Large Eddy Simulation (LES) method, and then the vortical evolutions were analyzed by the Dynamic Mode Decomposition (DMD) method. The vortical evolution, variation in impingement pressure, the characteristic frequency of pressure fluctuations on the strip surface, and the spatial structure of the jet corresponding to the typical characteristic frequency under different working conditions were compared. At H/d=9.1, vortices are fully developed with intense mixing, leading to the most unstable impingement wall structure. At H/d=5.0, vortex mixing is weak on both sides of the jet centerline, and large-scale vortices directly impinge on the surface, forming stable coherent structures. In contrast, at H/d=7.1, both the degree of vortex mixing and flow stability lie between the other two cases. The difference in vortex evolution is a potential factor influencing the uniformity of liquid‑film thickness.