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of the coating thickness for long wavelength perturbations. Ellen and Tu 3) have suggested a non-dimensional model that takes into account the surface shear stress and their model gives significantly more accurate prediction of the final coating thickness. Also, Tu and Wood 4) have experimentally demonstrated that the final coating thickness depends on both the pressure and shear stress distributions on the strip surface. Although Tuck and Vanden-Broeck.5) and Yoneda 6) suggested that the final thickness depends on the surface tension and the shear stress on the strip, recently Gosset and Buchlin 7) have shown that the surface tension does not play any role in determining the final coating thickness.There have appeared several numerical simulations of the gas wiping process to predict the final coating thickness by using CFD. One of the recent studies is that by Lacanette et al. 8,9) who numerically simulated the final coating thickness by using volume of fluid-large eddy simulation (VOF-LES) modeling. Their results were satisfactorily compared with their experimental data. They found that the coating thickness is independent of the nozzle-to-plate distance when it is smaller than 8 times of the nozzle slot width. Myrillas et al. 10) tried to validate a model for the gas jet-liquid film interaction in the gas wiping process. In their study, they found that two-phase numerical simulations using LES is more suited to model of the complex interaction than k-e turbulence model. Through these previous studies it can be concluded that the dominant factors affecting the final coating thickness are pressure gradient and shear stress on the strip.So far, most of the previous studies were focused on the average final coating thickness along longitudinal direction and their numerical and mathematical models are made in two-dimensional domain. In spite of its good productivity and easy control of the zinc coating thickness, however, the coated film surface after gas wiping has frequently three dimensional surface defects such as dents, blow lines, peculiar features and sag-lines.11) Therefore, in order to obtain a uniform coating on the strip, it is necessary to investigate in details the three dimensional character of the coated surface after the gas wiping process.Among these surface defects, the sag lines (or snaky coating) cause many problems such as irregularity in the electrical and thermal characteristics and the diffused reflection on the coated surface. The snaky coating is that with an oblique patterns appearing on the coated film surface after the gas wiping process. Depending on its seriousness, the snaky coating is usually classified into five grades as shown in Fig. 2. The arrows in Fig. 2 indicate the moving direction of steel strip. The first grade indicates that no sag line is observed on the surface. In the surface pattern of the 2nd grade, rather short sag lines appear irregularly, thus a pattern of sag lines is vaguely discernible. On the other hand, the 3rd-5th grade snaky coatings reveal oblique...

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When galvanized steel strip is produced through a continuous hot-dip galvanizing process, the thickness of the adhered zinc film is controlled by a gas wiping process. In the gas wiping process there is a technically serious problem which is called a “check mark problem”. The check mark is caused by non-uniform coating on the steel strip surface. Such a non-uniform zinc coating lowers the quality, productivity and profit of the end products. In the present study, to find out the causes of the check mark and technical methods to resolve the check mark problem, the flow field of the high speed rectangular nitrogen gas jet which is impinging on the moving steel strip in the continuous hot-dip galvanizing system has been investigated numerically by using a commercial 3-D flow analysis code, FLUENT. LES (Large Eddy Simulation) is used to obtain instantaneous flow field in the region under consideration. Numerical studies were conducted for two ratios of the plate distance (d) to the nozzle width (x) d/x = 6.7, 10.5 under the same jet Reynolds number of Re = 20000. It was found that the check mark is caused by the alternating vortices which are generated on the jet impinging line (stagnation line). The center of the alternating vortex has a relatively low pressure compared with the periphery of the vortex. The high impinging pressure removes the adhered molten zinc more than the low pressure. Hence the non-uniformity of the zinc coating appears on the strip surface. Such the alternating vortices move periodically to the right and to the left sides on the impinging line due to the jet flow instability and the pressure force balance. In addition since the strip moves upward at a constant speed, the non-uniform coating results in a variety of patterns like “W”, “V” and “X”. This pattern is collectively called as “check mark” in the production field. The angle of the check mark was calculated by using both the moving speeds of the steel strip and the vortices. It was favorably compared with the experimental measurement.