Coating film profiles generated by fluctuating location of the wiping pressure and shear stress

Impinging planar jets are a widely used means of removing excess drag-out coating material from steel strip in order to control the final thickness of the applied coating. A wide range of possible coating defects are known to occur for this process, many of which are suspected to have their origin in the spatio-temporal characteristics of the air jets. It is therefore of interest to improve understanding of the link between the unsteady flow behaviour inherent to impinging jets and the evolution of the coating free surface produced by the gas-wiping process. In this paper, the coating response, characterised by the amplitude and frequency of the coating thickness fluctuation, throughout both the active region of the gas-wiping jets and the region immediately downstream, is investigated using a numerical model. The pressure and shear stress profiles acting on the coating surface along the strip are imposed as time-varying inputs such that for both the pressure and shear the vertical location of the entire profile undergoes sinusoidal oscillation parallel to the strip. A range of amplitude-frequency combinations for the vertical oscillation of the profiles are employed to assess the combined effect of these parameters on the coating response. Additionally, the strip speed is a varied parameter. Both the magnitude of the coating thickness fluctuation and the corresponding shape of the coating surface profile along the strip are found to be dependent on the strip speed and the oscillation amplitude and frequency of the vertical location of the pressure and shear stress profiles.