Coating Film Profiles Generated by Fluctuating Location of the Wiping Pressure and Shear Stress

Dynamics of the jet wiping process via integral models

“…Hocking and coworkers concluded that the coating film is neutrally stable and incapable of producing the undulation patterns observed in the wiping lines without the presence of disturbances produced by the gas jet. Using the same quasi-steady formulation, Johnstone et al (2019) investigated the response of the liquid coat to a set of possible unsteady behaviour of the impinging jet. The formulation presented in these works neglects the role of inertia in the liquid film, disregarding the nonlinear contribution of advection.…”

Vortex Dynamics and Fluctuations of Impinging Planar Jet

“…This indicates that H/d ≥ 6.0 is a non-ideal condition for CGL operations if the objective is to minimise the non-uniformity of the coating thickness produced by the jet wiping process; under this condition impingement pressure fluctuations are present at frequencies to which the coating thickness has been identified to be sensitive. 12,13) The presentation of Fig. 12 together with Eq.…”

“…Numerical studies of air-coating liquid interaction showed that jet oscillation is reflected on the final coating thickness 9,10) with the low frequency components of the impingement wall pressure fluctuation correlated to the waviness of the coating surface. 11) Furthermore, an analytical coating model 12,13) demonstrated the coating thickness variation is sensitive to the fluctuation frequencies of both the magnitudes and positions of pressure and shear stress profiles. Using non-dimensional frequency parameter, df jet /U, where f jet is the fluctuation frequency of the wall pressure and wall shear stress profile (either magnitude or position), it was shown that thickness variation is small (less than 1% of the average coating thickness) when df jet /U > 1 and, in addition, the thickness variation increases as df jet /U reduces with a peak at df jet /U ~ 0.05 for both magnitude and position fluctuation, respectively.…”

“…Using non-dimensional frequency parameter, df jet /U, where f jet is the fluctuation frequency of the wall pressure and wall shear stress profile (either magnitude or position), it was shown that thickness variation is small (less than 1% of the average coating thickness) when df jet /U > 1 and, in addition, the thickness variation increases as df jet /U reduces with a peak at df jet /U ~ 0.05 for both magnitude and position fluctuation, respectively. 12,13) A possible cause of jet oscillation is attributed to the dynamics of the jet shear layer vortices. Vortices are formed at the jet shear layers due to the large velocity gradient between the high-speed jet and the surrounding quiescent air.…”

“…To investigate the ripple formation, studies have focussed on one-way coupling or two-way coupling between the unsteady jet flow and the liquid coating layer. Fluctuating wall pressure and wall shear stress were either assumed 6,13,29) or obtained from numerical investigation [30][31][32] and applied in an air knife model. Two-way interaction between an unsteady jet and the coating layer has been studied using LES-VOF (Large Eddy Simulation combined with Volume of Fluid).…”