Experimental analysis of the stability of the jet wiping process, part II: Multiscale modal analysis of the gas jet-liquid film interaction

Abstract

This second part of a two-part article on the stability of the jet wiping process focuses on the interaction between the impinging gas jet and the coating liquid film. Using a simplified laboratory model, the first part of the study (Gosset et al., 2019) has shown that this coating process is unstable and leads to wavy coating films. The scaling laws of the problem suggest that this coating nonuniformity does not originate from the liquid coating stability itself, but from a hydrodynamic feedback between the liquid and the impinging jet. This hydrodynamic feedback is herein investigated by combining several optical techniques and data processing methods. Time-Resolved Light Absorption (LAbs) technique is used for the 3D characterization of the coating layer thickness downstream the wiping region, while Time-Resolved Particle Image Velocimetry (TR-PIV) is used to analyze the dynamics of the impinging gas jet, in combination with liquid interface tracking via image processing and Laser Induced Fluorescence (LIF). A link between the frequency content in the liquid film and the jet flow is established. Specifically, the dynamics of the gas jet is analyzed via Multiscale Proper Orthogonal Decomposition (mPOD) of the TR-PIV data and reveals large-scale oscillations which are coupled with waves in the liquid film upstream the wiping, referred to as runback flow. The interaction between the flow structures responsible for the oscillation and the runback liquid film is discussed, showing how this self-sustained mechanism is potentially responsible for the nonuniformity of the final coating.