PRINCIPLES OF HYDRODYNAMIC INSTABILITY - APPLICATION IN COATING SYSTEMS .2. EXAMPLES OF FLOW INSTABILITY

With new high-speed paper machines, coating rheology and flow instability are critical issues. The ideal flow in a coating device is steady-state and two-dimensional. In other words, it would have no velocity components or momentum transfers in the cross-machine direction. In coating, complex fluids are applied to paper at high speeds under extreme flow conditions, so the problems of fluid mechanics and instability are often unique, requiring fundamental, in-depth analysis. Flow-through coating devices as nonlinear dynamic systems can only behave in a specific and consistent manner. By knowing the fundamental physical and dynamical principles (such as general behavior and transitions between steady and time-periodic states), we are better equipped to deduce and interpret information from experiments or computational analysis of instability problems. Mathematical examples are included to provide simple but practical working tools.