A hybrid spectral/finite-difference to transient free-surface flow inside thin symmetric cavities of longitudinally varying thickness

Abstract

The lubrication theory is extended for transient free-surface flow of a viscous fluid inside three-dimensional symmetric thin cavities of thickness that varies in the flow direction. The problem is first formulated for a cavity of arbitrary shape. The moving domain is mapped onto a rectangular domain at each time step of the computation. The pressure, which in this case is governed by the modified Laplace's equation, is expanded in a Fourier series in the spanwise direction. The expansion coefficients are obtained using the finite-difference method. Only a few modes are usually needed to secure convergence. The flow behaviour is strongly influenced by the cavity thickness. The flows inside a straight, contracting, expanding, and modulated cavities are examined. It is found that while the evolution of the front is always monotonic with time, that of the velocity at the front can be oscillatory if the degree of contraction of the cavity (whether modulated or not) is significant. The velocity of the contact point along the lateral walls is usually larger than that at the front, leading to the straightening of the front. However, for modulated cavities, the front may advance at a faster rate, leading to its own undulation. Copyright © 2005 John Wiley & Sons, Ltd.