Stability of the channel flow—new phenomena in an old problem

Abstract

In this article, we show an unexpected effect of side walls on the stability of gravity-driven viscous film flows over flat substrates in an open channel. Until now, it was assumed that side walls have a stabilizing effect on the flow, but the qualitative structure of the neutral curve remains the same. We show a fragmentation of the stability chart that was only known from undulated substrates. The cause of the fragmentation is a distinct damping of waves with a certain frequency that is independent of the underlying instability of the two-dimensional flow. This paper provides a detailed parameter study that identifies the channel width as the decisive parameter for the damping frequency. In addition, the damping is not an effect of flat side walls exclusively, since corrugated side walls have qualitatively the same impact on the stability chart. Furthermore, we demonstrate that the curvature of the emerging wave’s crest line has a non-monotonous dependency on the frequency and shows a distinct maximum. The frequency of this maximum shows the same behavior as the damping frequency when the channel width is varied. We therefore assume a relation between the wave’s curvature and its growth rate, both massively affected by the side walls.