The present article contributes detailed experimental results on the linear stability of gravity-driven viscous films flowing down periodically corrugated inclines. We asymptotically left the well-known Nusselt flow by gradually increasing the topography’s amplitude. Systematic variations of the channel’s inclination and the fluid’s viscosity followed. That way, we revealed non-trivial stability charts and phenomena far beyond the limits of the Nusselt regime. For the sake of understanding these phenomena, we thoroughly measured the steady-state free surfaces and velocity fields of the respective flows. This comprehensive approach provided us with the exceptional opportunity to unveil that the complex shape, which stability charts of film flows over strongly corrugated inclines exhibit, can be attributed to the simultaneous presence of stabilizing as well as destabilizing effects provoked by the topography. We proved that the stabilization of the flow due to an increased film thickness and the destabilization of the flow due to resonant standing waves are competing effects. Which one dominates in this competition depends on the amplitude and inclination of the substrate and on the viscosity of the fluid.

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