We aim at studying the stability and the dynamics of liquid films sheared by a confined counter-current gas flow within vertical and inclined channels.
Such a two-phase flow is commonly employed in several engineering applications, such as distillation processes.
The nature of the interfacial dynamics is the key to intensify heat and mass transfer between the liquid and the gas: the more the interface is disturbed by waves, the greater the inter-phase transfer is. This is valid as long as the liquid does not flood the channel, which represents an unwanted situation for the industrial processes.

In this work, we focus firstly on the linear stability of such a two-phase flow. We show that the Kapitza instability, responsible for the development of long waves on the liquid film, can be fully suppressed by confining the gas phase with an upper wall. This is valid in both the configurations of counter-current laminar gas and aerostatic gas, i.e. when the pressure gradient balances the gravity in the gas, which displays a Couette-like velocity profile.
The critical confinement at which the suppression of the Kapitza instability occurs depends on the angle of inclination of the channel as well as on the liquid Reynolds number.

At weaker confinements instead, the cut-off wavenumber displays a non-monotonic trend by increasing the gas velocity, meaning that the imposed gas flow rate is able to control the transition between stable and unstable regions in falling films.

These results have been obtained by means of the numerical resolution of the temporal two-phase Orr-Sommerfeld problem, and have been also validated by our experimental observations.

In a second step, we study the non-linear interfacial dynamics in the configuration of gas-liquid flows in confined inclined and vertical channels. For this, we employ both direct numerical simulations and a two-phase long-wave integral model.
At low inclination angles and under aerostatic gas conditions, increasing the level of confinement provides a non-monotonic behaviour of the maximum film thickness, i.e. the crest of travelling waves. By increasing instead the velocity of the counter-current gas flow at very strong confinements, the maximum film thickness simply decreases. This occurs both at low inclination angles and in vertical channels, although for the latter case the effect of the gas flow is less pronounced.