As an example from the broad field of wavy falling films we take a simulation created with OpenFOAM. The picture displays a water falling film of a Reynolds number of 60 being harmonically excited with a frequency of 13 Hz. Gravitation points from the left to the right. The control volume is 600 units long (in the direction of the stream). One unit is the thickness of the corresponding smooth film. The height (perpendicular to the stream) is equal to 4 units being magnified by a factor of 50.
The red colour marks the water, the blue the air. Downstream a certain pattern can be recognized: A large solitary wave and in front of it small capillary waves.
In process engineering a falling film is a thin liquid film which flows down an inclined plate by the action of gravity. Falling films are employed in various fields, like the food industry, the pharmaceutical industry or in power plants. They have the advantage of a good heat transfer and a small hold up. Furthermore, they have a large surface for the exchange between gas and liquid (like in gas scrubbing).
A falling film flowing down a vertical wall is always hydrodynamically unstable. However, if the Reynolds number is very small, the waves may not be seen.
In the EU project EasyMED I simulated evaporating seawater falling films. Turbulence wires were employed to improve the heat transfer.