Our experimental study proves the existence of recirculation areas underneath sufficiently large solitary waves that run over gravity-driven film flows. This evidence was obtained by setting up a measurement system that moved with the wave's speed and displayed the streamlines underneath the wave. Solitary waves of different height, length, and speed, which ran over basic flows of different Reynolds numbers and inclination angles were examined systematically. We found that recirculation areas appear underneath waves, whose height exceeds a critical value. This value depends on the basic flow's Reynolds number, irrespective of the inclination angle and the distance from the inlet. Furthermore, the size of the recirculation area grows with the height of the wave. (C) 2015 AIP Publishing LLC.
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