Origin of roll waves in horizontal gas‐liquid flows

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Abstract
The mechanism of formation of roll waves on a horizontal liquid layer sheared by a gas flow is examined. Measurements of the wave amplitude spectrum show that, for liquid film Reynolds numbers greater than about 100, roll waves evolve from disturbances that initially have wave‐lengths much longer than the film thickness and which grow slowly with distance. Predictions of linear stability analysis are shown to agree well with the observation of these instabilities and not the visual transition to roll waves as was suggested in earlier studies. For very thin films, the data do not give conclusive proof of the origin of roll waves, but it is speculated that these are actually solitary waves which have begun to break.