Wave structure of liquid films during the transition to the turbulent flow mode

The results of an experimental study of the waves on the surface of a vertically falling liquid film in the range of the Reynolds numbers of film flow 80 $<\operatorname{Re}<$ 420 have been presented. The experiments have been carried out with the use of the field optical diagnostic methods with high spatial and time resolution. Generation of capillary ripples with a small wavelength has been detected at the crests of large three-dimensional waves. It is in these areas of three-dimensional waves where so-called “turbulence spots”, the occurrence of which is associated with a transition to a turbulent flow mode, are observed, as noted in the literature. Long-lived rounded depressions with characteristic transverse dimensions of 1–3 mm, which are formed in the region of the interaction of capillary precursors and then propagate as independent structural elements of the flow, have also been recorded.