STAT IST ICAL CHARACTERIST ICS OF FREE FALL ING F ILMS AT H IGH REYNOLDS NUMBERS

Abstract--The characteristics of films flowing inside a vertical pipe are studied experimentally. Using an accurate wire conductance t chnique the film thickness is measured over the Re range 509-13,090. The mean film thickness data are in good overall agreement with established relations. Extensive statistical analysis hows that film thickness fluctuations have a stochastic character. The description given by Dukler and co-workers, of a relatively well-defined substrate in between the large waves, is confirmed up to Re ~ 7000. At higher Re the substrate tends to lose its identity, due to amplification of the small waves on its surface. Moreover, data on standard deviation, hm ~ x and hm ~. indicate that at Re> 5000 the amplification of large waves essentially stops, and that the substrate thickness tends to increase. The calculated probability density functions of film thickness, the spectral density functions, the skewness and kurtosis provide new reliable information for the statistical description of film thickness fluctuations. Finally, by identifying the wave peaks it is found that the total number of waves is nearly constant, in the range of Re studied, and that the distribution of wave peaks becomes bimodal at Re ~> 4000