In a previous publication, Dietze, Leefken & Kneer (J. Fluid Mech., vol. 595, 2008,p. 435) showed that flow separation takes place in the capillary wave region offalling liquid films. That investigation focused on the mechanistic explanation ofthe phenomenon mainly on the basis of numerical data. The present publicationfor the first time provides clear experimental evidence of the phenomenon obtainedby way of highly resolving velocity measurements in a specifically designed opticaltest set-up. Characteristically, the refractive index of the working fluid was matchedto that of the glass test section to provide optimal access to the cross-section ofthe film for the employed optical velocimetry techniques, namely, laser dopplervelocimetry (LDV) and particle image velocimetry (PIV). Using LDV, time tracesof the streamwise velocity component were recorded in high spatial (0.025 mm) andtemporal resolutions (0.4 ms) showing negative velocity values in the capillary waveregion. In addition, simultaneous film thickness measurements were performed usinga Confocal Chromatic Imaging (CCI) technique enabling the correlation of velocitydata and wave dynamics. Further, using PIV the spatio-temporal evolution of thevelocity field in the cross-section of the film was measured with high spatial (0.02mm) and temporal (0.5 ms) resolutions yielding insight into the topology of the flow.Most importantly these results clearly show the existence of a separation eddy in thecapillary wave region. Due to the high temporal resolution of the PIV measurements,enabled by the use of a high-speed camera with a repetition rate of up to 4500 Hz,the effect of wave dynamics on the velocity field in all regions of the wavy film waselucidated. All experiments were performed using a dimethylsulfoxide (DMSO)–watersolution and focused on laminar vertically falling liquid films with externally excitedmonochromatic surface waves. Systematic variations of both the Reynolds number(Re = 8.6-15.0) and the excitation frequency (f = 16-24 Hz) were performed. Resultsshow that an increase in the wavelength of large wave humps, produced either byan increase in the Reynolds number or a decrease in the excitation frequency, leadsto an increase in the size of the capillary separation eddy (CSE). Thereby, the CSEis shown to grow larger than the local film thickness, assuming an open shape withstreamlines ending at the free surface.
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