Regular large waves on thick falling liquid films

Here we investigate the possibility to create regular waves on falling liquid films that would survive for long distances. The flow is investigated in a 20-mm vertical pipe at the distance of 145-180 cm from the inlet for Reynolds numbers from 140 to 400. The regular waves were created by periodic pulsations of liquid flow rate with preset frequency and amplitude at the inlet. The spatiotemporal records of local instantaneous film thickness were obtained with brightness-based laser-induced fluorescence technique. Creation of regular waves is possible within a relatively narrow range of small (up to 10 Hz) frequencies, which expands as liquid flow rate is increased. These waves survive for at least 90 pipe diameters, in contrast to wider studied cases of high-frequency excited waves on low Reynolds number films. The excited waves are similar to the disturbance waves in gas-liquid annular flow, but they do not generate fast ripples on their surface and produce no entrainment.