Hydrodynamics of three-dimensional waves in laminar falling films

Experiments were performed to investigate the flow and surface structure in laminar wavy films over a Reynolds number range from Ref=ρūδf/η=27–200. Measurements of the velocity distribution by particle image velocimetry and film thickness by a fluorescence technique enabled to gain detailed information on the transient conditions within the three-dimensional wavy flow. In the entire range of Reynolds numbers, the flow in the wave crest is in a decelerated state, as its momentum is partially transferred into the near wall region, which results in acceleration of the wave back above the equilibrium state. This also affects the residual film behind the waves and causes subsequent waves to collide with their predecessors. ree-dimensional effects and the wave collision frequency increase with increasing flow rate. Transitions from streak-like to surge-like waves and the development of turbulent spots are first observed to occur at Ref≈75. The wave shapes at Ref≈200 become completely unsteady and approximately every second wave collision causes the formation of a turbulent spot.