| Experiments have been conducted to investigate instantaneous hydrodynamic characteristics of laminar falling films of 20 cS silicon oil on an inclined plate using a photochromic dye activation (PDA) technique over Reynolds numbers of 11-220.;The film thickness was measured directly from the video pictures and the data showed good agreement with those of previous investigations. Time-averaged mean velocity data were over-predicted by classical theory, while time-averaged wall shear stress data were in reasonable agreement. Instantaneous velocity distributions indicated that the over-prediction of the time-averaged mean velocity was a result of severe over-estimations of velocity in waves. Good agreement with classical theory was obtained with instantaneous velocity profiles in non-wavy regions, while wave-front and wave-back regions exhibited limited agreement. Non-dimensionalized velocity distributions did not follow classical laminar film theory, due to the existence of interfacial fluctuations causing unsteadiness in the local velocities, or turbulent models. Waves were determined to carry between 74%-85% of the liquid mass. Comparisons of instantaneous wall shear stress to classical theory showed under-prediction in wavy regions and good agreement in non-wavy regions. A steady-state force balance suggests that wavy regions experience acceleration, non-wavy regions experience deceleration, and wave-front and wave-back regions exhibit both acceleration and deceleration. |
