The slowest soap-film tunnel in the Southwest

Gravity-driven soap-film channels offer a convenient way to study two-dimensional hydrodynamics in the laboratory. With recently developed quantitative soap-film diagnostic techniques, velocity and thickness field information can be acquired to provide a complete description of the flow. We present a study of soap-film flow in a state-of-the-art tilted soap-film tunnel which can sustain a mean freestream velocity about 0.5 m/s with velocity fluctuations on the order of 1%. Our investigation concentrates on the evolution of the velocity and thickness profiles with downstream distance. We observe a hydrodynamically fully developed flow within a substantial range of distances and compare our results with the analytical solution based on the assumptions of linear air drag and constant film thickness. We find air drag to be the dominant dissipative mechanism in the flow, although the role of internal viscous dissipation is also apparent. Direct measurements of the air drag coefficient are in good agreement with the values inferred from the analytical solution. (C) 2002 American Institute of Physics.