Surface-active agents, present in water running at low velocities in laboratory flumes, will adsorb to the water surface and form a film on the upstream side of a surface barrier in the flow. This film has an impact on velocity measurements at the water surface. A force balance on the film indicates that the shear stress of the moving fluid below the film is equal to the negative gradient of surface tension within the film. Previous empirical measurements did not agree with laminar boundary layer theory used to predict the surface tension gradient with distance in the film. New experiments were conducted, and flow visualization showed that transition from a laminar to a turbulent boundary layer occurred. A new theory was formulated to include transition, allowing the steady-state film length to be estimated. Since the film length does not scale with Froude number, surface velocity measurements in the film region for laboratory flumes and hydraulic model studies will not scale properly. The estimate of film length from the theory formulated and tested in this paper may be used to determine if representative surface velocity measurements may be made in hydraulic model studies or experimental laboratory flumes.
Bibliographical noteFunding Information:
This research was funded by a grant from the Legislative Commission on Minnesota Resources, Minnesota State Legislature. The authors thank Professor Ray Krone for his pre-submittal review of the manuscript and the suggestions offered.