Thin water films driven by air through surface roughness

The interaction between thin films and roughness surfaces has been studied when the thin viscosity-dominated films are driven by the air shear stress in the context of a high Reynolds number boundary layer theory. A number of properties of this model are examined, such as transport and pooling of water in a roughness field, heat transfer of film/roughness combinations, and rivulet formation. For rivulet formation due to the instability of two-dimensional film fronts, a general formula for the largest unstable wavelength, the fastest temporal growth rate, and the neutral wavelength has been developed from the linear instability analysis. This formula is validated using experimental data for film fronts on flat surfaces which are driven by constant surface tension gradients. This formula is also validated using numerical simulations of film fronts moving through various roughened surfaces.; To describe a water bead on a precursor film, a new disjoining pressure model is developed from a modified classical long-distance disjoining pressure model. This model satisfies the requirement that the disjoining pressure on the precursor film is larger than zero. Another advantage of this modified model is that an effective distance used in classical long-distance disjoining pressure models is avoided even when a water bead is on a dry surface. This model is validated using experimental data from aircraft icing tests.