An experimental study on the transient runback characteristics of wind-driven film/rivulet flows

Abstract

A comprehensive experimental campaign was conducted to characterize the dynamic runback process of wind-driven water film/rivulet flows for a better understanding of the transient surface water transport process pertinent to aircraft icing phenomena. The experimental study was conducted by using an open-circuit wind tunnel to generate laminar boundary layer airflows with different freestream wind speeds to drive thin water film/rivulet flows over a flat test plate. A digital image projection (DIP) technique was used to achieve non-intrusive, temporally and spatially resolved measurements of the film thickness distributions to characterize the dynamic runback process of the wind-driven film/rivulet flow under different test conditions. Important characteristics of the wind-driven water runback process—such as the generation of well-organized two-dimensional (i.e., 2D) and more complicated three-dimensional (i.e., 3D) surface waves at air/water interfaces, stumbling runback motion of the film/rivulet heads in “acceleration-and-deceleration” cycles, breaking up the front contact lines to form multiple rivulet flows, meandering and merging of the rivulet flows—were revealed clearly and quantitatively based on the DIP measurements. A comprehensive force balance analysis was also performed to examine the variations of the relevant forces (i.e., the excess pressure forces built inside the film/rivulet flows, aerodynamic drag forces acting on the rivulet heads, and the restraining forces due to the surface tension along the liquid contact lines) and evaluate their importance in the breakoff of the stagnated film/rivulet heads to re-start the runback process of the wind-driven film/rivulet flows.