An experimental study on the transient behavior of wind-driven water runback over a flat surface

Under glaze icing condition, the stagnation of rivulet/film flows will provide enough icing time and water mass for local ice accretion. Previously investigations about the rivulet stagnation usually ignore the air-water interaction over the transient-complicated rivulet surface. In the present study, an systematic experimental investigation was conducted to elucidate the force balance criterion during the rivulet stagnation process. Instantaneous and time-average rivulets thicknesses were quantitatively measured by the digital image projection (DIP) technique. The details of micro-structures like rivulet breaking, surface waves and rivulet meandering were clearly presented. The obtained rivulets thicknesses were further processed to get transient rivulet front velocities. The force-balance rivulet breaking criterion was refined by those micro-physical phenomena. It is found that the inertia force of rivulet flow can not be neglect even the Weber number of the flow is very small. The surface waves affect the aerodynamic drag by changing the area difference between the rivulet body and the rivulet front. Therefore, the transient rivulet stagnation behaviors are significantly influenced by the surface waves. However, surface wave will not influence the moving velocity of the rivulet front for a relative long time scale. The other interesting topic in the current study is wind-driven rivulet meandering. Rivulet meandering phenomena were initiated during processes of the film to rivulet breaking, and merging of the rivulets. A novel rivulet meandering instability threshold was developed based on the experimental results. We find the capillary force due to meander curvature and the centrifugal term can be neglected under small Weber number condition. The force balance of a meandered rivulet is simplified as equilibrium between surface tension and aerodynamic drag. The yaw angle of the meandered rivulet was predicted with the new theory, and the calculation result match well with experimental result.