Aerodynamically Driven Surface-bound Liquid Flows: Characterization and Modeling of Wetting Patterns
The interaction of rain with a moving vehicle leads to the partial deposition of water drops on its surface. This results in thin surface bound film or rivulet flows moving under the combined action of gravity and aerodynamic forces. In the transportation industry, an actively engineered guidance o...
| Summary: | The interaction of rain with a moving vehicle leads to the partial deposition
of water drops on its surface. This results in thin surface bound film or rivulet flows moving under the combined action of gravity and aerodynamic forces. In the transportation industry, an actively engineered guidance of these flows has been identified as an effective measure to influence for example the soiling process of cars or the accretion of ice on airplanes, when flying through clouds of supercooled liquid drops. However, methodologies presently applied in the design process are mainly restricted to empirical testing in environmental wind tunnels. A simulation of such aerodynamically driven wetting is desirable, but to date involves extremely high computational efforts and costs and is in general not feasible.
A significant decrease in this computational cost can be achieved by identifying and physically describing recurring elements of aerodynamically driven thin liquid flows in the form of models. These models can then be employed in computational schemes, allowing a coarser grid resolution. The development of such models for a selection of exemplary surface bound liquid flows is the goal of the present experimental and analytical study.
To this aim a wind tunnel experiment has been designed in which a freely developing wetting pattern on a surface under controlled input conditions can be examined using visualization techniques. The acquired information about the flow of films, rivulets and drops allows changes in the wetting patterns under variation of input parameters to be identified, most on a statistical basis. The derived dependencies can then be used as input for model formulation.
The modeling that has been carried out concentrates on the dynamics of aerodynamically driven film flows and on instability mechanisms which can explain the breakup of films and the formation of dry patches on the surface. The dynamics of the resulting film is then analyzed, exhibiting its characteristic V-shaped contraction into a rivulet. Finally, a criterion for the onset of aerodynamically driven rivulet meandering is introduced. The models developed for these flows exhibit good agreement with the experimentally captured data indicating that the main influencing factors have been properly identified. This offers the possibility to improve the numerical code used in vehicle development.
In an outlook, it is proposed that the categorization of wetting patterns can serve as a framework to allow more detailed investigation on constitutive elements of a wetting pattern. In particular, it is suggested that future research concentrates on the dynamics of rivulet flow, as this element is prevalent in wetting patterns and existing explanations of rivulet stability and propagation are sparse, especially when an outer airflow is involved. |
|---|