External water management: A predictive challenge

Predictive capabilities of film, rivulet and drop transport on vehicle surfaces are desirable, since experimental investigations are complex and only possible at late stages of vehicle design. However, simulations and/or experiments remain essential, either related to wing-mirror sight demands or, more recently, to insure proper operation of sensors and/or imaging devices. Major difficulties are presently apparent with such predictions, because physical models describing basic wetting phenomena encountered on typical vehicle surfaces and geometries are lacking or are not implemented in numerical codes. These include processes such as wettability influence on drop impact and splashing, shear-driven film-rivulet-drop transition, incipient motion of drops, drop interaction with grooves and material discontinuities, film/rivulet/drop stripping from edges, etc. Improvements to present models implemented in simulation codes can only be expected once a basic physical understanding of these processes is acquired. The present contribution summarizes the main challenges in developing models and outlines a series of generic experiments with the express purpose of meeting these challenges. The structure of such models at the micro through to the macro scale is described. Implementation of these models in existing codes is also discussed and the outlook for a prediction of external water management is drawn.