Predicting Exterior Water Management is important for developing vehicles that meet customer expectations in adverse weather. Fluid film methods, with Lagrangian tracking, can provide spray and surface water simulations for complex vehicle geometries in on-road conditions. To cope with this complexity and provide practical engineering simulations, such methods rely on empirical sub-models to predict phenomena such as the film stripping from the surface. Experimental data to develop and validate such models is difficult to obtain therefore here a high-fidelity Coupled Level-set Volume of Fluid (CLSVOF) simulation is carried out. CLSVOF resolves the interface of the liquid in three dimensions; allowing direct simulation of film behaviour and interaction with the surrounding air. This is used to simulate a simplified wing-mirror, with air flow, on which water is introduced. The film shows very different behaviour on the in-board section, where a film is developed which eventually breaks to rivulets, and the end of the mirror, where the water is rapidly stripped off the surface due to the higher shear stress from the air. The same case is simulated using a fluid film method, which shows that a simple film stripping model based on film height is not capable of predicting the different regimes observed with CLSVOF. However, a model based on wave stripping due to Kelvin-Helmholtz instability is seen to give good agreement, as was a model based on local film velocity, surface curvature and body force. As well as informing the development of a film stripping model, this also illustrates how a high-fidelity simulation can be used as a tool for developing practical engineering software.
