It is well-known that when driving in rainy conditions the sight of the driver is reduced. As a result, either the risk of accidents rises or a higher concentration, accompanied with an increased stress level, is demanded by the driver. With the aim for an earlier development of appropriate water management concepts in advance of wind tunnel tests, a numerical strategy is elaborated in this study, based on the CFD-Software Star-CCM+. In order to consider both the small scales of interfacial physics and the large length and time scales of a full vehicle, a combination of the different modelling strategies is proposed: Lagrangian Multiphase, Fluid Film and Volume of Fluid. One essential phenomena is splashing due to droplet impingement on wetted surfaces. To enable the development of an impingement model specifically designed for the influencing factors on the wing mirror, spray impingement on a hemisphere is studied fundamentally in a small vertical wind tunnel. Apart from the deposited mass ratio, the primary droplets are characterized by the Phase Doppler technique above the hemisphere and the secondary droplets next to it. By a numerical calculation of the trajectories between measurement location and hemisphere, their direct correlation at the hemispherical surface is determined. Comprehensive investigations are also performed for the aerodynamic force on rivulets located in the boundary layer. Following a theoretical approach for the force calculation, the possibility of considering the dominating form drag force in the two-dimensional Fluid Film model is elaborated. This force is now implemented in Star-CCM+ and can be used to predict the propagation of rivulets on the side window. For different contact angles and wind speeds, very good agreement is achieved with the corresponding wind tunnel tests, where a point injector introduces well-defined rivulets on the side window. Finally, a new state-of-the-art for water management simulations of passenger cars is extensively described for the wing mirror, windshield with wiper, a-pillar and side window and their results are compared with appropriate wind tunnel tests.