The investigation of vehicle soiling by improvement of vehicle parts to optimize
the surrounding airflow is of great importance not only because of the
visibility through windows and at mirrors but also the functionality of
different types of sensors (camera, lidar, radars, etc.) for the driver
assistance systems and especially for autonomous driving vehicles has to be
guaranteed. These investigations and corresponding developments ideally take
place in the early vehicle development process since later changes are difficult
to apply in the vehicle production process for many reasons. Vehicle soiling is
divided into foreign soiling and self-soiling with respect to the source of the
soiling water, e.g., direct rain impact, swirled (dirty) water of other road
users and own rotating wheels.
The investigations of the soiling behavior of vehicles were performed
experimentally in a wind tunnel and street tests. The investigations of
self-soiling are assisted numerically by computational fluid dynamics (CFD)
simulations at the early development stage. An investigation of droplet
formation behind a side mirror was done with a generic side mirror. These
information are important for the simulation of the side window and side mirror
soiling to speed up the simulation process and to validate the simulation
results. A full-size car was examined in a wind tunnel to achieve an extensive
understanding of the processes of droplet formation behind a side mirror. The
identification of droplet breakup and detachment processes and the measurement
of the resulting droplet sizes are the main goals of these studies. The
corresponding experimental measurements of droplet velocity and size were
carried out by using high-speed cameras and a shadowgraphy measuring
technique.
A detailed insight into the physical processes of droplet detachment and breakup
behind the side mirror on a complete vehicle is shown, which can be used in the
future for the development of soiling countermeasures but especially for the
validation and comparison of complex three-dimensional (3D) soiling
simulations.
The droplet detachment process can be divided into the position and the physical
process. The detachment position is velocity dependent. At higher air
velocities, the detachment position shifts outwards toward the edge of the
mirror housing. Different physical processes can be observed for the different
air velocities. At the lower velocity of 80 km/h, a ligament formation process
can be observed, which changes to a sheet formation (laminar and later
turbulent) with increasing air velocity. The type of secondary breakup processes
occurring is independent of the air velocity. In addition, droplet sizes were
measured, and the distributions show two main peaks at 35 μm and 55 μm
independent of the airstream velocity, which could be attributed to the
break-off process and secondary decay.