The improvement of vehicle soiling behavior has increasing interest over the past
few years not only to satisfy customer requirements and ensure a good visibility
of the surrounding traffic but also for autonomous vehicles, for which soiling
investigation and improvement are even more important due to the demands of the
cleanliness and induced functionality of the corresponding sensors. The main
task is the improvement of the soiling behavior, i.e., reduction or even
prevention of soiling of specific surfaces, for example, windows, mirrors, and
sensors. This is mostly done in late stages of vehicle development and performed
by experiments, e.g., wind tunnel tests, which are supplemented by simulation at
an early development stage. Among other sources, the foreign soiling on the side
mirror and the side window depend on the droplet detaching from the side mirror
housing. That is why a good understanding of the droplet formation process and
the resulting droplet diameters behind the side mirror is necessary, including
all the possible influencing factors. A first fundamental study was already
investigated by the authors at a vehicle side mirror using a modified wind
tunnel soiling test rig [1].
In the current study, a generic plate with a defined edge for droplet detachment
is placed inside the open jet of a calibration wind tunnel. The aim of the
investigation is to test different parameters of the detachment process and to
analyze the resulting droplet sizes behind the generic edge. Parameters such as
the radius of the detachment edge, the liquid, and the volumetric flow rate are
varied and their influence on the atomization as well as soiling processes is
analyzed using shadowgraphy measuring technique and high-speed video
recording.
Our results show that the droplet detachment behind a generic plate can be
divided into two separate topics: the detachment position and the detachment
process. The detachment position is dependent on the air velocity and the edge
radius. With an increased radius and an increased velocity, the water can stay
longer wall-bound and detach further downstream. The detachment process at the
edge shows two different ligament formation processes independent of the air
velocity: ligament formation and ligament-bag formation. A secondary breakup
process may occur for droplets in the air, where four different processes are
observed, covering vibration breakup, bag breakup, bag-and-stamen breakup, and
sheet thinning. In all tested variations, most of the droplets show a diameter
of about 65 μm.