Side Mirror Soiling Investigation through the Characterization of Water Droplet Formation and Size behind a Generic Plate

Abstract

<div class="section abstract"><div class="htmlview paragraph">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 [<span class="xref">1</span>].</div><div class="htmlview paragraph">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.</div><div class="htmlview paragraph">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.</div></div>