Experimental analysis of spray topology in the wake of an automotive body

Advanced driver assistance systems rely on external sensors that encompass the vehicle. The reliability of such systems can be compromised by adverse weather, with performance hindered by both direct impingement on sensors and spray suspended between the vehicle and potential obstacles. The transportation of road spray is known to be an unsteady phenomenon, driven by the turbulent structures that characterise automotive flow fields. Further understanding of this unsteadiness is a key aspect in the development of robust sensor implementations. This paper outlines an experimental method used to analyse the spray ejected by an automotive body, presented through a study of a simplified vehicle model with interchangeable rear-end geometries. Particles are illuminated by laser light sheets as they pass through measurement planes downstream of the vehicle, facilitating imaging of the instantaneous structure of the spray. The tested configurations produce minor changes to the flow field, the impact of which is observed in time-averaged views of the spray. Analysis of the instantaneous data depicts a highly dynamic spray plume, characterised by independent "clusters"of particles, as well as long, tendril-like structures, the development of which may result in misidentifications by classification algorithms. Proper orthogonal decomposition identifies comparable structures in all three configurations, with the fluctuating energy content distributed across many modes, in a further indication of the dynamic nature of the spray. The discussed method aims to aid in the understanding of the physics responsible for downstream spray transport as the industry looks towards the widespread adoption of fully autonomous vehicles.