On-road vehicle aerodynamics with a large-scale stereoscopic-PIV setup: “the Ring of Fire”

This paper presents the first full-scale particle image velocimetry (PIV) measurements to analyze the flow field of a car under real driving conditions. The Ring of Fire (RoF) measurement concept, introduced by Terra et al. (Exp Fluids 58:83, 2017. https://doi.org/10.1007/s00348-017-2331-0), is adapted to automotive demands to validate CFD simulations for further improvements of vehicle aerodynamics. The experiment consists of a tunnel setup, where neutrally buoyant helium-filled soap bubbles are used as flow tracers and are illuminated by two high-speed lasers. Four high-speed cameras captured the particles motion in two separate Stereo-PIV configurations with fields of view of 1.3 \(\times \) 0.6 m² and 2.8 \(\times \) 2.2 m². Data for a Volkswagen up!, while driving on a test track at a constant speed of 33.33 m/s, was acquired for the wake and the side mirror region and processed with standard multi-pass PIV algorithms, in order to quantify the flow field and estimate limits of the described measurement principle for on-road car aerodynamics. The resulting ensemble averaged velocity fields are compared with CFD simulations, showing agreement for the here considered cases within 7.0–9.7%, based on the root-mean-square error between the experimental and the numerical results. Furthermore, drag calculation from the obtained velocity fields based on moment conservation is performed and the percent difference to wind tunnel measurements reaches values below 3.0%.