Underbody ground vehicle aerodynamics with the Ring of Fire

Abstract This study presents an experimental investigation of ground vehicle underbody aerodynamics using the Ring of Fire (RoF) approach, enabling unprecedented quantitative three-dimensional flow visualizations. A novel facility is introduced where the Lagrangian particle tracking system is installed underground beneath a transparent floor, providing full optical access to the underbody region of moving vehicles. Helium-filled soap bubbles are employed as flow tracers in combination with high-speed imaging, LED illumination and the Shake-The-Box particle tracking algorithm to reconstruct the three-dimensional velocity fields. The methodology is first validated on a radio-controlled car equipped with interchangeable flat floor and diffuser configurations, with and without vortex generators. The results confirm that the diffuser induces significant flow acceleration and pressure reduction at its inlet, enhancing downforce, while vortex generators promote momentum exchange near the wall and suppress localized separation. Pressure fields reconstructed from the velocity data are consistent with known theory and literature on diffuser aerodynamics. The approach is subsequently applied to a full-scale open-wheel race car, demonstrating its capability to resolve complex three-dimensional flow structures under realistic operating conditions. Strong acceleration beneath the front wing and diffuser is observed, along with coherent vortical structures shed from flow control devices. The results highlight the potential of the RoF technique for detailed aerodynamic characterization and validation of high-performance vehicle designs in on-road conditions.