Race cars use aerodynamic downforce to reduce the lap times by increasing grip. Diffusers, upswept ramps located at the rear of a vehicle, are often used to enhance downforce. This investigation proposes a novel LPT facility featuring HFSB, LED illumination, and high-speed cameras to characterize the flow inside automotive diffusers. An RC car, fitted with a custom floor and diffuser, traverses a region of seeded air following the Ring of Fire methodology. Underground cameras view the car through a transparent panel, providing unparalleled optical access to the diffuser of the car. The on-site construction of the setup and the intrinsically realistic interaction with the ground, contribute to realism and fidelity while potentially reducing testing costs associated with wind tunnel operation. The setup was shown to be a valid alternative to conventional testing grounds to capture separation, 3D flow evolution and differences in the flow field between the diffusers with varying angles. The 15° diffuser led to the largest velocity (u/U=1.3) under the car, the 10° diffuser produced the most downforce overall while the 20° diffuser showcased the most prominent separation, heavily affecting its ability to sustain low pressures under the car. The results highlighted the impact of the tyres in disrupting the mechanism of downforce generation through mass flow leakage through the sides of the car.

This investigation proposes a novel LPT facility featuring Helium-Filled Soap Bubbles flow tracers, LED illumination and two high-speed cameras to characterize the dominating flow patterns within automotive underbodies. A remote control (RC) car model, fitted with custom-made floor and diffusers, traverses a region of seeded air following the Ring of Fire methodology. Underground-placed cameras view the car through a transparent panel, providing unparalleled optical access to the underbody of the car. The on-site measurement setup and the interaction between car model and ground enhance the realism and fidelity of the experiments, while potentially reducing testing costs associated with wind tunnel operation. The setup is shown to be a valid alternative to conventional testing approaches to capture flow separation, 3D flow evolution and differences in the flow field between the four tested configurations, whereby the diffuser angle was varied in the range between 5° and 20°. The 15° diffuser led to the largest velocity and pressure peaks under the car, whereas the 10° diffuser produced the most downforce thanks to the diffuser “pumping” effect, leading to a large region of low pressure under the vehicle. Notably, the 20° diffuser featured the most prominent flow separation at the diffuser’s leading edge, heavily affecting its ability to sustain low pressures under the car. The results show that the wide tyres have a major impact on the underbody flow, because their large wakes induce mass flow leakage through the sides of the car, thus disrupting the mechanism of downforce generation and impairing the generation of streamwise vortices.