Implementation of the Ring of Fire system for on-site car aerodynamics

Abstract

Aerodynamic research in the automotive industry is an essential part of the reduction in resistance during driving, as it leads to an increased fuel efficiency and higher top speed. State-of-the-art aerodynamic research is conducted in wind tunnels, through numerical simulations (Computational Fluid Dynamics) and on-site coast down techniques. In recent year, a novel quantitative flow measurement technique for full-scale, on-site transiting objects has emerged: the Ring of Fire. This system is a non-intrusive, large-scale, stereoscopic particle image velocimetry measurement technique, which provides insight in the on-site flow topology around transiting objects and simultaneously determine their drag.

In this project, the Ring of Fire measurement system is implemented for the automotive industry in collaboration with Volkswagen. The Volkswagen Tiguan and Up are tested, where for the Tiguan a configuration with the radiator front open and with the radiator front closed are tested at a speed of 120 km/h. The objective of this research is to assess the applicability and feasibility of the Ring of Fire system for full-scale cars, comparing the flow visualisation in the wake of a car between different models and configurations, and additionally determining the drag and its uncertainty through the control volume approach.

A 3x3 meter field of view is used to obtained flow fields up to 5 meters in the wake. Up to 1 meter in the near wake the flow shows a clear upwash due to a vortex pair for both cars. After 1 meter, the wake of the Up transitions into a downwash, lowering the overall wake contour and causing a lateral expansion. In contrast, the wake of the Tiguan shows four vortices, of which the upper pair consists of two strong vortices maintaining upwash up to the far wake. A 'mushroom' kind of shape is the consequence, which is in accordance with the typical wake structures found in literature for estate cars. The Up however, shows a wake structure comparable to a notchback type car.

The control volume approach shows that the drag up to 2 meter is highly underestimated, which can be attributed to the incorrect reconstruction of the near wake pressure using the 2D Poisson equation. After 3 meter, the wake starts to diffuse in small scale structures which cannot be resolved by the window sizes. Therefore, the drag followed from the measurements between 2 and 3 meter, resulting in a mean value comparable to those found in wind tunnel tests and with an uncertainty of the mean at a 95\% confidence level below 3\%.

The Ring of Fire system setup in this project is capable of showing the differences between the Up and Tiguan, in terms of flow topology as well as drag values. However, a statistical distinction between the two Tiguan configurations was not established, likely due to the limited number of runs that were of sufficient quality for the Tiguan open configuration. Nonetheless, this first research towards the implementation of the Ring of Fire system for the automotive industry showed that its a promising technique, capable of defining on-site aerodynamic characteristics of cars and potentially serve as a validation tool for numerical simulations. Moreover, future research could investigate its applicability to accelerating, drafting or cornering cars, which cannot be investigated using the state-of-the-art techniques.