Accuracy assessment of the Ring of Fire system for on-site aerodynamic drag measurements

Abstract

The continuous pursuit of reducing drag in many speed sports, such as cycling and ice-skating, demands novel approaches to gain further insight into flow phenomena around athletes. In recent years, the Ring of Fire measurement technique has emerged as a feasible option to visualise and analyse flow structures of transiting objects based on particle image velocimetry. The accuracy of this on-site measurement technique has not yet been validated under equal test conditions.

This master's thesis aims to compare drag area values of a cyclist from Ring of Fire measurements to simultaneously acquired power meter data. Tests with the cyclist in upright and time-trial posture, as well as different helmet types and various drafting distances, are envisaged to assess the correlation between the two measurement techniques in multiple drag area regimes, and to gain insight into large distance drafting above 3m, which, to the best knowledge of the author, has not yet been studied in academic research. In addition, the campaign plans to remove any user operations during the test, which would be another step towards a fully autonomous Ring of Fire system, as envisioned in the future. A spacious indoor facility is suggested as the testing site to minimize environmental effects and to allow for the continuous motion of the cyclist. The Ring of Fire method shows great potential, as the measurements are conducted under simulated racing conditions and wake visualisation allows the operator to locate origins of drag. Validating the drag area results could further attest to the Ring of Fire's viability as an optimisation tool in the upcoming years.

The conducted campaign, within the framework of this thesis, indicates good agreement between the power meter and Ring of Fire techniques when assessing the relative drag area delta of a small-scale helmet change and a large-scale posture change. In terms of absolute values, the power meter model shows a high dependency on underlying model constants. Using literature-based coefficients, the absolute CdA values are within 5% of the Ring of Fire derived values.

Furthermore, the feasibility of evaluating long distance drafting effects with the Ring of Fire system is demonstrated. Measurable drag area savings of 15% are obtained by the trailing cyclist at front wheel to front wheel distances of 7-9m. Due to non-uniform inflow conditions in front of the trailing cyclist, a wake contouring algorithm needed to be employed to satisfy mass preservation within the control volume by resizing the inlet and outlet plane. In addition, enclosure of the wake structure and contouring of a representative inlet plane is achieved. The flow topology in the wake of the trailing cyclist is acquired by a stereo-PIV system. Primary wake structures, as well as in-plane velocity fields, are comparable to those observed behind isolated riders.