Aerodynamics plays a significant role in the field of cycling as it is the dominant resistive force at racing speeds of 50 km/h. Sporting events involving multiple cyclists leads to aerodynamic interactions between the cyclists and these interactions have been exploited for performance benefits. One such aerodynamic interaction is drafting, where cyclists closely follow each other and experience a reduction in aerodynamic drag. The benefits of drafting are profound with drag reductions as high as 49\%. The aerodynamics of drafting have been studied previously using qualitative and quantitative techniques such as wind tunnel test, numerical methods and track testing. However, large discrepancies exist between various studies partly due to the fact that most investigations focus either on flow visualisation or drag measurements, and thus a complete picture is not obtained. The Ring of Fire technique is an innovative flow measurement system that provides both flow field information as well as aerodynamic drag force for full-scale on-site transiting cyclists.

The Ring of Fire technique is used in the current study to investigate the aerodynamics of on-site drafting cyclists in an outdoor environment. The effect of drafting distance and cyclist size on drag reduction are investigated using different configurations of two cyclists in drafting formation. Large-scale time-resolved stereoscopic Particle Image Velocimetry (PIV) is conducted using Helium Filled Soap Bubbles (HFSB) at cycling speeds of 13.3 m/s. Planar pressure fields are reconstructed from velocity data using the 2D pressure Poisson equation (PPE). Modifications and improvements are made to existing data reduction techniques utilised in previous Ring of Fire experiments.

Qualitative examination of ensemble averaged flow fields is performed for the full wake of an individual cyclist and the near wake of the three cyclists are compared as well. Quantitative analyses of drag area, focusing on variation with distance, sensitivity to wind and statistical uncertainty are conducted. Measurements of individual cyclists show that the size of the cyclist is a qualitative indicator of relative aerodynamic performance between cyclists, provided cycling equipment and skill are reasonably common between them. Statistical uncertainty of the individual measurements are improved from previous outdoor Ring of Fire experiments.

Flow fields from drafting obtained in-between the cyclists and behind both the cyclists, which are compared with the wake of individual cyclists and the main mechanism of drag reduction for the trailing cyclist is addressed. Quantitative analyses of drag area of the leading, trailing and the two cyclists as a group are performed, with a particular emphasis on drag reduction of the trailing cyclist and its dependence on longitudinal and lateral drafting distances. Anomalies in the drag data indicate towards a complex interaction between the leader wake, phase difference in crank angles and the wake of the trailing cyclist. ... Read more

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. ... Read more