On-site drafting aerodynamics of cyclists using the Ring of Fire

Abstract

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.