On-site drag analysis of drafting cyclists

The aerodynamic drag of a trailing cyclists in a tandem formation is investigated at different lateral and longitudinal separations. A Ring of Fire (RoF) experiment is conducted at the Tom Dumoulin bike park of Sittard-Geleen in the Netherlands. The method is based on stereoscopic Particle Image Velocimetry (Stereo-PIV) measurements followed by invoking the conservation of momentum expressed in a control volume to evaluate the drag force of the cyclists. Three cyclists perform a series of individual tests as well as four different drafting tests, varying their order in the group. All tests were performed at a nominal riding speed of 45 km/h; the longitudinal displacement of the drafters varied between 0.32 m and 0.85 m and the lateral displacement varied between ±0.20 m among different runs. The results from the RoF measurements show the flow field interactions between the two drafting cyclists as well as the wake of the second cyclist. It is observed that the amount of drag reduction for the trailing rider is mainly caused by the change in inflow conditions. The drag reductions of the trailing cyclist are in the range from 27% to 66% depending on the longitudinal and lateral separation from the leading rider. The aerodynamic advantage of the drafting rider decreases with increasing lateral and longitudinal separation between riders, with the lateral separation found to be more relevant. Furthermore, based on the analysis of the individual wakes, the drag reductions found by the RoF, and the drag reduction measured by Barry et al. (2014), a model is introduced that predicts the aerodynamic gain of the trailing rider based on his or her position with respect to the leading rider. Validation of the model with data from literature shows that in the near wake the model prediction is in line with literature, with an overestimation of the drag reduction when the longitudinal distance is between 0.1 m and 0.3 m.
Aerodynamics