CFD simulations of the aerodynamic drag of two drafting cyclists

The aerodynamic drag of two drafting cyclists in upright position (UP), dropped position (DP) and time-trial position (TTP) is analysed by Computational Fluid Dynamics (CFD) simulations supported by wind-tunnel measurements. The CFD simulations are performed on high-resolution grids with grid cells of about 30μm at the cyclist body surface, yielding y^* values well below five. Simulations are made for single cyclists and for two drafting cyclists with bicycle separation distances (d) from 0.01m to 1m. Compared to a single (isolated) cyclist and for d=0.01m, the drag reduction of the trailing cyclist is 27.1%, 23.1% and 13.8% for UP, DP and TTP, respectively, while the drag reduction of the leading cyclist is 0.8%, 1.7% and 2.6% for UP, DP and TTP, respectively. The drag reductions decrease with increasing separation distance. Apart from the well-known drag reduction for the trailing cyclist, this study also confirms and quantifies the drag reduction for the leading cyclist. This effect was also confirmed by the wind-tunnel measurements: for DP with d=0.15m, the measured drag reduction of the leading cyclist was 1.6% versus 1.3% by CFD simulation. The CFD simulations are used to explain the aerodynamic drag effects by means of the detailed pressure distribution on and around the cyclists. It is shown that both drafting cyclists significantly influence the pressure distribution on each other's body and the static pressure in the region between them, which governs the drag reduction experienced by each cyclist. These results imply that there is an optimum strategy for team time trials, which should be determined not only based on the power performance but also on the body geometry, rider sequence and the resulting aerodynamic drag of each team member. Similar studies can be performed for other sports such as skating, swimming and running.