Slipstreaming in Gravity Powered Sports: Application to Racing Strategy in Ski Cross

Aerodynamic effects and performance improvements of running in drafting formations

Drafting as a process to reduce drag and to benefit from the presence of other competitors is applied in various sports with several recent examples of competitive running in formations. In this study, the aerodynamics of a realistic model of a female runner is calculated by computational fluid dynamics (CFD) simulations at four running speeds of 15 km h^-1, 18 km h^-1, 21 km h^-1, and 36 km h^-1. Aerodynamic power fractions of the total energy expenditure are found to be in the range of 2.6%-8.5%. Additionally, four exemplary formations are analysed with respect to their drafting potential and resulting drag values are compared for the main runner and her pacers. The best of the formations achieves a total drag reduction on the main runner of 75.6%. Moreover, there are large variations in the drag reduction between the considered formations of up to 42% with respect to the baseline single-runner case. We conclude that major drag reduction of more than 70% can already be achieved with fairly simple formations, while certain factors, such as runners on the sides, can have a detrimental effect on drag reduction due to local acceleration of the passing flow. Using an empirical model for mechanical power output during running, gains of metabolic power and performance predictions are evaluated for all considered formations. Improvements in running economy are up to 3.5% for the best formation, leading to velocity gains of 2.3%. This translates to 154 s (≈2.6 min) saved over a marathon distance. Consequently, direct conclusions are drawn from the obtained data for ideal drafting of long-distance running in highly packed formations.