On-site cycling drag analysis with the Ring of Fire

Abstract: The Ring of Fire (RoF) measurement concept, introduced by Terra et al. (Exp Fluids 58:83. https://doi.org/10.1007/s00348-017-2331-0, 2017; Experiments in Fluids 59:120, 2018), is applied to real cyclists to enable the aerodynamic drag determination during sport action. This principle is based on large-scale stereoscopic particle image velocimetry (PIV) measurements over a plane crossed by the athlete during cycling. The momentum before and after the passage of the athlete poses the basis for the control volume analysis in the athlete’s frame of reference, which returns the aerodynamic drag. This approach extrapolates aerodynamic studies towards more realistic conditions, compared to experiments performed in wind tunnels with scaled or stationary athletes. The measurement concept is termed Ring of Fire as the rider crosses a region of intense light. Two experiments are conducted, indoor and outdoor, with attention placed on the effects of the environmental conditions and the confinement of the measurement region. Stereo-PIV measurements feature a plane of approximately 2 × 2 m ² , using neutrally buoyant sub-millimeter helium-filled soap bubbles (HFSB) as flow tracers. The drag measurement is obtained examining the wake produced by the athlete. It is observed that the drag value becomes independent of time after about 5 torso lengths from the passage. A statistical estimate of the drag is produced combining the results of several passages. Fluctuations of the drag value during a single passage are associated with the unsteady wake flow. Overall fluctuations among different transits are ascribed to the varying conditions of the airflow prior to the passage of the athlete. The experiments conducted outdoor exhibit significantly larger dispersion of the drag value, compared to the quieter conditions indoor. Repetition of the transit 10–30 times yields a basis for statistical convergence of the average drag value. The flow topology past the cyclist compares satisfactorily between both experiments and with wind tunnel experiments reported in literature. The current measurements clearly separate drag values from upright and time–trial athlete’s positions, indicating the suitability of this principle for aerodynamic analysis and optimization studies. Graphical abstract: [Figure not available: see fulltext.].