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.]. @en

A procedure is proposed to reconstruct the instantaneous velocity field from full particle trajectories in a data assimilation framework that includes the vorticity transport equation. The technique is christened as time-segment assimilation (TSA). The work addresses the common problem of low seeding concentration in 3D experiments, usuallThe Ring of Fire measurement system is deployed for the measurement of the aerodynamic drag of transiting cyclists. The drag force is evaluated using large-scale stereoscopic PIV and invoking the conservation of momentum within a control volume in a frame of reference moving with the athlete. Two experiments are carried out that yield the cyclist aerodynamic drag in time-trial and upright position in indoor and outdoor conditions. The rider cycles at a velocity of approximately 5 m/s and 8 m/s for respectively the indoor and outdoor experiment, corresponding to a torso based Reynolds number of 2.1 × 105 and 3.2 × 105. The indoor measurements are conducted at a rate of 8 Hz within a measurement plane of approximately 1.8 × 2.4 m2. The outdoor measurements are conducted at a rate of 2000 Hz within a measurement plane of approximately 1.8 × 1.8 m2. Neutrally buoyant helium-filled soap bubbles are used as flow tracers. Despite the fact that two different cyclists and two different bikes were used and that the local angle of attack of the body was different, the streamwise velocity and vorticity fields compare well between both experiments and to literature. Results from both experiments show the same peak momentum deficit as well as the same main and secondary vortices. A clear distinction in upright vs. time-trial ensemble–averaged drag area is found for both experiments. Furthermore, the indoor experiment shows it is possible to distinguish smaller variations in the drag area between two postures, namely between a time-trial asymmetric and symmetric configuration. Small drag differences (≈ 5%) with less than twenty samples per case are detected. y leading to limited spatial resolution. In the present study the measurement fidelity and spatial resolution are increased by considering finite time-segments as a whole for instantaneous velocity reconstruction. The use of a time-segment for velocity field reconstruction from measurement data extends previously proposed data assimilation techniques that consider only instantaneous measurement data (e.g. VIC+ and FlowFit), to use finite measurement time-segments. The assessment with sinusoids indicates lower errors due to modulation. However, the appearance of a range of amplified peaks is not fully understood. In the case of a simulated turbulent boundary layer measurement more vortical structures are recovered when a longer time-segment is used for the velocity field reconstruction. @en