An assessment of the Ring of Fire approach for indoor and outdoor on-site sports aerodynamic investigation

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.