Computational and experimental investigations of flow over athletes are key elements of sports fluid dynamics to analyse performance and equipment design. However, the use of athlete-specific models often limits reproducibility, collaboration, and data sharing due to ethical and competitive constraints. This paper introduces and defines Generic Athlete Models as openly available, standardized geometries accompanied by benchmark flow datasets. The case is made that such models are essential to improve the reliability, comparability, and transparency of fluid dynamics research in sports. While only a few generic athlete models currently exist, this paper outlines clear directions for their further development and broader adoption.

Research in cycling aerodynamics is performed using mannequins of different geometries, which are usually not shared, thus hampering the advancement of our understanding of the flow around a rider on the bike. The primary outcome of this work is to introduce and openly share two anthropometrically realistic generic cyclist models, one in time-trial and one in sprint position. These two models are obtained by averaging the scans of 14 male elite cyclists. The average cyclist geometries are published and openly accessible, making them unique in the field of cycling aerodynamic research. The second objective of this work is to better understand how the difference between the sprint and time-trial position affects the velocity and vortex topology in the wake of a cyclist and, in turn, the aerodynamic drag. Robotic volumetric particle image velocimetry measures the time-average velocity for each mannequin within a wind tunnel. One meter downstream of the lower back, the wakes of the two mannequins are dominated by strong hip/thigh streamwise counter-rotating vortices, which induce a downwash behind the riders’ backs. The strength of these vortices downstream of the sprint model is significantly larger than that of the vortices of the mannequin in the time-trial position. The same holds for a secondary vortex pair that originates from the upper arms and hips. In addition to the vortex strength, the aerodynamic drag area of the sprint model exceeds that of the time-trial model. Hence, it is presumed that stronger vortices relate to higher aerodynamic drag. In contrast to the drag area, the drag coefficient of the two models is the same. Further research is necessary to understand the relation between the cyclist position, the flow topology and the drag coefficient. Finally, the flow around the time-trial model is described in further detail to understand the origin of the different vortex structures. Through comparison to the literature, a vortex topology classification is postulated for the mid-wake and upper-wake. The arm spacing and shoulder width play a critical role in the development of this vortex system.

An aerodynamic assessment is presented of two elite skaters, each in two different skating postures, at the ice-rink Thialf in Heerenveen, the Netherlands, via on-site Ring of Fire (RoF) measurements. This experimental approach adopts stereoscopic Particle Image Velocimetry (Stereo-PIV) to measure the flow upstream and downstream of the skaters. Both skaters transit through the RoF 20 times, 10 in each skating configurations. Athlete A skates with two hands on the back and with one arm on the back and one loose. Athlete B skates with one arm loose in a normal deep sit and in an extreme deep sit. All tests are performed at a nominal skating speed of 11 m/s. Firstly, the wake velocity fields of skater A with two hands on the back are presented throughout five different phases of the skate stroke. Significant variations in the distribution of the velocity deficit downstream of the athlete are observed, which suggest corresponding variations in the skater's aerodynamic drag. These velocity fields are also compared to literature and the similarities and differences are discussed between the flow around a static skater and that in the natural skating motion. Secondly, average streamwise velocity and vorticity fields for all 4 different postures are presented and compared. It is observed that for all cases the maximum velocity deficit in the wake is in the range of 0.45 ≤ u_x^* ≤ 0.55 and is located behind the lower back and upper legs. Furthermore, a characteristic vortex pair is observed downstream of the skater's hips for all four skating configurations, indicating it is independent of the athlete, the posture, and skating phase. The ensemble average aerodynamic drag is evaluated via a control volume approach along the wake behind the skater, accounting for the non-uniform flow conditions prior to the skater's passage. The uncertainty of the average drag measurements from the present RoF is about 5%. The results show that the optimization of the deep sit, e.g. the trunk and knee angle, yields a reduction by 7.5% of the skater's aerodynamic drag. Conversely, the difference in drag between two arms loose and one arm loose is not statistically significant.

In this work, we investigate the flow field around speed skating helmets and their associated aerodynamic drag by means of computational fluid dynamics (CFD) simulations. An existing helmet frequently used in competition was taken as a baseline. Six additional helmet designs, as well as the bare-head configuration, were analysed. All the numerical simulations were performed via 3D RANS simulations using the SST k-w turbulence model. The results show that the use of a helmet always reduces the aerodynamic drag with respect to the bare head configuration. Besides, an optimised helmet design enables a reduction of the skaters aerodynamic drag by 5.9%, with respect to the bare-head configuration, and by 1.6% with respect to the use of the baseline Omega helmet.

Elite level cycling events are performed at speeds in excess of 50 km/h. At these speeds, over 90% of the resistance forces come from aerodynamic resistance (C_DA). Recently bicycle-mounted pitot tubes, such as the Notio Konect (NK) have become more commercially available making C_DA easier to measure. Its reliability and sensitivity would be useful for riders and coaches to be able to understand what constitutes as a change in C_DA. Accordingly, the aim of this study was to establish the intra- and inter-effort reliability and sensitivity of the C_DA measures of the NK. Seven elite level track riders were used in this study which was broken into two parts: (1) Reliability and (2) Sensitivity. For both parts of the experiment, riders performed identical efforts, riding at ∼50 km/h for six laps of a 250 m indoor velodrome. For reliability, the riders performed six efforts without any changes in position or resistance. For sensitivity, they performed the efforts with a rod with discs of a known diameters attached at each end to vary the C_DA by a known amount. For the reliability assessment, low coefficient of variation of intra–(0.47%) and inter-effort (0.9%) reliability were measured. With regards to sensitivity, the smallest changes in resistance (from 5–6 cm, i.e. 1.2% or 0.002 m²) was identified by the NK. The data in this experiment suggests that the NK is a highly reliable in measuring C_DA can detect changes up to at least 1.2% in an indoor velodrome using elite level track riders. Highlights The Notio Konect showed high levels of inter- and intra-effort reliability. The Notio Konect could detect a change as small as 1.2% in aerodynamic drag. The findings suggest that the Notio Konect is suitable for detecting small changes in aerodynamic drag in a velodrome setting.

The Reynolds number effects on body limbs of a cyclist model, namely leg and arm, are investigated via robotic volumetric Particle Image Velocimetry measurements in the velocity range from 5 ​m/s to 25 ​m/s. The near wakes of such body limbs feature recirculation regions whose width and length are governed not only by the taper of the body parts, but also by the presence of coherent streamwise vortical structures. Moreover, the interaction with the wakes of the upstream body parts plays a role in the local wake properties. While reductions of the wake width are observed on both lower leg and arm with increasing free-stream velocity, the wake of the upper leg follows an opposite trend increasing in size at higher velocity. Such variations of wake width with the Reynolds number are related to the behaviour of the local drag coefficient, indicating a drag crisis behaviour on both leg and arm. The distribution of the so-called critical velocity upon these body segments is discussed, as it determines the freestream speed where a minimum value for the drag can occur.

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.].

Abstract: The aerodynamic drag of a human-scale wind tunnel model is obtained from large-scale particle tracking velocimetry measurements invoking the conservation of momentum in a control volume surrounding the model. Lagrangian particle tracking is employed to obtain the velocity and static pressure statistics in a thin volume in the wake of a cyclist mannequin at freestream velocities between 12.5 and 15 m/s, corresponding to Reynolds numbers from 5 × 10⁵ to 6 × 10⁵ based on the torso length. The spatial distributions of the time-average streamwise velocity and pressure coefficient match well with previous works reported in literature. The streamwise velocity fluctuations in the wake of the cyclist’s model are presented, clearly demonstrating the unsteady nature of the main wake flow structures. Furthermore, the obtained aerodynamic drag follows the expected quadratic increase with increasing freestream velocity. The accuracy of this drag estimation is evaluated by comparison to force balance data and corresponds to 30 drag counts. The three terms composing the overall drag force, ascribed to the mean and fluctuating streamwise velocity and the mean pressure, are also evaluated separately, demonstrating that the resistive force is dominated by the contribution of the mean streamwise momentum deficit, whereas the contribution of the pressure term is negligible.

The accuracy of the “PIV wake rake” method to measure the drag of transiting objects is evaluated. Tomographic particle image velocimetry measurements are conducted on a sphere towed at different speeds, within a Reynolds number range where the drag coefficient is constant. In contrast to PIV wake rake application on steady models in wind tunnels, where the upstream conditions can be accurately controlled and known a priori, measurement of the flow field prior to the passage of the model is essential for an accurate estimation of the drag for towed models when control of the undisturbed conditions is more challenging. The drag resolution of the technique is estimated to prospect the use of the technique in large-scale applications. A resolution of approximately 20 drag counts is obtained which is coarser than wind tunnel experiments but comparable to techniques used for field measurements. Graphical abstract: [Figure not available: see fulltext.].

A method is introduced to measure the aerodynamic drag of moving objects such as ground vehicles or athletes in speed sports. Experiments are conducted as proof-of-concept that yield the aerodynamic drag of a sphere towed through a square duct in stagnant air. The drag force is evaluated using large-scale tomographic PIV and invoking the time-average momentum equation within a control volume in a frame of reference moving with the object. The sphere with 0.1 m diameter moves at a velocity of 1.45 m/s, corresponding to a Reynolds number of 10,000. The measurements in the wake of the sphere are conducted at a rate of 500 Hz within a thin volume of approximately 3 × 40 × 40 cubic centimeters. Neutrally buoyant helium-filled soap bubbles are used as flow tracers. The terms composing the drag are related to the flow momentum, the pressure and the velocity fluctuations and they are separately evaluated. The momentum and pressure terms dominate the momentum budget in the near wake up to 1.3 diameters downstream of the model. The pressure term decays rapidly and vanishes within 5 diameters. The term due to velocity fluctuations contributes up to 10% to the drag. The measurements yield a relatively constant value of the drag coefficient starting from 2 diameters downstream of the sphere. At 7 diameters the measurement interval terminates due to the finite length of the duct. Error sources that need to be accounted for are the sphere support wake and blockage effects. The above findings can provide practical criteria for the drag evaluation of generic bluff objects with this measurement technique.