The effect of drafting on the aerodynamic drag of a long-track speed skater is investigated in-field, at the 400m ice-rink of Thialf, Heerenveen. The Ring of Fire system is used to measure the flow downstream of an elite, isolated skater at approximately 11 m/s, transiting repeatedly through a tunnel filled with Helium-filled soap bubble flow tracers. Large-scale stereoscopic particle image velocimetry is used at an acquisition frequency of 500 Hz to obtain the near to far wake up to 11 m distance behind the skater. Over these 11 m, the center of gravity of the wake can shift up to 10 cm laterally, depending on the phase of the skating motion, and it moves about 15 cm to the floor. The former suggests that a trailing skater should slightly adapt its trajectory to achieve the lowest aerodynamic drag by drafting. The drag reduction of a trailing skater is estimated from the measurements on the isolated rider, assuming that the trailing rider's drag reduction only stems from the loss in total pressure in the wake of the first rider. The drag reduction is obtained with varying lateral and longitudinal distance between the leading and hypothetical trailing rider. It is observed that the peak reduction (∼40%) steeply decays with increasing lateral offset: at an offset of 50 cm the reduction is negligible. Instead, with increasing longitudinal offset, the decay is more gradual: at a distance of 11 m the reduction of 17% remains significant. The in-field estimations of the drag reduction are supported by wind tunnel measurements conducted on scaled skater models. Finally, the results obtained on the ice-rink indicate that a trailing skater should follow a slightly wider trajectory of about 20 cm, in comparison to the leading skater, to achieve the peak drag reduction during the entire skating stroke.@en

The Ring of Fire (RoF) measurement concept to perform on-site experiments of aerodynamic drag for transiting objects is investigated with a study that identifies the main sources of uncertainty. The main contributors to the uncertainty of the drag measurement are examined for the case of a cyclist riding through the measurement domain. A sensitivity analysis is conducted that assesses how the estimated drag is affected by the choice of particle image velocimetry image processing parameters. The size of the cross-section considered in the control volume formulation is also investigated. It is found that the accuracy of the estimated drag depends on the procedure used to detect the edge of the momentum deficit region in the wake. Moreover imposing mass conservation yields the most accurate drag measurements. The drag estimation has little dependence upon the spatial resolution of the measurement as long as the interrogation window size stays within 5% to 25% of the equivalent diameter of the object cross section. Experiments are conducted in a sport-hall, where the aerodynamic drag estimates from the RoF are compared to a conventional torque power meter installed on the bicycle, and different rider's postures as well as equipment variations are considered. Although the discrepancy in the absolute value of drag can be as high as 20%, power metering and RoF agree within 4% on relative drag variations.@en

An on-site Ring of Fire (RoF) experiment is performed at the Tom Dumoulin bike park in Sittard-Geleen, the Netherlands. The current work investigates the aerodynamic drag of a cyclist following a lead cyclist at different lateral and longitudinal separations; additionally, the athletes’ skills to maintain their position and distance with respect to the preceding riders are evaluated. The effect of the relative size of the lead cyclist on the drag area of the drafting cyclist is also investigated. The results show drag reductions of the trailing cyclist in the range from 27% to 66% depending on the longitudinal and lateral separation from the leading rider. The aerodynamic advantage of the drafting rider decreases with increasing lateral and longitudinal separation between riders, with the lateral separation found to be more relevant. Besides this, the drag reduction of the drafting cyclist benefits from an increase in drag area of the leading cyclist. @en

The research presented in this thesis introduces a new measurement concept for on-site aerodynamic measurements based on large-scale stereoscopic particle image velocimetry (stereo-PIV) measurements past an athlete, a vehicle or an object travelling through a quiescent environment. The analysis of the momentumdeficit past the transit poses the basis to estimate the aerodynamic drag. For such an approach, where the object crosses the illuminated measurement plane, the experimental method is referred with the name “Ring of Fire” (RoF). The first part of this work presents the development and assessment of the Ring of Fire concept through the study of cycling aerodynamics. A feasibility study is performed in which two RoF experimentswith a cyclist are conducted, indoor and outdoor,mimicking respectively track and road cycling. During these experiments attention is placed on the effects of the environmental conditions and the confinement of the measurement region. Furthermore, the experiments cover different postures of the cyclist (time trial and upright) with the aimto directly measure the effect of posture on aerodynamic drag and its detectability with the RoF. Despite differences between the two experiments in the cyclist geometry, bike geometry, and the cycling speed, the flow fields in the near wake of the riders compare well between both experiments and to literature. In terms of drag estimation, a clear distinction in upright vs. time trial drag area is found for both experiments, with the upright posture yielding higher drag area by about 20-35% with respect to the time trial posture. The comparison of these drag values with literature data, however, could not yield a conclusive assessment, given the large dispersion (approx. 50%) of the literature data due to many varying parameters, like rider posture, bikes geometries and testing conditions. Furthermore, the uncertainty of the measured drag and its dependency upon experimental conditions and the image processing parameters have not yet been addressed. Knowledge of the minimum detectable drag variation is relevant when measurements are intended to perform aerodynamic optimisation, therefore, a sensitivity analysis is conducted that assesses how the estimated drag is affected by the choice of PIV image processing parameters. The size of the cross-section considered in the control volume formulation is also investigated. It is found that the accuracy of the estimated drag depends on the procedure used to detect the edge of the momentum deficit region in the wake. Moreover imposing mass conservation yields the most accurate drag measurements. The drag estimation has little dependence upon the spatial resolution of themeasurement as long as the interrogation window size stays within 5% to 25% of the equivalent diameter of the object cross section. In addition, the drag values obtained with the RoF are compared against the drag estimates from simultaneously acquired power meter data. To assess the agreement between the two approaches in different regimes, drag variations are introduced by different cyclist postures, as well as varying garments. Regardless of the underlying input parameters in the power meter model, both small- and large scale deltas are well captured by both the Ring of Fire technique and the power meter approach. The uncertainty on the average drag measurements from the RoF is within 5%. The second part of this work implements the findings and conclusions from part 1mand presents two applications in speed sports studied with the Ring of Fire. Firstly, themeffect of drafting in cycling is investigated. More precisely, the amount of drag reduction experienced by a trailing cyclists in a tandem formation is investigated at different lateral and longitudinal separations. The longitudinal displacement of the drafters varied between 0.32 m and 0.85 m and the lateral displacement varied between +/- 0.20 m among different runs. The results show that the amount of drag reduction for the trailing rider is mainly caused by the change in inflow conditions and that its aerodynamic advantage decreases with increasing lateral and longitudinal separation between riders, where the lateral distance is found to produce a more rapid effect. Based on these results a model is introduced that predicts the aerodynamic gain of the trailing rider based on his or her position with respect to the leading rider. Validation of the model with data from literature shows that in the near wake the model prediction is in line with literature, with an overestimation of the drag reduction when the longitudinal distance is between 0.1 m and 0.3 m. Secondly, the applicability of the RoF to speed skating is demonstrated. An aerodynamic assessment is presented of two elite skaters, each in two different skating configurations at the ice-rink Thialf in Heerenveen, the Netherlands. Both skaters transit 20 times through the RoF, 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 both arms loose for all the runs, but was varying his knee and trunk angles. All tests were performed at a nominal 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. Secondly, average streamwise velocity and vorticity field for all 4 different postures are presented and compared. Finally, the results show that the difference in drag between two arms loose and one arm loose was found to be not statistically significant. Conversely, the optimization of the trunk and knee angles results in a reduction by 7.5% of the skater’s drag.@en

The aerodynamic drag of a trailing cyclists in a tandem formation is investigated at different lateral and longitudinal separations. A Ring of Fire (RoF) experiment is conducted at the Tom Dumoulin bike park of Sittard-Geleen in the Netherlands. The method is based on stereoscopic Particle Image Velocimetry (Stereo-PIV) measurements followed by invoking the conservation of momentum expressed in a control volume to evaluate the drag force of the cyclists. Three cyclists perform a series of individual tests as well as four different drafting tests, varying their order in the group. All tests were performed at a nominal riding speed of 45 km/h; the longitudinal displacement of the drafters varied between 0.32 m and 0.85 m and the lateral displacement varied between ±0.20 m among different runs. The results from the RoF measurements show the flow field interactions between the two drafting cyclists as well as the wake of the second cyclist. It is observed that the amount of drag reduction for the trailing rider is mainly caused by the change in inflow conditions. The drag reductions of the trailing cyclist are in the range from 27% to 66% depending on the longitudinal and lateral separation from the leading rider. The aerodynamic advantage of the drafting rider decreases with increasing lateral and longitudinal separation between riders, with the lateral separation found to be more relevant. Furthermore, based on the analysis of the individual wakes, the drag reductions found by the RoF, and the drag reduction measured by Barry et al. (2014), a model is introduced that predicts the aerodynamic gain of the trailing rider based on his or her position with respect to the leading rider. Validation of the model with data from literature shows that in the near wake the model prediction is in line with literature, with an overestimation of the drag reduction when the longitudinal distance is between 0.1 m and 0.3 m.@en

A novel measurement system, the Ring of Fire, is deployed which enables the aerodynamic drag estimation of transiting cyclists. The system relies upon the use of large-scale stereoscopic PIV and the conservation of momentum within a control volume in a frame of reference moving with the athlete. The rider cycles at a velocity of approximately 8 m/s, corresponding to a torso based Reynolds number of 3.2 × 105. The measurements upstream and in the wake of the athlete are conducted at a rate of 2 kHz within a measurement plane of approximately 1000 × 1700 mm2. The non-dimensional, ensemble-averaged streamwise velocity fields compare well to literature and the ensemble-averaged drag area shows a rather constant value along the wake with an uncertainty of 5%. A comparison with wind tunnel force balance measurements shows discrepancies which may be partly attributed to the bike supports and stationary floor in the wind tunnel measurements. The 25% drag difference measured between a rider in upright and time-trial position, however, matches literature well.@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

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

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 ≤ ux* ≤ 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.@en