Ring of fire as a novel approach to study cycling aerodynamics

Abstract

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.