Flow topology & large-scale wake structures around elite cyclists

2017-02-22T23:34:51Z (GMT) by Crouch, Timothy
This work, for the first time, shows the dominant flow structures in the wake of a competitive cyclist geometry, and provides a general picture of the steady and unsteady near-wake flow topology, in considerable detail and from a fundamental perspective. In contrast to accepted practices in the sport, wind tunnel investigations with a full-scale anthropometric cycling mannequin show that the fundamental flow physics is the dominant driver of the drag experienced by a cyclist, rather than just the frontal area. With cycling currently at the point where even small reductions in drag are required to be competitive at the elite level, more efficient means of optimising the aerodynamics of cyclists are required. This research shows that a comprehensive understanding of the nature of the flow around cyclist geometries can be gained by investigating the wake structures, and the degree to which they influence surface pressures and the aerodynamic forces acting on the rider. In doing so, a more systematic approach to reducing the aerodynamic drag force of cyclists can now be sought by targeting high drag areas associated with the large-scale flow structures through adjustments to rider position and modifications to equipment. Wake structures are identified and investigated using a series of detailed three-dimensional velocity field wake surveys and flow visualisation studies for the time-trial racing position. The aerodynamic forces and wake structure variants are considered for static leg positions around a full 360$^circ$ crank cycle. It is found that there are significant variations in the drag force with leg position associated with different flow regimes. Thus, these different flow regimes must be considered when optimising the aerodynamics of cyclists. Two characteristic flow regimes are identified, corresponding to symmetrical low drag and asymmetrical high drag regimes. The primary feature of the wake is shown to be a large trailing counter-rotating streamwise vortex pair, orientated asymmetrically in the centre plane of the mannequin. The primary flow structures in the wake are the dominant mechanism driving the large variations in the aerodynamic drag force around the crank cycle. This is also found in numerical flow simulations that were performed in parallel with this experimental research. From the analysis of time-averaged skin-friction patterns, topological critical points have also been identified on the suction surface of the mannequin's back. These are discussed with velocity field measurements to elucidate the time-average flow topologies, showing the origin of the primary flow structures for the low and high drag regimes. The proposed flow topologies are related to the measured surface pressures acting on the mannequin's back. These measurements show that most of the variation in drag is due to changes in the pressure distribution acting on the lower back, where the large-scale flow structures develop that have the greatest impact on drag. The influence of changes to rider position (e.g., arm and torso angles) and Reynolds number on time-averaged force and surface pressure measurements are also investigated to see how resilient the dominant flow structures are to these changes. Findings show that the aerodynamic drag force can effectively be reduced by influencing the formation of the large trailing streamwise vortices. Specifically, drag is influenced by changes in the magnitude of the pressure distribution where vortices originate. Despite these changes, the overarching shape of the distributions are preserved and the dominant wake structures are present for all positions and test velocities analysed. This suggests that the dominant wake structures identified in this thesis are a critical feature applicable to a wide range of positions and cycling speeds. A frequency analysis of wake probe data and time-accurate surface pressure measurements have revealed a complex wake exhibiting a variety of time and length scales associated with unsteady wake structures. This is expected considering the relatively high Reynolds numbers involved and the complex three-dimensional geometry of the mannequin and bicycle. Areas on the body and in the wake are associated with dominant shedding frequencies that are found to be significantly higher than typical pedalling frequencies. Due to the fact that the speed of the legs during pedalling is slow relative to the forward speed of the cyclist, the different wake states corresponding to different static leg positions analysed in this thesis are still likely to be representative of the pedalling case.