Flow topology in the wake of a cyclist and its effect on aerodynamic drag

Abstract

Three-dimensional flows around a full-scale cyclist mannequin were investigated experimentally to explain the large variations in aerodynamic drag that are measured as the legs are positioned around the 360 • crank cycle. It is found that the dominant mechanism affecting drag is not the small variation in frontal surface area over the pedal stroke but rather due to large changes in the flow structure over the crank cycle. This is clearly shown by a series of detailed velocity field wake surveys and skin friction flow visualizations. Two characteristic flow regimes are identified, corresponding to symmetrical low-drag and asymmetrical high-drag regimes, in which the primary feature of the wake is shown to be a large trailing streamwise vortex pair, orientated asymmetrically in the centre plane of the mannequin. These primary flow structures in the wake are the dominant mechanism driving the variation in drag throughout the pedal stroke. Topological critical points have been identified on the suction surfaces of the mannequin's back and are discussed with velocity field measurements to elucidate the time-average flow topologies, showing the primary flow structures of the low-and high-drag flow regimes. The proposed flow topologies are then related to the measured surface pressures acting on the suction surface of 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 having the greatest impact on drag develop.

Aerodynamic drag of a transiting sphere by large-scale tomographic-PIV

“…Furthermore, relating changes in the leg orientation of a cyclist to the wake flow topology recently provided new insights for cycling aerodynamics (e.g. Crouch et al 2014).…”

“…Despite the simple geometry, the flow exhibits an unsteady, turbulent wake with complex vortex interactions (e.g. Achenbach 1972;Brücker 2001) mimicking conditions also encountered behind ground vehicles (Ahmed et al 1984) or cyclists (Crouch et al 2014). The experiments determine the time-average drag and a detailed comparison is made with data available from literature.…”

The Influence of the Phase Difference between the Crank Angle of the Pilot and that of the Stoker on the Drag Acting on a Tandem Bike

“…The oscillating frequency of 2.5 Hz is the lowest possible limit in this experimental set-up. In order to make the frequency of pedaling less than 2 Hz (The practical pedaling frequency is less than 2 Hz [2]), the present motor should be replaced for the more powerful motor in the future.…”

“…It was reported that the dominant mechanism affecting drag is due to large changes in the flow structure on the basis of three-dimensional flows around a full-scale cyclist mannequin [2]. Two characteristic flow regimes are identified, corresponding to symmetrical low-drag and asymmetrical high-drag regimes, in which the primary feature of the wake is shown to be a large trailing streamwise vortex pair, orientated asymmetrically in the center plane of the mannequin.…”

Drag kings: characterizing large-scale flows in cycling aerodynamics

“…Cyclists are painfully aware that legs are not very aerodynamic resulting in, one assumes, a tremendous amount of drag. Thanks to the recent article by Crouch et al (2014) we may finally have an answer to this conundrum.…”