Aerodynamic interactions between multiple cyclists
thesisposted on 01.03.2017 by Barry, Nathan Raymond
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This research provides a detailed insight into the aerodynamic interactions that occur between cyclists. The force interactions between multiple cyclists are first identified for a range of practical scenarios and encompassing the scope of real world formations. This is followed by an investigation of the flow field around two cyclists in a tandem formation and how this flow structure differs from a single cyclist. Experiments included static cyclist models and extended through to pedalling athletes. As such this work also covers an insight into the dynamic flow behaviour of a cyclist that has not received significant focus previously. Aerodynamic interactions are a fundamental component of cycling, with cyclists travelling in close proximity in many competition events as well as on the road. To date the investigation of aerodynamic interaction effects in cycling has been limited, especially regarding the flow field. This research sought to provide a more detailed understanding of aerodynamic interactions, the changes to forces and the flow mechanisms responsible. Loads were measured on full scale cyclists, primarily athletes, in the Monash University large open-jet wind tunnel. Both tandem and transverse formations were studied. Following the quantitative mapping of force interactions, especially drag, a series of experiments were proposed to characterise the flow field around tandem cyclists. By identifying changes from the single rider wake profile, this would lead to an identification of the flow mechanisms responsible for the observed changes in drag. Full scale flow visualisation was conducted on a pair of athletes in tandem in the wind tunnel. This was followed by flow mapping using PIV in the Monash FLAIR water channel using replica reduced scale model cyclists to capture high resolution cross sections of the flow. Flow fields were then linked back to full-scale athletes by capturing wake profiles for a pedalling athlete in both single and tandem cases. Force measurements showed that the drag of a cyclist varies as a strong function of relative spatial position. Positioned inline to the flow, at small separation there is a small drag saving for the lead rider (2.5%) and a large drag reduction for the trailing cyclist (40%). These reduce with increasing axial and lateral separation distance. Despite the large changes to the inflow conditions, the primary streamwise vortices formed from the hips of a cyclist remain dominant features in the wake of a trailing rider. Some reduction in peak streamwise vorticity was observed and this was found to be proportional to the reduction in streamwise velocity. However, the general distribution of the wake maintains similarity with the single rider case. The similarity in the wake conditions of the single and trailing rider indicate that the large drag reduction is not a product of disruption to the primary wake vortices. Instead it was found to be dominated by the reduction in streamwise velocity at the inlet for the trailing rider, thus reducing the momentum loss over the cyclist. As separation between the cyclists is increased, energy is recovered from the freestream and the inlet momentum impacting on the trailing rider is increased, thus the reduction in drag is smaller. This research has provided a more detailed understanding of the aerodynamic interactions between cyclists. This understanding can be applied to better exploit the energy savings possible for riders travelling in groups and applied to both high performance and commuters alike. The case of multiple cyclist interactions also presents valuable insights for the study of interactions between other complex, and moving, geometry bluff bodies.