Richard Young, Technology and Innovation Coordinator, UKSI, London, England

Dr. Richard Young at the UKSI competed in the sport of cycling at the 1988 and 1992 Olympics while completing a degree in biomechanics

Today, victory in sport is a matter of a fraction of a second or a few millimeters separating first and second place. Therefore any legal, cost-effective, and performance-enhancing technology has to be taken seriously, especially given the amount of money associated with winning. Whole new scientific disciplines like sports psychology, sports nutrition, and sports bio-mechanics have developed over the last 30 years, and have become part of the supporting framework behind elite sportsmen and women around the world. During the last five to ten years, rather late into the fray, sports engineers and technologists have also emerged, and their contributions to the engineering and technological aspects of sports equipment and athlete biomechanics have gained increasing acceptance. All of these disciplines have combined to help continually improve elite performance in sport.

It has long been accepted that an understanding of fluid flow phenomena could lead to performance enhancements for certain competitive sports, especially those dominated by aerodynamics and hydrodynamics. Over the years, FLUENT has been used for a number of pioneering simulations of this type, such as motor racing, ski jumping, yachting, and sports ball modeling. Results have been used to optimize the balance between drag and downforce (motor racing), to illustrate why one posture is better than another (ski jumping), to perfect the design of a winged keel (yachting), and to better understand the impact of laces and geometric patterns on flight (sports balls). Performance enhancements that result from analyses like these will undoubtedly lead to the continued expansion of sports engineering in the years to come through the use of CFD.

In the United Kingdom, the concept of a sports institute, dedicated to understanding and improving performance, was first discussed in 1995. In October 2000, the idea became a reality as the United Kingdom Sports Institute (UKSI) opened in London. Sports institutes of this type are not new; many have been established around the world during the last ten years. All, and especially the Australian Institute of Sport, have helped contribute to notable sporting successes. These government funded organizations, which are primarily aimed at helping Olympic athletes, seek to provide elite competitors with the facilities and leading edge support necessary to help them excel at the pinnacle of their sport.

It was with this ideal in mind that the UKSI has begun to investigate some of the fundamentals of flow applications in Olympic sports using FLUENT, with the hope of helping elite athletes on the British Olympic and Paralympic teams. To date, technological advances have played a major role in many Olympic sports, such as pole vaulting, cycling, and skiing, resulting in better equipment and refined techniques. Many of these advances have not been systematically studied, however, and some of the underlying engineering phenomena have never been fully understood. Through the use of CFD, many of these knowledge gaps can be filled. At the UKSI, this technology has been identified as having the potential to produce significant performance gains for elite athletes. Fluent’s software has been proven to be successful in other competitive sports and is head and shoulders better than other CFD codes for sports applications.

Crosswind effects on cyclists

Cycling is one Olympic sport where CFD can help illuminate several flow phenomena. Applications for CFD in this sport are many, including cycle aerodynamic design, cyclist posture, helmet design, and optimal cyclist drafting positions during pursuit races. One area where cyclists do not agree, however, is on the selection of rear wheel type in a crosswind. While disk wheels become unmanageable for the front of a bicycle on windy days, the choice between disk and the traditional spoked wheels for the rear continues to undergo vigorous debate.

It has been speculated that the rear disk wheel could act as a sail in certain circumstances, providing a forward force in the rolling direction opposite the drag force, and hence reducing the net drag experienced by the cyclist. Although many cyclists use rear disk wheels to try to capitalize on this lift, there has been little clear evidence to support its existence. An analysis of wheel performance would add to the growing body of knowledge that CFD has provided to date for cycling applications, much of which cannot be easily obtained from wind tunnel tests.

In the CFD study carried out, simulations using FLUENT were applied to a generic geometrical representation of a cyclist and bike created in GAMBIT. All crosswinds were simulated as constant and steady at 90° to the direction of motion of the cyclist. Calculations were performed for a cyclist using a spoked front wheel at a forward speed of 25 mph, in crosswind speeds varying from still air to 30 mph, with spoked and disk rear wheels. Since the same CFD mesh was used for each simulation, it was felt that it should lead to the predicted trends being accurately resolved.

Flow path lines around a cyclist with a spoked rear wheel in a 20 mph crosswind (top) and a disk rear wheel (bottom)

In crosswinds, the cyclist experiences a drag force (opposing the direction of motion) and a side force. While the cyclist only has to work against the drag force, the CFD calculations showed an increase in the magnitude of the drag force for both types of rear wheels when a crosswind is present. The net drag force predicted by FLUENT as a function of wind speed shows that in still air, the advantage of using a rear disk wheel over a spoked wheel is negligible (about 2%). As the wind speed increases, however, the advantage of the disk wheel improves dramatically owing to the “sail effect.” In a 20 mph cross wind, the net drag experienced by the cyclist is 17% lower with the disk wheel than with the spoked wheel, suggesting that the disk wheel gives an apparently overwhelming advantage.

There are practical disadvantages to disk wheels though. For example, a disk wheel creates significantly larger side forces. In a 20 mph crosswind, the side force acting on the cyclist plus bicycle with a rear disk wheel is approximately double that for a cyclist using a spoked rear wheel. The trade-off for the cyclist is, therefore, one of stability, especially in a gusting wind. In reality, the situation is complicated further by variability of wind and rolling directions, and shielding by surrounding objects (including, in stage races, the other cyclists). The message from the simulations is clear, however. The cyclist can move moderately to significantly faster for the same power output, using the rear disk wheel rather than a spoked wheel, confirming the empirical observations experienced by many topnotch cyclists.

Graph of relative drag difference between a cyclist using a rear wheel with and without a disk in a range of crosswinds

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