CFD and bicycle rider
http://www.cd-adapco.com/press_room/dynamics/25/tourdefrance.htmlCD-adapco makes the Tour de France less of a Drag
Stephen Ferguson, CD-adapco, UK
As anyone who has ever ridden a bicycle on a windy day will testify, aerodynamics play a big part in cycling. Perhaps more than any other sport, top-level cycling is dominated by aerodynamics and, more specifically, the art of drafting. Drafting occurs when one cyclist rides in the wake of another, reducing their exposure to the oncoming air and ultimately the energy expended in cycling. The question of “how much energy?” is the topic of some debate. CD-adapco has recently undertaken a comprehensive CFD simulation in order to answer this question. The influence of aerodynamics is most visible in the time-trial stages of the Tour de France. Here cyclists can be seen riding special carbon-fiber bicycles on which, at a cost of tens of thousands of dollars, each component is specifically designed to minimize the aerodynamic drag. Clad in aerodynamic clothing and helmets, riders are forced to adopt uncomfortable crouched position on their bikes, minimizing their frontal area and reducing their exposure to the oncoming air.
In the Tour de France, the time-trial comes in two distinct flavors: an Individual Time Trial (ITT) and a Team Time Trial (TTT). In the Individual Time Trial, each rider competes alone. With no other riders to draft behind, the ITT is known as "the race of truth", a brutal contest of man and machine against the clock. In the Team Time Trial, riders compete as a team of (up to) nine riders. Each rider takes it in turns to ride at the front of the line, taking the full force of the oncoming air and providing a wake in which teammates can draft. As each rider tires they swing off the front of the line and drift backwards to the rear, recovering in the wake of the other riders in preparation for the next turn at the front.
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Although time-trialing ability alone is not enough to win the Tour de France, each July the final recipient of the yellow jersey is almost certainly a time-trialist of supreme ability. Recent Tours have almost always been won by riders that dominate the timetrial stages; 5 times winner Miguel Indurain and now 7 time winner Lance Armstrong typically based their victories around large time gaps opened up in the time-trial stages.
Cycling is a sport in which every second really does count , and small aerodynamic advantages can be the difference between winning and losing the Tour. In 1989, Greg Lemond trailed French rider Laurent Fignon by 50 seconds prior to the final stage, a 24.5km ITT. To most observers, this gap seemed insurmountable - requiring Lemond to ride each kilometer 2 seconds faster than Fignon, himself no mean time-trialist. On a warm Paris afternoon, Lemond, wearing an aerodynamic helmet and riding a special aerodynamic bicycle, beat Fignon, riding a normal road bike, by 58 seconds, and won the Tour by just 8 seconds. Subsequent analysis has suggested that the drag on Fignon's ponytail alone was enough to slow him down by the critical 8 seconds by which he lost the race. The TTT played a large part in deciding the winner of the 2003 Tour; of his 61 second margin Lance Armstrong won 43 seconds in the Team Time Trial.
By performing CFD simulations of a nine-man TTT, and a single cyclist ITT, it is possible to directly evaluate the influence of drafting. Although many individual cyclists have been subjected to windtunnel testing, the sheer size of a nine-man chain of cyclists makes physical testing impractical. In the CFD world too, entire TTT calculations were until recently, prohibitively expensive, requiring a great number of computational cells in order to sufficiently resolve the flow past the cyclists.
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The CFD models were constructed and run using the STAR-CAD Series, a range of CAD-embedded CFD products. Unlike other CFD codes, which are restricted to using hexahedral or tetrahedral elements, STAR-CAD Series has the unique ability to create and solve upon meshes of arbitrary cell topology. Using specially created polyhedral elements (which typically have between 12 and 14 faces), CD-adapco's CFD technology can provide near hexahedral accuracy with at least 5 times fewer cells than a typical tetrahedral calculation. The mesh for the TTT comprised almost 7 million polyhedral cells, equivalent in accuracy to a mesh of approximately 30 million tetrahedra.
The results of the simulations were illuminating. Compared with the lead cyclist, the drag of the rider in second place is reduced by 21% - a significant saving. The third rider feels a further small decrease in drag over the second, but from the third rider back all other cyclists experience almost identical drag. As the riders are continually progressing towards the front of the chain, taking a short turn on the front, before freewheeling to the rear of the line, on average (assuming a constant rate of rider rotation and ignoring the effect of dropping back) the drag coefficient of a rider in the TTT is around 27% lower than experienced by an individual rider.
Perhaps the most surprising conclusion from the CFD simulation is that, despite feeling the full force of the oncoming air, the lead rider experiences lower drag than if he were riding an ITT at the same speed. The drag coefficient of the leading TTT rider is 0.277, while that of an individual rider is 0.285 . This rare example of "something for nothing" occurs because the second place rider reduces the influence of the lead rider's wake, increasing his base pressure and consequently reducing the drag force that he experiences.
Despite the predicted reduction in drag coefficient afforded to riders in a team time-trial, the increase in speed over an individual time trialist is not large. In the 2004 Tour, Armstrong completed the 55km individual time trial with an average speed of 49.39 km/h, while his team won the 64.5km TTT at an average speed of 53.71 km/h. This increase in average speed of just 9% seems relatively small in comparison with the effort exerted by eight extra cyclists. However, as drag increases with velocity squared, a 4 km/h increase would require a greatly increased effort for an individual rider. One would expect the difference in speed to be much greater if all nine riders were time-trialists of Armstrong's ability. In reality even the top teams consist of riders whose competence lies in areas other than time-trialing, so that the contribution of all nine riders is not equivalent. Because of this, the lead rider is also forced to temper his effort, so that the lesser time-trialist on the team can keep up, despite the reduction in drag. If gaps open up in the line, riders are liable to be left behind, cycling in the wind and unable to catch up. Although CFD calculations are so far not routinely used in perfecting time-trialing tactics for the Tour de France, CD-adapco's Dennis Nagy believes that they soon will be. "It's only a matter of time", he said, "the rewards in professional cycling are increasing all of the time, while winning margins are coming down. In the near future CFD could be the difference between winning and losing the Tour de France".
[ 本帖最后由 FreddyMusic 于 2008-2-4 11:50 编辑 ] CD-adapco help Felt Racing to design "the most aerodynamic, UCI-legal bicycle frame ever created"
Stephen Ferguson, CD-adapco
http://www.cd-adapco.com/press_room/dynamics/27/images/felt1.jpg
Felt Racing recently unveiled their new DA Carbon Fibre racing bicycle that, according to company founder Jim Felt, is designed to be “the most aerodynamic, UCI-legal frame ever created.” Although Felt’s claim is a bold one, he has a sheaf of wind-tunnel data to prove it, and can point to a twoyear design process for the bicycle that involved extensive CFD simulation right from the start.
Founded in 2000, Felt Racing is an American manufacturer of high-end racing bicycles, particularly aimed at the demanding Triathlon and Time- Trial markets. Under the leadership of Felt, an internationally renowned frame-building guru with a host of world-championship winning designs under his belt, Felt Racing have quickly established a reputation for technological innovation and aerodynamic design, with a stated mission “to design, develop, and deliver the best bicycles in the world. Period.”
The DA is a significant step in that direction, featuring a remarkably narrow (25mm) frame, with aerodynamically optimised tubing shape and innovative wind-defying features such as a revolutionary brake-mounting that sits inside the
seat tube, and a unique bayonet steering system.
According to Felt Racing Frame Designer Tim Lane, who was responsible for most of the CFD simulation, aerodynamics play a crucial role in Time Trial racing: “With no team-mates to pull you through and no wheel to draft, Triathlon and Time Trial require not only a strong engine, but also a vehicle that is ergonomically and aerodynamically advantaged. Racers must convert every last ounce of energy into raw speed, and slice through the wind like a razor.”
In order to make sure that wind-tunnel resources are exploited to their full potential Felt adopt a complimentary approach, using CFD simulation to determine which designs are the most aerodynamically efficient, and only testing the best in the windtunnel.“As a company we’ve invest heavily in wind-tunnel testing”, says Lane, “but we recognize that wind-tunnel testing is both expensive and time consuming. By using CFD simulation right from the start of the design process, we can ensure that by the time we get to the tunnel, we are finetuning an already aerodynamically efficient design.”
Tim Lane and his colleagues at Felt Racing have established an impressive process for CFD modeling so that “right from the start of the design process”, literally means from the moment that first CAD models are generated, usually many months before prototypes are built. Through a process called CAD-embedding, Lane and his team can access CD-adapco’s CFD software directly from their Pro/ENGINEER CAD package. This enables designers to perform CFD simulations of their current design by expending just a few minutes of effort, with all the CFD functionality available from a small number of additional menus in the CAD tool.
Results of the CFD simulation (which typically take less than an hour to compute using a standard desktop computer) areautomatically presented to the designer in terms of drag-coefficients, for the whole bicycle.
The results are not always what the designer originally expected: “Bicycle aerodynamics is about the interaction between all the different components that make up the complete bicycle”, says Lane. “Just because a component or concept looks good on the CAD-screen or seems viable in theory doesn’t mean that it will work out on the road. Unless you are very careful, an aerodynamically optimized component can sit in the dirty air generated by someone else’s beautifully designed, yet aerodynamically inconsiderate component, thus still generating a whole heap of drag.”
Lane and his team investigate any unusually good, or unusually bad results by examining a predefined set of flow-visualization plots that are automatically generated and stored for each design simulated. “The beauty of CFD is that if we want to, we can investigate every single component, and look in detail at the flow-features that it generates”, says Lane.
This thorough investigation of the design envelope is warranted because competitive cycling, like Formula 1 motor racing, is bound by a very strict set of regulations, which are defined by the sport’s governing body (the Union Cycliste Internationale or UCI). The regulations are specifically designed to maintain the traditional shape of a bicycle and to limit the scope for manufacturers such as Felt Racing to gain significant competitive advantage for their riders. As the DA proves, this doesn’t mean that there’s nothing that can be done: “In designing the frameset we took advantage of every possible lenience permitted within the UCI rules” , says Lane, “it’s not just a frame – but a completely thought out frameset comprised of a frame, fork and stem, blended together as a single unit”.
Because the wind-tunnel mock-ups were unable to support a riders weight; when the basic bicycle design had been decided upon, Lane used additional CFD modeling to see check that the bicycle performed with a rider in a number of aerodynamic riding positions. Rider and bicycle were combined in CD-adapco’s STAR-CCM+ and joined together using advanced surface meshing, that creates a single contiguous surface suitable for CFD modeling, while respecting the complex geometry of the bicycle – right down to every gear-tooth on the groupset.
Importantly, using CFD Felt Racing were able to speed up their design process: “Of all the CFD technology we tried, only CD-adapco’s combination of CAD-embedding and surface wrapping provided a robust and efficient process by which we could optimise our designs without delay to our demanding production schedule”, says Lane. “Ultimately, using CFD, we were able to build a more aerodynamically optimised bicycle at less expense, because of the cost and time saved in reducing the number of wind-tunnel prototypes.”
Although it is difficult to say whether Felt Racing have achieved their aim “to design, develop, and deliver the best bicycles in the world”, every triathlete and time-triallist that manages to race faster, using lessenergy, because of Felt Racing’s investment in CFD technology, will probably agree that the DA is a significant step towards it.!
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