This Sunday one of the most popular sporting events for tens of million people around the world begins. The Tour de France starts in Utrecht, the Netherlands. We will again see the world’s best top athletes fighting for the stage victory every day. We’ll admire them as they climb the steepest slope at an amazing speed and be impressed to see them completing a time trial at an average speed above 50 km/h. Throughout the past years, the regulations have continuously improved to guarantee a clean and fair race. As an example, during time trials, neither cars nor motorbikes are allowed in front of the cyclists as this would obviously reduce air resistance. Similarly, if a cyclist is catching up to the one ahead, they must stay on different sides of the road. However, there is no regulation to prevent a vehicle from following the athlete as it is commonly believed that a car riding behind a cyclist cannot influence him.
But is this really true?
In a previous blog, I reported the work of my team studying the mutual aerodynamic effect of two cyclists. We published this work in 2013 in the leading scientific journal Computers & Fluids. We quantified that the cyclist riding behind a leading cyclist enjoys a 15-30 percent drag reduction. But we also found that the leading cyclist also experiences a small (2 to 3 percent) reduction in drag. In fact, the second cyclist pushes the air in front of him thereby reducing the pressure loss behind the front-runner and reducing the drag the front-runner experiences. In scientific terms, we called this effect “subsonic upstream disturbance”.
Pressure coefficients in a vertical plane, for (a) single rider and (b) two riders closely behind each other. Overpressure in red and yellow, underpressure in blue. The following cyclist pushes the air helping the front runner (2.7%); a following vehicle during a time trial will have a much bigger impact.
After some brainstorming, we concluded that a larger follower would increase the drag reduction further. A following car, provided it is not too far behind, would also push the air and help the cyclist. As the vehicle size increases, so does the effect. To make sure we were correct, we asked one of my best students, Yasin Toparlar, to run a few CFD simulations with ANSYS Fluent to validate our idea.
CFD analysis performed by TU/e with ANSYS Fluent: velocity field in vertical centerplane
Yasin carefully modeled the system — cyclist, car and surrounding air — to compare drag experienced by the cyclist alone with drag for the same cyclist with a mid-size vehicle following him. The results were amazing. If a medium-sized car follows the athlete at a realistic 5 meter distance, the cyclist will enjoy a drag reduction of almost 1.44 percent.
CFD analysis performed by TU/e with ANSYS Fluent: pressure coefficients in vertical centerplane, for three distances between cyclist and car: 5, 10 and 20 m.
If the distance is a safe 10 meter, the drag reduction is 0.23 percent. (It should be noted that this 10 m is exactly the minimum distance between cyclist and following car specified by the International Cycling Union (UCI) because of safety reasons (ruling 2.4.023 in the UCI Cycling Regulations on Road Racing).
While this 1.44 % might seem small, if we translate this value into time saved on the finish line for a typical 50 km time trial, this would save close to 24.1 seconds compared to a competitor not adopting the same tactic. Because time trials are often won by seconds, and sometimes less than a second, this drag reduction advantage can clearly be decisive.
Wind tunnel testing performed by TU/e at the von Karman Institute for Fluid Dynamics
Because of our long experience with the ANSYS Fluent simulation tool, we knew our CFD results were reliable. However, to convince more skeptical readers and to be able to publish the results in a top scientific journal, we experimentally measured the drag reduction by using a cyclist and a car in the large wind tunnel of the Von Karman Institute of Fluid Dynamics in Belgium. The numerical results are in close agreement with the experimental results, confirming that a closely following car is helping the athlete. Having a larger car with signs and bicycles on the roof would further magnify the impact and the time savings.
It should be noted that the impact of the following car quickly decreases and becomes negligible if the car is more than 20 meters behind the cyclist. That is why we have contacted the International Cycling Union with our findings and with the suggestion to shift the official distance from 10 m to at least 30 m.
With this research, it is not our intention to criticize. On the contrary, we want to advance the scientific knowledge in cycling aerodynamics and to contribute to fairer competition.
EDITORS NOTE: Want to hear more about this topic? Join us on Thursday, July 9th when today’s author, Bert Blocken, will show us how engineering simulation and CFD are increasingly being used in cycling, not just to improve the performances of the bicycles themselves but to help the athletes adjust their positions for optimal results. REGISTER NOW.
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