Welcome back to Sports & Building Aerodynamics. This is week 6 on cycling aerodynamics. At the end of this week, you will understand the importance of form drag in cycling. You'll understand drafting effects in cycling. You will also know which position in a group of cyclists has the lowest aerodynamic drag, and you will understand how, in the future, this knowledge can be decisive in team time trials. Also this week, we start again with a quote, this time from Aristotle saying: 'Probable impossibilities are to be preferred over improbable possibilities.' Let's have a look at the contents of week 6. First we are going to address the question: why study cycling aerodynamics? Then we are going to focus on wind-tunnel testing for a single cyclist, followed by CFD simulations for a single cyclist. Then we'll go and have a look at aerodynamics of two drafting cyclists. Then the aerodynamics of larger drafting cyclist groups, followed by aerodynamics between a car and a cyclist, and finally, we'll offer you an interview with two professional cycling coaches. But before that, we are going to set the scene by watching a short movie from the World Road Championships of 2013, and especially the team time trial. >> Well, they're definitely going to be on the podium. The question is, which color of medal they're going to get. Is it going to be gold? Is it going to be silver? Four riders left. Tony Martin on the front, the world time-trial champion of last year. He is sitting there in the lead. On his wheel is Sylvain Chavanel. It's going to be so, so close. I thought it was over. Niki Terpstra and also Peter Velits, the four riders in Omega Pharma-Quick Step. Heading towards the finish line, are they going to do it? 1 hour, 4 minutes, 17.62, 300 meters to go. The four riders sprinting for the finish line. The time taken on the fourth rider across the line. It looks like they're going to do it. Tony Martin and the rest of his squad, race-up to the finish line now. And they are the world champions. [LAUGH] They have only just done it. By .88 of a second. That is absolutely incredible. >> We start again with a module question. This graph shows the power curves for three cyclists. The question is, which curve belongs to elite cyclists? So this graph, as you can see here, shows power as a function of the cadence or the pedaling frequency of the cyclist. Is that only the top curve, is it the top two curves, is it the bottom two curves, or is it all three curves. Hang on to your answer and we'll come back to this question later in this module. At the end of this module you will understand the importance of aerodynamic drag in cycling. You will understand how cycling performance in time trials can be further optimized. And you will understand the typical output of wind-tunnel tests or CFD simulations of cycling aerodynamics. Let's focus on a cyclist cycling on level road. Then he would like to achieve an acceleration of his body and the bicycle. That is done by a propulsion force which is counteracted by aerodynamic drag and by non-aerodynamic resistance forces, and then in the vertical direction, of course, there's also gravity and reaction forces by the road on the tires. So then we can apply Newton's second law, which states that the sum of the forces equals mass multiplied with acceleration, and if we do that, in the horizontal direction, it is clear that this acceleration indeed needs to be provided by the propulsion force. But there are different ways in which we can achieve this acceleration or achieve an increase in speed. That is, increasing the propulsion force or decreasing the aerodynamic resistance. And in addition, also some efforts can be devoted to decreasing non-aerodynamic resistance, but generally, this is of lesser importance. So the answer to the question, why study cycling aerodynamics, actually lies in those two aspects. Because there is for every cyclist an optimum between power output and aerodynamic drag or low aerodynamic drag. Because you can be in a position that allows you to generate maximum power, but that will generally not be a very aerodynamic position, while on the other hand, you can be in a very aerodynamic position, but that would not be a position where you can generate the maximum power. So there is indeed an optimum, and that optimum can be investigated. And that was the focus of a research project that we carried out some time ago. It was commissioned by the Flemish Cycling Union and led by Leuven University, in which both Eindhoven University, and ETH in Switzerland were partners. And in this project indeed we focused on this optimum between power output and low aerodynamic drag, and here you see one of our collaborators in this project, Dr. Erwin Koninckx, who is testing Tom Boonen on the isokinetical cycling ergonometer. And then for different cyclists, when you do this test, you can get indeed this kind of curves. So this is the power curve as a function of pedaling frequency. And then we can go already back to the module question, which of these curves belongs to elite cyclists? Well maybe a bit surprisingly, all 3 of them. So you can see that these are curves from three top cyclists and unfortunately I cannot reveal who they are but they're all top cyclists, but you see that the curves are completely different. This also reinforces the knowledge statement that for every cyclist there will be a different optimum between cycling at low drag and at maximum power output. Let's focus briefly on aerodynamic drag. While the cycling resistance on level terrain consists for the largest part of aerodynamic drag, although there is also wheel-bearing and drive-train friction and rolling resistance. But because aerodynamic drag is most important, certainly the greatest potential for improvement in cycling speed is aerodynamic. And at racing speed, which is about 54 kilometers an hour, the aerodynamic resistance or drag makes up to about 90% of the total resistance. And of that 90%, about 60 to 70% is due to the body of the cyclist. So this aerodynamic drag actually is the sum of form drag and friction drag. And these are topics that we discussed actually in the first week of this MOOC. The form drag is related to the shape of the cyclist and there flow separation plays a major role, while the friction drag or viscous drag is actually the boundary layer, which will often be turbulent, that exerts indeed friction on the surface of the cyclist. Let's focus on three positions. We have the up-right position, the dropped position and the time-trial position. And these positions will of course give a different aerodynamic resistance. The drag force can be written as follows. It's a product of the frontal area of the cyclist, the drag coefficient, then the air density and the cycling speed squared, that is, if we assume that the wind speed is 0. So the only movement of air is caused by the movement of the cyclist, and then finally, we divide that by two. So you see there, the dynamic pressure actually occurring in this equation. But often these results, when we do wind-tunnel tests or computational simulations, they are important in terms of the so-called drag area; the product of the frontal area and the drag coefficient. So let's look at a few examples from wind-tunnel testing. These are, for the three different positions that you see here, typical values of the drag area. For the up-right position 0.27, for the dropped position, 0.24, and for the time-trial position, 0.21. And that's the cyclist and the bicycle combined. If you only look at the drag area of the cyclist, you see that the values clearly are lower, and that they are about indeed 60 to 70% of the resistance of the cyclist and the bicycle together. In this module, we learned about the importance of aerodynamic drag in cycling. How cycling performance in time trials can be further optimized, and we've seen what a typical output can be of wind-tunnel tests or CFD simulations on cycling aerodynamics. In the next module, we're going to focus on wind-tunnel testing for a single cyclist. Thank you for watching, and we hope to see you again in the next module.
