Welcome back to Sports & Building Aerodynamics, in the week on cycling aerodynamics. Let's in this module focus on drafting cyclist groups. That are groups larger than two cyclists. We start again with a module question. Eight cyclists with the same body geometry and the same position on the bicycle are riding behind each other in a team time trial in zero-wind conditions. Which position has the lowest aerodynamic drag? Is that A) Position 2. B) Position 3. C) Position 4. D) Position 7. Or E) Position 8. Hang on to your answer and we'll come back to this question later in this module. At the end of this module, we will have addressed some of the key questions that I would like to mention here. Can we understand and improve team time trial cycling? Which position in a group of cyclists gives the lowest aerodynamic resistance? And in the 2013 Tour de France team time trial, nobody could beat team Orica Green Edge. Nobody? Well, maybe science can. Let's have a look. Previously in this MOOC, we focused on the validation of CFD simulations with wind-tunnel measurements of real and dummy cyclists. We've also looked at the mutual aerodynamic drafting effects of cyclists, two cyclists riding closely behind each other. And we found that there is indeed a substantial effect for both cyclists on each other, and the reason was the same as we saw in building aerodynamics: subsonic upstream disturbance. But this will probably also play a role when eight cyclists are riding behind each other in a team time trial. Therefore, we started CFD simulations of cycling groups. The distance wheel-to-wheel between those cyclists is one centimeter. They are all in time-trial position. We assume that all the cyclists have the same body shape, size, and position on the bicycle. And for the CFD simulations, we used the similar parameters as mentioned in the previous study, in the previous module, both for steady RANS and for Large Eddy Simulations. So I'll not go into the computational details again, you can see them in the previous module. But we are going to look immediately at some results here. The results I will show you now are percentages in drag reductions, or reduction of the drag force, compared to the situation for a single cyclist. So if we have two of them in time-trial position, the first one has a reduction in his aerodynamic drag of 2.7% and the second one, of course, a much larger reduction of 13.9%. Let's add a third one. Then you see that the benefit for all three of them will increase. The first one now has a benefit of 3%. Also, the second one will have a larger benefit now because he has somebody riding in his wake that actually fills up to some extent this underpressure area. And then, you see that the third one has clearly an advantage over the second one. Let's add another one, and then you see that in this configuration, four cyclists, it is the last one that has the lowest aerodynamic resistance. And you even see that adding a fourth cyclist still has an effect on the first one although indeed this effect is very small. What actually is happening here, and what is the reason for the fact that the last one has the lowest aerodynamic resistance, is what is visualized in this animation. So this is the Large Eddy Simulation of four cyclists in the team time-trial position riding behind each other. And you see in both views, both the side view and the top view, that as you move downward along the road, so downstream over the row of cyclists, you see that the wake gets higher and the wake gets wider. This means that the last cyclist is actually in the position that is most protected from the wind. However, at some point down this line, the widening and the increasing height of this wake will stop, and at that point, the aerodynamic resistance of being more down the line will also stop, and you will have the same benefit as the one in front of you. And this happens actually, so we can continue building our line of cyclists, this actually happens almost at the moment when you have five cyclists. If you have six of them, then it's not the last position that has the lowest aerodynamic resistance, but it is the one but last. The reason is that the fifth one and the sixth one here have the same benefit, from an equally wide and equally high wake, but that the fifth one still has a cyclist riding behind him or her filling up the underpressure area. And this is something the last one doesn't have, and that's why the last one, or the last position, is not the most aerodynamic position. What about the practical consequences of this research? Well, I think it's unlikely that this 2.7% or maybe even 3.1% benefit for the leading rider, that this would really make a difference in regular races because those are often very unpredictable, very chaotic. They might end indeed in a chaotic sprint as shown here on this photograph, or in a long adventure by a lonely rider or maybe even more riders in any group that have set up an escape. But in team time trials, however, it might be a different story because these are much more organized. And the differences with which these races are sometimes won are, well at least in 2013, they were unbelievably small. And this means that even minor improvements to organization of team time trials in aerodynamics, for example, in view of aerodynamics can really decide who wins and who loses. Let's look at some of these races. These are the results of the 2013 Tour de France team time trial, won by Orica Greenedge. And Omega Pharma-Quick Step was very close behind Orica Greenedge. It's indicated here that it's one second, but it was not even one second. And then it's maybe a bit cynical that this is called a gap, because the distance was, or the difference was so small, it was 0.75 seconds on a total of more than 25 minutes, and that's 0.048%; unbelievably small. But then later that year, we had the 2013 World Road Cycling Championships. And then the order was reversed. Here Omega Pharma-Quick Step won the prestigious team time trial and Orica Greenedge was the second one. And here it says it was 0.81 seconds although some media have reported 0.88 seconds. Regardless, the difference again is extremely small. Let's have a look at what is the aerodynamic benefit of the trailing rider on the leading rider, what that has for consequences on time differences. If you look at this 2.6 or 2.7% difference then you see that if you have a team time trial or a time trial for let's say, 30 kilometers then this makes a difference of 26 seconds. Of course, it's to some extent clear that if you have a team time trial, well, your benefit by using this knowledge will not be 2.6%, because in all team time trials, of course, cyclists are riding behind each other. Nevertheless optimum strategies and optimum sequences can be determined. And given the order of magnitude of the time differences in this table and the order of magnitude with which these races have been won, you clearly see that this can have a substantial effect. So, the conclusion is that CFD simulations in combination with detailed biomechanical and biomedical knowledge of the cyclist, can be used to determine the optimum sequence in team time trials and to even establish better times than the excellent times that have been established so far. So let's go back to the module question now. Eight cyclists, same body geometry, riding behind each other in a team time trial in zero-wind conditions. Which position has the lowest aerodynamic resistance? Well, here it's position seven. Although position six and seven are very close to each other. The reason for position seven is that, even though indeed the widening and increasing height of the wake stops almost at cyclist number five, it very very slightly keeps increasing a little bit. So that's why position seven has a better aerodynamic position than position six and then position eight actually doesn't have a cyclist filling up the wake behind him or behind her. So, that's why position seven here, very close to position six, is the best one. Some important comments to conclude this module. Here some important issues about cycling aerodynamics have been raised in particular concerning team time trials. Up to now, to the best of my knowledge, there's no team that has specifically been organizing team time trials based on this information. So you could say that teams have fought with equal arms. But it is also true that possible important improvements can be made and they might be decisive in the future. Nevertheless I would like to state that it's of course very easy to raise comments on how cycling should be performed from behind a computer desk, even a very good computer desk or a good computer, but of course it does not detract in any way from the great appreciation that we have for every cyclist, recreational or professional, who is fighting aerodynamic forces to make a great performance. So in this module, we've learned about how you can understand and improve the team time trial cycling, which position in a group of cyclists has the lowest aerodynamic resistance, and how this in the future maybe could change who wins and who loses in these very prestigious races. In the next module, the final module of week six, we're going to offer you an interview with professional cycling coaches about these new insights in cycling aerodynamics. Thank you for watching, and we hope to see you again in the next module.
