Welcome back to Sports & Building Aerodynamics, in the week on the 100 meter sprint aerodynamics. At the end of this week, you will understand the importance of form drag in running. You will understand the effects of wind on running performance. You will understand the effects of altitude on running performance. You will understand the importance of representative wind speed measurements during races, and you will understand how the geometry of the stadium can influence running performance. As any week, we'll also start this week with a quote. A quote from Aristotle this time saying: "The worst form of inequality is to try to make unequal things equal." First, we're going to have a look at a few short movies to set the scene for this week. >> Set, go. >> Welcome to the MOOC on Sports & Building Aerodynamics, broadcasting live from the IAAF Diamond League meeting in Brussels, Belgium. This is Eindhoven University of Technology reporting live from the Koning Boudewijn Stadium in Brussels, Belgium, for the memorial Ivo Van Damme. This is one of the highlights of the Diamond League of the IAAF. And athletes actually, top athletes spend a substantial part of their life trying to achieve the best possible performance in their discipline, setting new records. And then for us as scientists and engineers it's our responsibility to make sure that this performance is measured in the most accurate and most reliable way. While this might seem straightforward, it is actually not, because sprint races for example, and hurdles are run all over the world, always at different conditions, different temperatures, at different relative humidities, different altitudes, and different wind speeds, and especially, the wind speed is known to be a very important parameter influencing sprinting and hurdle times to a very large extent. That's why in those type of races, wind speed measurements are made along the track, and if the tail wind, so the wind assistance for the athletes, is larger than two meters per second, then the records are not validated, so they're not put into the record books. Well this seems very simple and also very fair, straightforward, it is unfortunately not it is not that simple, because reality is far more complex. And this often gives rise to unfair situations and an unfair comparison between athletes and especially between performances made at different athletes' conventions, or different races all over the world. The reason for that is that the wind speed, or the wind-flow pattern in and around the stadium is very complex. And that a single measurement at one position along the track is an oversimplification of reality. It is not representative, in most cases, of the actual wind speed that the athlete will experience. So this means that sometimes invalid records are validated, and that sometimes valid records are not validated. And this is exactly what has been the focus of our research, and it's also one of the focus points of this part of the MOOC. These are the contents of week five. First, we'll try to answer the question, why study sprint aerodynamics? Then we will present a mathematical-physical model of running. After that, we will look at wind effects and altitude effects. And then we'll see how stadium aerodynamics and sprint records are related. And we'll conclude this week with an interview with a professional athletics coach. We start again with a module question. An athlete runs in still air conditions, so at zero wind speed, at a speed of 10 meters per second. What are the fractions in percentage, of the form drag and the friction drag on the athlete's body? Is this proportion 0.4% versus 99.6%? Is it 4% versus 96%? Is it 50/50? Or is it 96% versus 4%? Or 99.6% versus 0.4%? Please 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 running. You will understand the typical output of wind-tunnel tests, or CFD simulations of running aerodynamics. And you will understand the relative importance of form drag and friction drag in running. So let's look at this runner, this athlete, who is running on level road in still air conditions, with the running speed, U. He can achieve a certain acceleration. And his acceleration is actually driven by the forces working in a horizontal direction on this athlete, which is the propulsion force P and the drag force D. And then, of course, there's also gravity and the reaction force by the ground. Then we can write Newton's second law, which, in this case, in the horizontal direction, just states that the mass multiplied to the acceleration is the result of the propulsion force, minus the drag force. So if you want to increase running speed, we should either increase the propulsion force, or decrease the aerodynamic resistance, so decrease the aerodynamic drag. The aerodynamic drag itself is composed of two components; it's a form drag and a friction drag. So the form drag is related to the shape of the runner. And the friction drag or the viscous drag, as also explained in the first week of this MOOC, is due to the skin friction in the boundary layer on the surface of the runner's body. And drag can be expressed in this way. So it's this equation with D, the drag force, A, the frontal area of the runner, Cd, the drag coefficient, Rho, the air density, and U, the relative air speed. In still air conditions, it is the running speed, but if there is wind then it's the sum of the running speed and the wind speed. Often, results on the aerodynamic drag, either from wind-tunnel tests or CFD simulations, are expressed in terms of the drag area, with unit square meters. And let's look at a few typical examples. These are values from wind-tunnel testing, on actually a standing person, so not a running person, where it was found that the drag coefficient is about 0.94 or 0.95 for this particular case. If you then have a running person the drag coefficient is a lot lower. And that's of course because the running position is not a fully straight-up position, and also because the arms and the legs are moving. But the drag coefficient here was found to be independent of the Reynolds number which is important. If then the area of a runner, the frontal area, is about 0.5 square meters, then you get these results for the drag area for the standing person and for the running person. However, these are actually quite high compared to other values that have been mentioned in literature. For a world-class male sprinter, Linthorne mentions a value of the drag area of 0.3 square meters. And Mureika mentions a value of only 0.23 square meters. And the reason actually for that, is that these values were determined based on models for the 100 meter sprint. And in the 100 meter sprint, actually, for quite some distance, starting from the beginning until about 25 to 30 meters, the athlete is actually not in a straight-up position. He starts from, or he or she starts from a crouched position and then gradually, as the athlete proceeds over the track, actually the body assumes a more vertical position. And that's the reason why actually averaged over these 100 meters this drag area is actually less than what is reported from wind-tunnel tests with the fixed position or a straight-up running position. These drag areas can also be determined from CFD simulations, and I will not explain the details here of these simulations, but you see some of them here. This is a simplified simulation because in this case, the person has a running position but it's a static position, so this is not including movement of arms and legs, and of little movement of the torso that also occurs during running. So, these are some images of contours of the wind speed, actually the relative speed, because this athlete is running in zero wind conditions, around the athlete's body in different planes. And if you look at the results that we get for the drag area here, they are about 0.37. Which is quite similar to the values that were reported before, and this is actually for what we call a frozen runner; in a running position, but not moving. If we then look, and that's the advantage of CFD, we can split up the drag force into the form drag and a friction drag component, and then we find that form drag is 96% and friction drag is only 4%, so we see here that friction drag is less, proportionally less, than we saw with the cyclist, and this is because a cyclist has a more aerodynamic position than a runner. Looking at some typical examples, so these are examples of drag forces expressed in Newton. This is for a drag area of 0.3 square meters, a running speed of 10 meters per second, and then for different air densities. Here, you see how the drag force changes as a function of air density. So typical values, 16.5 Newton up to 19.5 Newton. So let's turn back to the module question, about our athlete that is running in still air conditions, in zero wind speed and a speed of 10 meters per second. The question was, what are the fractions in percentage of form drag and friction drag? And thanks to CFD we can actually determine those fractions, and the right answer was answer D, it's 96% form drag and 4% friction drag. In this module we've learned about the importance of aerodynamic drag in running, the typical output of wind-tunnel tests or CFD simulations of running aerodynamics, and the relative importance of form drag and friction drag in running. In the next module, we're going to focus on the various stages of the 100 meter sprint. The background of a mathematical-physical model of the 100 meter sprint. And the time-velocity curve for this sprint. Thank you for watching. And we hope to see you again in the next module.
