Welcome back to Sports & Building Aerodynamics, in week six on cycling aerodynamics. In this module, we're going to focus on wind-tunnel testing for a single cyclist. And we're going to start again with a module question. Consider this cyclist, which is equipped with pressure sensors in the wind tunnel. Where on the body, does the lowest pressure occur? Is that A) At the front side of the torso, arms, and legs. B) At the sides of the torso, arms, and legs. Or C) At the back side of the torso, arms, and legs. Please hang on to your answer, and we'll come back to this later in this module. At the end of this module, you will understand how wind-tunnel tests for real cyclists are prepared and performed. You will understand some aspects of the flow around the cyclist body. And some aspects of the pressure distribution on a cyclist body. This is still a part that was part of a research project: optimization of power output and aerodynamic drag. And our goal, actually, here was to investigate, if time-consuming and also, sometimes expensive wind-tunnel testing could to some extent be replaced by high-quality CFD simulations. Of course this means that we have to validate those CFD simulations, and for that of course, some limited set of experiments would be needed. So detailed wind-tunnel tests to benchmark the CFD simulations. We did wind-tunnel tests in two phases, for a real cyclist and for a dummy cyclist. This module will focus on the real cyclist tests. The reason why we also used a dummy cyclist is that this one, as opposed to a real cyclist, remains static in fixed position. And that also, we have all freedom to embed quite some pressure sensors and tubes inside the model, which is quite hard to achieve with a real cyclist. More information about the wind-tunnel tests on this real cyclist and also about the CFD simulations can be found in this article. Okay, let's focus on the wind-tunnel tests on the real cyclist, the full-scale cyclist. These were performed in the closed-circuit wind tunnel in Marknesse in the Netherlands, which is part of the Dutch-German wind tunnel group. And you see here on the left side a photograph taken from outside the test section, so you see the test section there. On the right side you see the cyclist on the round plate mounted on the balance in the wind tunnel test section. So we tested here a standard racing bicycle with disc wheels, also a standard handlebar except for the time-trial position where of course we use the time-trial handlebar. And what is important here is that we used the bicycle, but that it was mounted first on a stand, and then together with this stand on this round plate. And the reason of this round plate actually is to put the bicycle and the cyclist, but especially the bottom part of the bicycle, outside the boundary layer on the floor, the wind-tunnel floor. Because what we want to simulate here is a cyclist riding in still air, and when there's still air, zero wind, there's no boundary layer at the ground surface. However, in the wind tunnel we reproduce that by generating wind flow in the tunnel, so we will generate a boundary layer. However, this is not what we want to be measuring in, and that's why we put the bicycle with the cyclist at a higher elevation, that's what this round plate is about. It's actually also mounted on the force balance below. Then we also use a positioning system and the reason for this, maybe at first sight rather strange system, is to be sure that we could reproduce the cyclist's positions on the bicycle to a very high accuracy. Some other details. We use three static positions, so up-right position, dropped position, and time-trial position, no pedaling. Then we got the frontal areas as indicated here. The maximum blockage ratio was 6%, slightly on the high side. The cyclist had a height of 183 centimeters, a weight of 72 kilograms. And he had an aerodynamic helmet, glasses, gloves, and a standard racing suit. And here you see the cyclist in a few positions. Not with the clothes that were actually tested, but this was just testing the positioning system. And you see that the positioning system makes sure that different body parts are always at the same position. So there's also one below the chin of the cyclist. Here you see him in the up-right position, the dropped position, and then the time-trial position. And here finally you see the cyclist in time-trial position sitting ready on the round plate on the force platform in the wind-tunnel test section. Some other important details. We have the wind direction parallel to the bicycle. Measurements were made at three speeds; 10, 15 and 20 meters per second, to investigate potential Reynolds number effects. We had a very low approach flow turbulence intensity of 0.02%. Then drag force measurements were made with, as you can see here, quite high accuracy and data were sampled at ten Hertz. Then we also had pressure plates, 30 pressure plates that we pasted on the body of the cyclist. And there sampling was done at one Hertz for 40 seconds. These here are the positions of the pressure plates. So you see we measured on the front side of the cyclist, and also on the sides and the back. And, well, it was quite a job to fit these pressure plates on the cyclist and you see that process going on here. Showing some other photographs. Here then you see him almost ready, on the force platform. This is the view from the back. View from the front. The side, fully equipped. Backside, you see that the pressure plates finally result in quite some tubing behind the cyclist. And here we are indeed ready to go and to start the measurements. Then also flow visualization with smoke was performed. And I want to show you a few photographs of that. Here you see indeed the very smooth approach flow, with a 0.02% turbulence intensity, but behind the cyclist of course we have flow separation, and a very pronounced transient vortex shedding. And that is also what you see here. So you see indeed the smoke being moved up and down, and back and forth, due to this, turbulent vortex shedding. While the flow over the helmet of the cyclist then, at larger altitude behind the cyclist, is still quite smooth. This is the same but then shown by a movie. Where you see the smoke actually being released in front of the helmet and then we are going to go down a bit, visualize the flow in the wake behind the back of the cyclist and you see indeed the flapping motion of the smoke, due to the vortex shedding and the interaction of vortical structures in the back. And this is also what later on in this week we will clearly illustrate with Computational Fluid Dynamics simulations. So let's have a look at some results. Well we did these simulations at different wind speeds and we could not detect any significant Reynolds number effects. Then we did measurements not only for the cyclist on the bicycle, and that on the stand, and that on the round plate, and that on a force balance, but also for the bicycle without cyclist, and then for the stand without bicycle and cyclist, and for the round plate, without stand and without bicycle and without cyclist. The reason was that we had to of course subtract the drag area of the round plate and of the stand, from the total drag area to get the values for the bicycle and cyclist alone. And then these are the values that were obtained, so the average values over a series of identical tests. In the up-right position the drag area is 0.27, dropped position 0.24, and time-trial position 0.21. For the cyclist alone, these values are of course lower, and as mentioned also in the previous module, this amounts to about 60 to 70% of the total drag area of cyclist and bicycle together. Often, the results in terms of pressures are expressed as pressure coefficients, and this is also what we do here. So we calculate the pressure coefficients as the difference between the pressure at the surface and static pressure in the approach flow and divide it by the dynamic pressure. And this is calculated with the approach-flow wind speed. And based on that, well, different types of graphical representations can be made, but I will only focus on a few results here. Saying that the highest value found here was 0.78 as pressure coefficient, the lowest was -0.88. But what is interesting is that the lowest values occurred here, on the side of the cyclist. And this might be surprising because one could imagine that this would occur at the back of the cyclist. But that is clearly not the case. So this is actually also the answer to our module question. Where on the body does the lowest pressure occur? Well it is not in the back, it is at the sides of the torso, arms and legs. And the reason for that is that also close to these positions flow separation takes place. This is also to some extent related to what we saw earlier in week one with the flow around a circular cylinder. And indeed as you see here, when you look at the pressure distribution around the circumference of the cylinder you see that the maximum negative pressures, so the lowest pressure, occurs at 90 degrees, which is exactly the side of the cylinder. And of course, a cyclist's body is not the same as a circular cylinder, but indeed there are some similarities in terms of shape and in terms of flow separation. In this module we've learned about how wind-tunnel tests for real cyclists are prepared and performed. We've seen some of the aspects of flow around the body of a cyclist, and some aspects of the pressure distribution on a cyclist's body. In the next module, we're going to focus on wind-tunnel tests for dummy cyclists. Thank you for watching. And we hope to see you again in the next module.
