[BLANK_AUDIO] Welcome back to Sports and Building Aerodynamics, in the week on wind-tunnel testing. In the second module, we're going to focus on wind-tunnel types and applications. The module question is, in which type of wind tunnel are tests for sports aerodynamics, for example cycling, usually performed? Is it A, an aeronautical wind tunnel, B, a climatic wind tunnel, or C, an atmospheric boundary layer wind tunnel? [BLANK_AUDIO] Please again 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 the different types of wind tunnels. You will understand the advantages and disadvantages of the different types, and which applications are performed in which type of wind tunnels. Almost every wind tunnel is a unique instrument. There is certainly no mass production of wind tunnels, they are almost always custom-made. There are two basic types; the open-circuit wind tunnel and the closed-circuit wind tunnel. And there are two basic test-section configurations, open and closed. This is an example of two open-circuit wind tunnels. And in such a wind tunnel, usually the air follows a straight path from the entrance to the exit. And there are two subtypes. A suck-down wind tunnel, which is shown at the top, or a blow-down wind tunnel as shown at the bottom. And the difference is the orientation of the fan in the wind tunnel itself. This is an example of a suck-down wind tunnel, where indeed here you see at end the fan being located. And this is an example of a blow-down wind tunnel, where indeed the fan, in this case a centrifugal blower, is at the entrance. And this is a photograph in a very large blow-down wind tunnel, where indeed the fan is at the end here of the image which is the beginning of the wind tunnel. For a closed-circuit wind tunnel this is a schematic indication and the concept here is that air recirculates continuously with little or no exchange of air with the exterior. Now this is an example, a plan view of a closed-circuit wind tunnel with the fan here, and the rectangular cross section here. And this is an outside view and an inside view of that tunnel operated with a closed test section but it can also be operated with an open test section. Open-circuit wind tunnels and closed-circuit wind tunnels have specific advantages and disadvantages. In open-circuit wind tunnels, a clear advantage is the much lower construction cost, and the fact that you don't have purging problems if you use smoke tests, for example. Disadvantages however, are that if it's located in a room, you often need extensive screening to get high flow quality. And we'll talk about screens later on. Wind and ambient temperature can affect your flow conditions in a tunnel. You need more energy to run than for a closed-circuit wind tunnel, and noise is sometimes also a significant problem. So this kind of wind tunnel is often built and used in schools and universities where the tunnel is used for research and educational purposes but not with a very high degree of utilization. Advantages of closed-circuit wind tunnels are that you actually have recycling of seeding particles if you use LDA and PIV. We'll talk about that later. You have a high flow quality because you apply corner vanes and screens. The conditions inside the tunnel are almost independent from those outside the tunnel, so independent from weather and building operation. You need less energy to run this wind tunnel compared to an open-circuit wind tunnel and you have much less noise. Disadvantages are however the higher construction costs, the fact that you need to find a way to purge the tunnel, and that cooling is often needed in high utilization degrees. So this kind of wind tunnel is often built and used by consultancy companies and by industry where the wind tunnel indeed is frequently, very frequently used. Let's look at a few types of wind tunnels. This is the aeronautical wind tunnel, often but not always, this wind tunnel works with scale models, and it's usually closed-circuit. The smoke tunnels are used for visualization purposes and they're usually open-circuit to allow purging. Then there's automobile wind tunnels. Often full-scale models are used here, although sometimes also reduced-scale models have been used. And these also include the climatic wind tunnels where rain, ice and snow can be reproduced. And these effects on the car can be tested. Then there's general purpose wind tunnels. For example, for testing wind effects on people, for sports, for aerodynamics. But also for birds, insects, wind turbines, and so on. And then finally there is the atmospheric boundary layer wind tunnel, sometimes also called environmental wind tunnel. So let's get back to the module question. In which type of wind tunnel are tests for sports aerodynamics usually performed? And this might a tricky question because Sports and Building Aerodynamics and sports aerodynamics in particular certainly take place in the atmospheric boundary layer. However, the type of wind tunnel where these experiments are performed is usually an aeronautical wind tunnel or a climatic wind tunnel. And the reason is the following; often when testing, for example, cycling aerodynamics, we want to focus on still air conditions, where actually there's no wind flow and the only airflow present is the one caused by the movement of the cyclist. And that actually in a wind tunnel, you need to reproduce a uniform, zero turbulence flow, which is also the case here in this movie. So you see the very smooth flow approaching the cyclist, and only behind the cyclist you see turbulence being generated and therefore the smoke being disturbed. So even though indeed cars and many sports take place in the atmospheric boundary layer, it's not in atmospheric boundary layer wind tunnels that these applications are tested. In this module, we have learned about the different types of wind tunnels, the advantages and disadvantages of the different types, and about the different applications that are performed in these different wind tunnels. In the next module, we're going to focus on the characteristics of an atmospheric boundary layer wind tunnel, on the differences between this kind of tunnel and other tunnels. On the requirements for successful atmospheric boundary layer testing. We are going to define wind engineering and the importance of this field, and we're going to look at the many applications of atmospheric boundary layer wind-tunnel testing. Thank you for watching and we hope to see you again in the next module. [BLANK_AUDIO]
