Welcome back to Sports and Building Aerodynamics. This is week two on wind-tunnel testing. At the end of this week, you will understand the importance of wind-tunnel testing, you will understand the different types of wind tunnels. The difference between aeronautical wind tunnels, and atmospheric boundary layer wind tunnels. The main components of wind tunnels, and the importance of flow quality and similarity. As any week, we also start this week with a quote. This time by Marcus Tullius Cicero: Asiduus usus uni rei deditus et ingenium et artem saepe vincit, which means: Constant practice devoted to one subject, often outdoes both intelligence and skill. And certainly in wind-tunnel testing, constant practice is important to be able to get accurate and reliable results. These are the contents of this week. We start with trying to answer the question, why wind-tunnel testing? Then we focus on the different types of wind tunnels, their applications and then in more detail, on the atmospheric boundary layer wind tunnel. Afterwards we will address wind-tunnel components, measurements and flow visualization, similarity and flow quality and finally some best practice guidelines for performing wind-tunnel tests. So, why wind-tunnel testing? Well, let's start again with the module question. Numerical methods such as CFD are getting ever more powerful and are increasingly applied, and the question is: Will CFD ever be able to replace the wind tunnel for Sports and Building Aerodynamics? Answer A Yes, certainly. Answer B No, never. Hang on to your answer, and we'll come back to this question later on in this module. At the end of this module, you will understand the definition of wind-tunnel testing, the important contribution that wind-tunnel testing has made and is still making everyday to our society. The many applications of wind-tunnel testing including sports and building aerodynamics. And here are some references that are certainly recommended reading material, and from which I've drawn to prepare this module. What is wind-tunnel testing? Wind-tunnel testing is accurately reproducing the wind flow, around either full-scale or model-scale objects in a tunnel-like construction, under controlled conditions, and in order to study their performance for design or research purposes. And here you see in the left figure an example of an atmospheric boundary layer wind tunnel, and on the right side, an image of a building model, urban model on a turntable in a wind tunnel. In this module, but also in this MOOC as a whole, we will only focus on low-speed applications, and low-speed wind-tunnel testing means that we are working at Mach numbers lower than 0.4. So in tunnels with with a maximum speed capability up to about 134 meters per second, which are also called subsonic wind tunnels. The key question in this module is the following, and I've taken it literally from Barlow et al. (1999): What has motivated the invention, development, and continuing uses of wind tunnels? Well in order to answer that question, let's have a look at the importance of wind-tunnel testing in our society. There have been claims first in the 1970's and 1980's, that the wind tunnel in the subsonic regime would rapidly disappear because CFD would become available, and it would be more cost-effective, and it could be equally accurate as experiments. However, we see now that CFD has improved quite a lot, but has not come close yet even to replace the wind tunnel. And this is a nice quote from Leschziner. It says that this, and there he refers to computer technology, evolution has given rise to the rather radical view expressed predominantly among the US aerodynamics fraternity, that the wind tunnel is destined to become a convenient storage cabinet for computer output. But a moment's contemplation leads to the conclusion that this view reflects a rather narrow interpretation of CFD focusing on a particular type of flows most relevant to high-speed external aerodynamics, and some turbomachinery applications. And this indeed is very true for flows in Sports and Building Aerodynamics, which are excessively complex. It's certainly not possible to use CFD as a standalone tool, without any experiments. This is another quote by Barlow et al., saying that there are now no credible predictions that computational simulation will replace the need for all data from physical experiments in any significant development projects. And he concludes by stating, turbulence continues to confound us in many respects. So, again, why is wind-tunnel testing so important? Well, our planet earth is engulfed by air and water, and fluid mechanics is everywhere in and around us. So it's not surprising that wind-tunnel testing has played a decisive role in shaping the society that we live in today, but also in shaping the tools that we use to operate in this society. And that can be buildings, cars, airplanes, ships, bicycles, and so on. So therefore, a week on wind-tunnel testing is certainly an essential component in a MOOC on Sports and Building Aerodynamics. What would the world be like without wind-tunnel testing? I just want to give you some examples. Imagine a world without airplanes that have been developed in a very wide range of sizes and shapes with excellent performance in strength and efficiency, production costs, and so on. And I want to show you here some examples of airplanes at different times in the past being tested in wind tunnels. [BLANK_AUDIO] But not only airplanes. For example also space shuttles that have to re-enter the atmosphere. Trucks that have to transport goods from one place to another. Cars that also have been tested extensively in wind tunnels. High performance ships which can be navy ships, but also ships drilling for oil. Aerodynamic bicycles. Top performances in cycling. And then last, but definitely not least, buildings that have to withstand the force of wind, at least where structural aspects are concerned. Wind-tunnel testing is used extensively for building aerodynamics. So coming back to the key question in this module: What has motivated the invention, development and continuing use of wind tunnels? Well, one answer is that this invention and the use and the ongoing evolution has been motivated by the wide interest in practical problems in aerodynamics. That's a first point. But second also by the fact that theoretical and computational methods have not been, are not now, and will probably never be capable of providing the full range of results needed to guide detailed design decisions in many practical problems. And Barlow et al. also state that the most successful attack on virtually any aerodynamic design problem, will be based on application of a combination of results from experimental, theoretical and computational methods, combined together and leavened by experience. And this includes indeed wind-tunnel testing or the use of wind-tunnel testing to validate CFD simulations. Which is something that we will do extensively in the rest of this MOOC. Barlow et al. also mention some specific advantages of wind-tunnel testing. Well they allow of course to use models that can be prepared already very early in the design stages. It is also said that wind tunnels can include the full complexity of real fluid flow, and here I have to slightly disagree. Because this is not really true for atmospheric boundary layer applications where it's very difficult, sometimes impossible in wind tunnels, to reproduce thermal stratification. But it's certainly true that wind tunnels can provide large amounts of reliable data. They're often also very rapid indeed, economical, accurate to get detailed aerodynamic information that can be used to support design decisions. And wind tunnels indeed have saved already money and lives and they will for sure keep doing that in the future. So coming back to the module question. Will CFD ever be able to replace the wind tunnel for Sports and Building Aerodynamics? The right answer here is definitely no. And just as a simple example, consider the fact that you would like to investigate the effect of textile texture and roughness on cyclist aerodynamics. This is something that you can certainly not resolve with CFD. You can try to model it, to approximate it, but if you want to get accurate and reliable results in a fast way, you will have to do this with wind-tunnel testing. In this module, we've learned about the definition of wind-tunnel testing. The important contribution that wind-tunnel testing has made to our society. And the many applications of low-speed wind-tunnel testing which include the two topics of this MOOC, Sports and Building Aerodynamics. So a week on wind-tunnel testing is certainly an essential component in this MOOC. In the next module, we will focus on the different types of wind tunnels, the advantages and disadvantages of different types. And on the different applications performed in these different wind tunnels. So thank you for watching, and we hope to see you again in the next module. [BLANK_AUDIO]
