Hello. Welcome to this MOOC on Sports and Building Aerodynamics. In the first week, we will focus on basic aspects of fluid flow. Learning outcomes for this week are the following. At the end of this week you will understand important physical properties such as density and viscosity. You'll understand why the location of the velodrome is important for world hour records in cycling, you'll understand why turbulent flows are so complex. And why there is not yet a good definition of turbulence. You will also understand boundary layers, including the special case of the atmospheric boundary layer, in which Sports and Building Aerodynamics take place. This week actually actually might be experienced as quite tough because it's a lot of information in a very short period of time. But please hang on, because it provides the basic background for the rest of the course. And you will only truly benefit from the weeks on sports and building aerodynamics, the application weeks, when you've also worked your way through the first three weeks. So, in this perspective, this quote by Aristotle is quite applicable. It says, the roots of education are bitter, but the fruit is sweet. This definitely applies to the first week in this MOOC. These are the contents of this first week. We will start with fluid properties in three parts. Then, flow properties in two parts. After that, we will focus of fluid statics, kinematics, and dynamics. And then on the important topic of boundary layers, also in three parts. And finally we conclude with a special case of the boundary layer, being the atmospheric boundary layer. So let's start with fluid properties. In every of these modules in this MOOC we will start with a module question and this question will then be answered later on in this module based on the knowledge gained from this module. So imagine a cyclist that is riding in still air. And the wind is only caused by the movement of the cyclist, so actually there is 0 wind speed but relatively speaking there is wind for the cyclist. So, the question is, where on the body does the dynamic pressure reach a maximum? Is it A, at the sides of the arms, torso and legs? B, at the front of the arms, torso and legs? C, at the back of the arms, torso and legs, or answer D, nowhere, it is zero at all positions on the body. So please, 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 fluid properties velocity, pressure and temperature. You'll understand the difference between static pressure, dynamic pressure, and total pressure. You'll understand the velocity distribution around a cyclist, and you'll also understand the static and dynamic pressure distribution around and on a cyclist. Let's start with a few definitions. Fluid actually derives from the Latin fluidus, meaning fluid, flowing, moist, and fluere, to flow. And there are different definitions possible. One is that it is a substance that continuously deforms under shear stress. Another is that it is a substance whose molecules move freely past one another. Yet another is that it is a substance that has a tendency to assume the shape of its container. So a substance that can flow has no fixed shape, and offers little resistance to shear stress. Fluids include liquids, gasses and plasmas, and a liquid is actually a fluid where particles move freely among each other, but they are not separated. So they are actually held together by intermolecular bonds, they have a definite volume, but not a definite shape. But they do not disperse to fill the space available for the liquid. Gas on the other hand there we have particles that move freely among each other, but they are separated, so they yield perfect mobility. They have no definite volume and no definite shape and they do disperse to fill any space available, irrespective of their quantity. The Greek word for liquids has actually provided the name for the word hydrodynamics. And the same can be said for gas and air which is given the name aerodynamics. A physical property and that's what we will start focusing on here in this module, is any measurable property which describes the state of a physical system. And that there are five, particular properties that we will focus on here. Velocity, pressure, temperature, density, and viscosity. But first, we have to make an important assumption. And this is the continuum assumption. A fluid actually consists of a large number of molecules, and these molecules are in constant motion. They undergo collisions with each other and with the solid boundaries. So a fluid, as shown in this animation, is discrete. A continuum on the other hand is a bulk material that can be subdivided into infinitesimally small volumes. And then properties of these volumes are the same of that of the bulk material. So these are clearly two different things. However, then the continuum assumption means that we're going to assume, that the fluid is continuous, rather than discrete. And that actually is an accurate assumption when the size of the flow system, is much larger than the mean free path of the molecules. Which is about 5 multiplied by 10 to the power minus 8 for air at standard atmosphere so there this is a good assumptions. This also means that fluid properties that you are going to describe such as velocity, pressure, temperature, density and viscosity are also assumed to be continuous and to vary continuously from one point to the next. So in this module we're going to focus on velocity, temperature and pressure. And then in later modules, we'll focus on the other properties. First some etymology, velocity derives from the latin velocitas, means speed or velox, fast. Pressure derives from latin pressura, a pressing, pressure. And temperatures derives from the Latin tempero, I temper. Velocity is defined as the rate of change of the position of an object. So it's a vector quantity. The magnitude is a speed and the direction is the direction of the motion. We indicate it with a symbol v and then it can be subdivided in components along the three coordinate axes, u, v, and w. Let's look at an example, we're going to focus on the cyclist that is in time-trial position, that is riding at 15 meters per second, which is a very good speed of 54 kilometers an hour in still air. So again, the only movement of air is caused by the relative movement around the cyclist. What you see on the left figure here is the velocity vector pattern in a vertical center plane, so a plane that cuts right through the middle of the cyclist. You see the cyclist here, and the bicycle was not included in these simulations. On the right side you see the wind speed contours in meters per second also in this vertical center plane. Where you see the wind speed is high between the legs of the cyclist and over the helmet. And that behind the cyclist, you have a low wind speed region, and that's the reason why it's actually quite beneficial to ride behind another cyclist in the low wind speed regions to reduce your aerodynamic resistance. Then pressure is defined as force per unit area, applied normal to the area. It can be indicated with unit Pascal or in bar, which is 10 to the power 5 Pascal. Or in standard atmosphere, this unit is quite often used. It's approximately equal to the air pressure that you find at earth and at mean sea level. Then there is an important distinction between static pressure and dynamic pressure. The static pressure is the pressure at a nominated point in a fluid which can always be defined, also when the fluid is in rest, so not in motion. And usually, when in science the word pressure is used, it's actually used to refer to static pressure. The dynamic pressure is the kinetic energy per unit volume of a fluid particle, and you can calculate it with this equation. Where rho is the density, and V is the velocity, or the speed, actually. And then the total pressure is the sum of static and dynamic pressure. So let's look again at an example, our cyclist, again riding in time-trial position at 15 meters per second in still air. You see here the pressure in Pascal indicated, so this is pressure relative to the atmospheric pressure. And I also have to mention that the color bar was cut at the bottom and top to make the overpressure and underpressure areas more clear. So here you see static pressure around the cyclist. You see overpressure in red in front of him and underpressure in blue behind him. And you can also make such a graph of contours of dynamic pressure and then you see that these indeed are directly related to the wind speed around the cyclist. So there's clearly a very large difference between both definitions of pressure. Let's turn back to the module question then. A cyclist is riding in still air, wind is only caused by the movement of the cyclist. Where on the body does the dynamic pressure reach a maximum? Well because dynamic pressure is defined by this equation. And the wind speed is actually zero at the body of the cyclist. This means that actually, also the dynamic pressure is zero at all positions on the body. So, the right answer is D. Let's have a look at static pressure then because static pressure is not zero at the body of the cyclist. This is the same cyclist again also here I cut the color bar to more clearly indicate the over- and underpressure areas. So you see in front of the cyclist clearly overpressure, which also causes part of this aerodynamic resistance. And then you see behind the cyclist, on the back of the cyclist, you see, well, maybe a slight underpressure but it's not totally clear so this pressure is actually quite close to zero Pascal. The lowest pressures actually are found on the side of the cyclist, and this is maybe surprising but this is also the area of flow separation. Flow separation is a very important aspect. We will focus on that later in this week. So then temperature finally, is actually a measure of the average kinetic energy of the molecules in an object or a system. And this animation shows actually what happens when you increase the temperature of a gas. Then the molecules will start vibrating or start moving with larger path lengths, but also with higher speeds. So temperature can be indicated with Kelvin, or with degrees Celsius or degrees Fahrenheit, and the conversion equation is also given on this slide. In this module, this first module, we've learned about the fluid properties velocity, pressure and temperature. About the difference between static pressure, dynamic pressure, and total pressure. The velocity distribution around a cyclist, and the static and dynamic pressure distribution around and on a cyclist. This week is an important week. It might be experienced as quite tough as mentioned before, but please know that it provides the basic background for the rest of the course. So please hang on, and you will benefit from it later. In the next module we are going to focus on the fluid property density. We are going to see how density changes as a function of temperature and pressure. How air density changes as a function of altitude. And also how cycling aerodynamics is influenced by temperature and altitude. And I will try to explain what these four seemingly unrelated pictures have in common with each other. Thank you very much for watching. And we hope to see you again in the next module. [BLANK_AUDIO]
