Welcome to this presentation on atmospheric turbulence with focus on wind energy. This is the first of the two presentations that we will go through. After this lecture you will be able to explain the relevance for wind energy, identify two factors that cause turbulence, and describe a couple of measuring instruments. So why is turbulence relevant for wind energy? It is because we have wind turbines, which will rotate when the wind blows in a direction perpendicular to the rotor plane. And wind has random variations, which causes loads on the wind turbines. In simple words, what it means is, it causes stresses on the blade root here. So what happens is, if we take an example of one blade that this blade simply bends as it rotates. And this causes fluctuations in the loads that the wind turbine experiences as is shown in this slide here. So on the x-axis, you have the time in seconds going from 0 to 1,000 seconds. On the y-axis, you have the blade root bending moment in kilonewton meter. We don't worry about the absolute numbers of the blade root bending moment for now, but it is clear that there is random variation of the blade root bending moment. Or in other words, the stressors as the time changes. And ultimately, what it causes is something called the fatigue damage, which you will also learn in some of the other lectures, specifically the lecture that deals with structural dynamics. And now we come to this question on what causes atmospheric turbulence. Basically what we have to imagine is that if we have air parcels in the atmosphere, which are stacked on top of each other, then the air parcels which are very close to the ground simply have no velocity. And the air parcels which are further away from the ground have a large velocity. Now what this causes is a gradient in the wind speed as one moves from air parcels which are closer to the ground to air parcels which are higher up in the atmosphere. And this gradient, or the difference in the velocity as one goes from the ground to high up in the atmosphere, is what causes atmospheric turbulence or mixing of the air parcels. An example here is given where we have a lot of trees on the ground which causes friction when the air parcels move. And therefore the velocity of the air parcels closer to the ground is almost zero, as compared to the velocity of air parcels which are high up in the atmosphere. Another reason why turbulence occurs is say for example, the air parcels, which I denoted by the small blue clouds, have more or less the same temperature, which is denoted by the blue color. And then during the day when the sun shines, then the ground heats up, which causes the air parcels which are closer to the ground to be heated up more than the air parcels which are away from the ground or higher up from the ground. And the difference in the temperature, so the warmer temperature is shown by the red color, and the cooler temperature is shown by the blue color. So, the colder the air parcel is, the larger the density of the air parcel. And hence the larger the mass of the air parcel. So simply because of the temperature differences and because the heavier air parcels, they mix with the air parcels which are lighter, which are near to the ground. It causes turbulence. One could think of an example of a hot air balloon on a nice warm day which rises up in the air because the temperature of the air parcels inside the balloon is much warmer. Now we talk about turbulence scales. When we talk about turbulence, we can talk about the velocity of the air parcels waiting from a fraction of a second up to several days. But we can also talk about the variation of wind velocity of air parcels which are scattered in space. So from fraction of millimeters to kilometers. In this lecture and also in the following lecture, the focus will be on micro-scale turbulence. That is, the variation of the velocity of the air parcels from a fraction of a second up to one hour. Let's take a look at instruments used to measure wind speeds. The first instrument that we see is the cup anemometer, which looks like this. And how does this work? So you install a cup anemometer on a meteorological mast, or some steel structure with beams or some steel, another steel structure protruding from the mast, and you simply mount this instrument on top of that. When the wind blows, for example, as a demonstration, [SOUND] then the cups, which are conical here in shape, they simply rotate with the wind speed, and the rate of rotation is proportionate to the wind speed. This is how you measure the wind speed using cup anemometer. Another commonly used instrument is the sonic anemometer. And this is a more reliable instrument to measure the fluctuations of the air parcels at much smaller time scales. And this is how a sonic anemometer looks like. So you have a transmitter and a receiver on three different axes, then you emit the acoustic pulses from the transmitter, which is received by the receiver and then sent back. So you measure the time that it takes for the acoustic pulse to travel from the transmitter to the receiver, and then from the receiver to the transmitter. And the difference in the transit times is then proportional to the wind speed. This is how you measure wind speed using a sonic anemometer. This is how a wind speed time series looks like as an example, a typical wind speed time series. On x-axis, we have the time in seconds going from 0 to 1,000 seconds. On the y-axis, we have the wind speed in meters per second. And it is very clear that there are random variations of the wind speed with time. Such typical time series is what we get out of a cup anemometer or a sonic anemometer. So in summary, in this lecture, we have learned about the importance of turbulence for wind energy, factors causing atmospheric turbulence, and two instruments, two very common instruments that are used for wind energy applications.
