Welcome back to Sports & Building Aerodynamics, in the week on building aerodynamics. In the second module on wind energy, we start again with a module question. What you see here is a top view of a converging building arrangement. The question is, at which position does the highest amplification of wind speed occur? Is that in position A, upstream of the narrowest part of the passage. In position B, at the narrowest part of the passage. Or at position C, downstream of the passage. Please hang on to your answer, and we'll come back to this question later in this module. At the end of this module, you will understand a particular case study of wind energy harvesting in the built environment. You'll understand how the aerodynamic design and wind energy output of the Bahrain World Trade Center could have been improved. So let's look again at this configuration. The two configurations actually, the converging arrangement and the diverging arrangement, where an earlier question in this MOOC was where you would get the highest amplification of wind speed. And as opposed to what one might think based on intuition this was actually the diverging arrangement. And there was a particular reason for that, which we have called the wind-blocking effect. Actually the slowdown, the upstream slowdown of the wind speed, or in other words: subsonic upstream disturbance in the wind-flow pattern. And this was also indicated here by this movie where on the left side, for the converging arrangement, you see the highest wind speeds occurring not in the passage, but around the buildings, because of the high flow resistance in the passage. While in the diverging arrangement, you see the highest wind speeds in the passage, and that is also indicated here. Okay, then as researchers it is important to publish your work, which is what we did. We received actually very positive reviews and quite a good number of citations and then you might wonder, who cares? This is just an academic example that is very far from reality because nobody would construct buildings in a V-shape you might wonder. Well that's not entirely true. Let's have a small trip to Bahrain, where, when we zoom in on Manama, you see here the Bahrain World Trade Center, which was opened in 2008, a $150 million project, 240 meters in height. What is very special about it is that in the passage between the buildings there are three 29 meters wind turbines with a rated power of about 225 kilowatts, and they're oriented towards the prevailing on-shore Gulf breeze. But if you look at this building from the top, this is a converging arrangement. And if you compare that to the simple academic example that we had before, it seems that the Bahrain World Trade Center is positioned in a converging rather than in a diverging arrangement. And we know from the simple case that the diverging arrangement is better, much better. So the question is, is this also the case for the Bahrain World Trade Center? And that's why I would like to pose this hypothesis. The Bahrain World Trade Center design would have yielded higher wind energy output if the buildings were positioned in diverging rather than converging arrangement. Or, in other words, in terms of wind energy, the towers should have been turned 180 degrees around. That's the hypothesis, let's know see what comes out of the investigation. So we did an investigation here, started by wind-tunnel testing and then also by detailed CFD simulations. And I would like to show you a short movie of those wind-tunnel tests. Today we are present in the Dutch city of Apeldoorn. And we're actually guests at the Wind Tunnel Laboratory of TNO, and together with our friends and colleagues of TNO we're going to perform measurements on the Bahrain World Trade Center. So we had a model made, a 1 to 300 scale, and we're going to test this here in one of the three atmospheric boundary layer wind tunnels in the Netherlands. And, this wind tunnel as you can see is equipped of course with a fan. It's a fan that actually sucks the air from outside. So it's a suck-down wind tunnel. There is also a fetch of roughness elements, which will be illustrated later. And in front of me, you see the model that we have made of the Bahrain World Trade Center and we'll do detailed measurements of mean wind speed and turbulence intensity in the passage between the towers at the positions of the three turbines. So let's get started, it's rather cold here and that's because the wind tunnel draws air from outside, and it's still middle of winter. So let's go. So what you see here first is that the contraction of the wind tunnel is situated outside. So the test section actually is in this small attached green building. The test section itself has a width of three meters and a height of two meters, and the building model is positioned on the wind tunnel turntable. Behind the contraction you see the spires, and other roughness elements that are meant to generate the atmospheric boundary layer wind speed and turbulence profiles. The fan is situate at the end, so it's a suck-down tunnel. And then measurements are made for different wind directions with hot-wire anemometry where the hot wire is carefully positioned with a traversing system, to be exactly in the center of the wind turbines, where we measure at their axis location. During these measurements we had excellent support from our colleagues at TNO and also from my PhD student Adelya Khayrullina, who you can also see sitting in the wind tunnel here. Let's have a look at the wind-tunnel results. We will express them as ratios, but not as ratios of velocity magnitude, but of the x-velocity component where the x-axis is parallel to the passage axis, and the wind-turbine axes because only the x-velocity component is relevant for the wind energy output due to the fixed position of the turbines. So the turbines themselves do not rotate as a function of wind direction. So then we defined a wind speed ratio Kx as the x-velocity component in the passage, divided by the wind speed at tower height; so 240 meters. So let's have a look at these results. We do that for the three different turbine positions and this is the first one, the lowest turbine, turbine number one. Where you see this wind speed ratio, at the position of the axis of the turbine for the three different wind directions that were investigated. And indeed it clearly shows that the diverging arrangement at this particular position, gives a much higher wind speed ratio, especially for the zero degree angle, which is the prevailing on-shore Gulf breeze. This is for the second turbine. Very similar results. And then also the third turbine where also there the diverging arrangement, clearly gives higher wind speed ratios. But of course, these are point measurements. So they are not representative of the wind speed that is experienced by the turbine. For that we actually need whole-flow field data and that can be obtained by CFD. So then we start with the CFD simulations. This is the grid that was finally used with 1.6 million cells, but we obtained that by grid-sensitivity analysis. Then, steady RANS simulations were performed. The computational domain and grid were made according to best practice guidelines and some information on boundary conditions is indicated on this slide. And then the realizable k-epsilon turbulence model was used to close the RANS equations, second-order discretization schemes, and so on. So let's then have a look at the validation first of the CFD simulations. So a very important part, as mentioned before, of CFD simulations is comparison with high-quality experimental data. In this case our wind-tunnel experiments. And what you see here is the wind speed ratio, in the horizontal axis experimentally determined and in the vertical axis numerically determined. And actually, the agreement is quite good. The deviations are not larger than about 10%. So then we can use the CFD simulations further to examine the flow field through the passage and around the buildings. This is the wind speed ratio but then not only for the x-component but the total wind speed divided by the reference value, for the converging, left, and the diverging, right, arrangement for the second turbine. And indeed here you see that along the horizontal black line, which indicates the turbines, or the swept area of the turbines looked at from the top, that in the diverging arrangement indeed the wind speed is much higher. That is clearly indicated here. This is for a wind direction of 15 degrees, where we see two remarkable facts. First of all, that the wind speed indeed in the diverging arrangement is again higher than in the converging arrangement. However, we have to look at x-components later on. But what you also see here is actually a very nice feature of the design, the current design of the Bahrain World Trade Center. So that the towers actually were shaped in such a way that they deviate the oblique flow, into a flow that is actually parallel to the wind turbine axis. And that you see very clearly here. And this actually is not the case in the diverging arrangement. But also here for the 30 degrees you see that actually the design in this respect functions very well, while here this is not the case. Okay let's then look at results of the amplification factor, the wind speed ratio integrated over the turbine swept area, which is more representative for the wind energy output that could be obtained. And here you see that for the first three vertical bars, that's for zero degrees, that the diverging arrangement clearly gives higher wind speed than the converging ones. For the 15 degree angle, the diverging arrangement is slightly better than the converging one, and for the 30 degree wind direction, the diverging arrangement is slightly worse than the converging one. That is again illustrated here. So let's have a more detailed look here at what happens with the converging arrangement. Because what you can see here, in the converging arrangement is that actually the highest wind speed occurs downstream of the passage. So maybe we should just shift the turbines a little bit more downstream so that they are in a higher wind speed area. And when you calculate that, and those are the gray bars here in the middle, you see that actually this arrangement would perform better than the current converging arrangement for all turbines and all wind directions. And for some cases, especially the 30 degree angle, it would very much also outperform the diverging arrangement. So then some intermediate conclusions. Well, what we have seen is that the diverging arrangement gives a higher amplification of mean wind speed compared to the converging arrangement and for the wind directions parallel to the turbine axis and at 15 degrees. However, we have seen that this is not the case for the 30 degree angle. But we've also seen that an improved position of the turbines, so a bit more downstream, but still in the passage, gives a much higher amplification of mean wind speed and that for all wind directions; so a very substantial improvement. So let's then look at the transformation of this wind velocity data to wind energy output. Of course, we need wind statistics and the orientation of the building. Here we have the wind statistics for two years. So this is actually not enough to make a complete assessment, but the availability of data was rather limited. So we are happy to have the two year data. And then we combine that with the wind turbine power curve. And this is the power curve, as published by the designers of the Bahrain World Trade Center, so the actual curve of the turbines that have been used. Then you can calculate the wind energy output, indicated here in megawatts-hour per year. And here you see it for the three turbines for the zero degree wind direction. And clearly the diverging arrangement is the best, followed by the improved converging arrangement, and then the existing converging arrangement. For 15 degrees, it is the improved converging arrangement that is the best, followed by the diverging one, and then the existing one. And for 30 degrees, we see that the wind energy output is much lower, but this is also related to the fact that this wind direction is not that common. You see that the best configuration is first the improved converging arrangement, then the existing converging one, and then, slightly less, the diverging one. So summing this information up for all the wind directions, this is what you get as wind energy output for the three turbines and you can compare these values with the values written at the right of this slide, and these are the values calculated by the designers. And you can see that, well, we're actually, for the existing converging arrangement, pretty close, a little bit less than the estimated values, but what is clear is that for every turbine the divergent arrangement is better than the existing one, the existing converging one. But also that the optimal converging arrangement outperforms the two other configurations. And then actually putting everything together, summing-up over the three turbines, what you see is that the diverging arrangement would have yielded a 14% increase in wind energy output compared to the existing converging one. But the converged optimal configuration, so the shift of the turbines more downstream, gives an improvement of 31%, which is very substantial. So the conclusion here is that the Bahrain World Trade Center definitely has a good design. It's a good idea, but it can be improved significantly. So let's turn back to the module question. We had a look at this converging building arrangement, and the question was: at which position does the highest amplification of wind speed occur? And as we've seen here in the slides, but also in the case for the Bahrain World Trade Center,. this actually occurs downstream of the passage. And that was shown in this simple configuration, but it was also very clear for the Bahrain World Trade Center, and the reason to actually propose an optimal converging solution with the turbine shifted more downstream. In this module we've learned about a particular case study of wind energy harvesting in the built environment. We've also learned how the aerodynamic design and wind energy output of the Bahrain World Trade Center can be improved. This actually concludes the week on building aerodynamics. It was a week in which we've learned about wind-flow patterns around buildings, why these patterns are so complex, and why they are often misinterpreted. We've seen how wind flow can create wind nuisance and even wind danger for pedestrians. We've looked at some important aerodynamic processes for buildings, such as natural ventilation and wind-driven rain. And we've seen why successful integration of wind energy in the built environment requires a detailed study of building aerodynamics. So in the next week, we are now going to focus on the 100 m sprint aerodynamics, where also building aerodynamics, but then the aerodynamics of the stadium, and the resulting impact of that on sprint records will be outlined. Thank you very much for watching, and we hope to see you again in the next week.
