Welcome back to Sports & Building Aerodynamics, in the week on building aerodynamics. In this first module on wind energy, we start again with the module question. What you see in this figure, is a venturi-shaped roof that was invented by Ben Bronsema to achieve a maximum amplification of wind speed in the contraction of this venturi-shaped roof. And what you see below are four different roof configurations. The question is, which one gives the highest amplification of wind speed in the center of the contraction? Is it A) The venturi-shaped roof without guiding vanes, and of course, this roof will not be floating in the air, but there will be slender columns supporting it. Is that B) The venturi-shaped roof with four vertical guiding vanes. C) The venturi-shaped roof with 36 vertical guiding vanes. Or D) The case without a venturi-shaped roof. So just a regular roof. 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'll understand the advantages and disadvantages of wind energy harvesting in the built environment. And you will understand a common misconception about wind energy in the built environment. Concerning wind energy in the built environment, well in general, the interest in renewable energy is driven by the depletion of fossil fuels, the uncertainty about traditional energy supplies, climate change, and growing environmental awareness. However, it is clear that most research efforts and practical applications of wind energy in the past have focused on the large-scale wind installations that we know very well and that have been placed in remote off-shore or on-shore areas. And much less attention has been given to wind energy in the built environment, which in itself is an interesting concept, because then energy can be produced close to the location where you need it, where you will use it. Actually, there are three types of integration of wind energy in the built environment that can be considered, you can allow siting of stand-alone wind turbines in urban locations. You can retrofit turbines onto existing buildings or you can integrate them completely into the architectural design. Two things that are certain about the built environment compared to open on-shore and off-shore areas is that you will have a lower mean wind speed, and you will have higher turbulence intensities, which are not very beneficial for wind energy. Nevertheless, if you want to apply wind energy in the built environment, it's very important to amplify, locally amplify the mean wind speed as much as possible. But in general we will not reach the high wind speeds that you would obtain, for example, at the same height over open sea. Nevertheless, as mentioned before, we want to achieve high local amplification factors of mean wind speed. And that's why actually the Ventec-roof was invented and designed by Bronsema. You see a perspective view here on the left side, and then a vertical cross-section on the right side. Where the position of the turbine is very schematically indicated by this vertical line. This is another view, a vertical cross-section on the left and a horizontal cross-section on the right, where we also see the potential location of the vertical guiding vanes which, in the beginning, by the designers, by architects, were thought to improve the amplification of wind speeds at the position of the turbine. So, we see the configuration with four vertical guiding vanes, in blue. And those with 36 vertical guiding vanes in red. But then the question was, when we started this research project in collaboration with the inventor, which type of roof would yield the highest amplification in the passage? This study is reported in more detail in this paper which you can find on the website of the MOOC, and I will briefly present a summary of it now. First, we started doing wind-tunnel measurements, because you always need those to validate CFD simulations. Otherwise, nobody will trust the result of the CFD simulations. So, we did this at the Peutz Laboratories in the Netherlands, for the different configurations that were shown earlier. Here you see, on the left side, the mean wind speed profile, on the right side, the turbulence intensity indicated. And then measurements were made, of surface pressure, and of velocities around the building and also inside the roof. The measurement positions are indicated here. Then the computational grid was made based on a grid-sensitivity analysis. Here you see the grid for the roof with 36 vertical guiding vanes. Here the grid for the roof with the four vertical guiding vanes, and here the one without vertical guiding vanes. And again, of course, this is no flying saucer, this is actually a roof that in reality will be supported by slender columns that provide minimal blockage to the flow. What we did in all these studies was actually only working with hexahedral and prismatic cells to allow the use of the required second-order discretization schemes, and also to promote faster and better convergence. These are some of the settings, I will not read it in detail, I will just mention the highlights. Steady RANS realizable k-epsilon model, second-order discretization schemes. And other parameters can be found in the paper. Then the grid-sensitivity analysis. We looked at the wind speed profiles in the center of the roof contraction, comparing the coarse grid with the middle grid and the fine grid, in terms of this wind speed. You see that actually the differences between the three grids were very small. And we continued therefore using the middle grid for this problem. Then the validation, comparing wind speed ratios, the experimental ones on the vertical axis, and the numerical ones on the horizontal axis. The deviations are about 10%, which is certainly acceptable for a RANS study. And then let's have a look at the more detailed results from the CFD simulations. This is the case with the 36 vertical guiding vanes. You see a vertical cross-section, with the contours of the wind speed ratio, and also a horizontal cross-section. And you see that, there is some amplification, but it's only present in the middle of the roof. If you take the case with the four vertical guiding vanes, you see that there is also an amplification in the middle of the roof, but it extends also a little bit more downstream. However if you have the roof without vertical guiding vanes, you see actually that the amplification is much larger and it is also present over a much larger area of the roof. Actually, almost over the entire area of the roof. And this is, at least to some extent, counter-intuitive. What happens actually in the case with the vertical guiding vanes, is that these guiding vanes impose so much resistance to the flow through the roof, that the air approaching the roof will rather flow around the roof and over the roof rather than being forced through it. And that is something that doesn't happen to that extent in the case without vertical guiding vanes. And this is actually a LES simulation showing the similar effect. On the left side, you see a vertical cross-section, with wind speed indicated in meters per second for the case without guiding vanes, and there the flow very smoothly goes over and through the roof construction. While on the right side, you see actually quite substantial separation above the roof, which is due to the higher flow resistance in the roof and in the passage. And actually this resembles a little bit what we discussed before. This example of the buildings in V-arrangement, where one could expect maybe that the converging arrangement would give the highest wind-speed amplification, but it was actually the diverging one. And also there, the issue was, if you add too much resistance, the flow will rather flow around the building rather than through the passage. And this is very similar to that what is happening here. It's also about the Venturi-effect not applying to open flows, but only applying to confined flows and indeed, you have some confined flow here when you only focus at one of these channels, bounded by the roof upper and lower surface, and by the vertical guiding vanes. But you don't have that, upstream of the building. So what actually happens here in this case, with the many guiding vanes is that the wind-flow rate entering one of these ducts already has reduced so much that the amplification will be very limited. Of course, then we need to estimate how much wind energy this construction would give us. So we again look at these three configurations, we take a vertical axis wind turbine and we can make some calculations for different geometrical parameters. And we find for a wind turbine in the middle of configuration C, so without vertical guiding vanes, about 4000 kWh. And this is not that impressive you might mention, because it will certainly not be enough to actually justify the installation of such a roof. However, you can multiply this by a rather large factor, because, as opposed to the cases with the vertical guiding vanes, here in the roof without the vertical guiding vanes you cannot put one vertical axis wind turbine but you can put quite a lot of them. And of course, there will be some wake effects, but you can substantially increase the wind energy output from this roof, making it a possibly interesting investment. Back to the module question. This is the venturi-shaped roof. The question is, which configuration gives the highest amplification in the contraction. In this presentation I did not focus on roof D, but clearly it is roof A, the one without the vertical guiding vanes, that provides much higher amplification than the three other configurations. So in this module, we have learned about the advantages and disadvantages of wind energy harvesting in the built environment. And we have also looked at a common misconception about wind energy in the built environment, being that the so-called Venturi-effect would apply to open flows, which it does not. In the next module, we'll focus on a particular case of wind energy harvesting in the built environment. And we'll see how the aerodynamic design, and wind energy output of the Bahrain World Trade Center, can be improved. Thank you for watching, and we hope to see you again in the next module.
