Welcome to Sports & Building Aerodynamics in the week on building aerodynamics. In this module, the second module on pedestrian-level wind, we start again with the module question. Consider this building which has a through-passage in which serious wind discomfort is experienced, so strongly amplified wind speed. Which of these remedial measures would be most effective and the answer can be more than one. A) A canopy above the through-passage. B) Screens in the passage. C) A four-wing revolving door in the passage, or D) Sliding doors in 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 how the assessment of wind comfort and wind danger is performed based on CFD for a complex case study. You will also understand what type of remedial measures can be applied in building projects if you have to improve wind comfort and wind danger or wind safety. So this is the first case study, in the next module we'll focus on another one, which is the so-called Silvertop Towers in Antwerp, Flanders, in Belgium, which is a study that we performed in 2002-2003. So what is the problem statement? Well as you can see here, it's a group of three high-rise buildings. The height is about 60 meters. They are residential buildings, located south of Antwerp, but, at that stage, when we started the study, the buildings and neighborhood had really declined. And you can see that here, from the daytime view, it are dark-grey buildings, with also a lot of other building physical problems, considering, or including thermal bridging and rain penetration, mould growth and so on. And that's why actually the housing department organized an architectural contest. The final contest-winning design actually scored very high on the safety aspect, which was indeed one of the most important criteria. And this, you see actually the new design on the right-hand side and it might seem quite similar to the left-hand side but, actually yes, the integrity generally of the buildings was preserved, but what has been done is actually creating through-passages through each of the towers. And what you see there on the right-hand side are small canopies sticking out of these through-passages. So what was important actually for this project to win the contest is that well, due to these passages, you create social control to the site, that the intention was also to have pedestrian walk-ways going through the towers, which indeed also would increase the social control to the site and this was also actually a specific requirement by the policemen on the beat to be able to easily access different parts of the terrain. However, also the entrances were actually placed in these passages. And you see a perspective drawing of that situation on the right-hand side, where you see also the canopy sticking out of the through-passage. You see the building entrances indicated in the passage. You have a lower passage and at that point surprisingly to us there was also an upper passage, which we weren't really sure about whether that would be useful. Of course, because the lower passage would be used by people entering and exiting the building. So because this through-passage and the safety aspect were so important, these passages are really an important aspect of the design. So it's also important of course that wind comfort and wind safety are preserved. But of course, we know that the most problematic case in terms of wind discomfort and wind danger are through-passages through buildings, so that needed to be assessed. So the research mission was first to assess wind comfort and wind safety in the passages in the new configuration and if needed, to suggest effective remedial measures. So we started the CFD simulations, and of course quality assurance is very important. This requires verification and validation, and because we did not have any wind tunnel measurements to validate the CFD simulations with, and of course the buildings with the through-passages were not yet realized, we also did not have on-site experimental data, so we resorted to so-called sub-configuration validation. And this means that you subdivide the actual complex configuration in sub-components that have specific parts of the flow field around them and on which you can do then validation because often for these simplified cases, experimental data, high-quality experimental data is available in literature. So that is indeed the approach that was followed here and then this validation is performed based on those data. And then it is generally assumed, or can be assumed that if the model performs well, so the chosen approach, steady RANS for example and a certain turbulence model and wall functions perform well for each of the sub-configurations, they will also perform well for the superposition of these sub-configurations, which is the actual complex situation. So that is the approach followed here. There is quite a lot of high-quality experimental data available for sub-configuration validation. These are just three online sources indicated here. This is not a complete list, but definitely a lot of very valuable data can be found in these databases. So first we focused on a through-passage through a generic building. This was experimental data obtained in Sweden in 1975 and you see here on the right-hand side the comparison between the simulations in the solid lines and the dotted lines indicate the experiments. Where we have quite a good agreement at least for the highest wind speed ratios, which is the focus of our research. Then we also looked at buildings that are actually parallel, but shifted towards each other because, indeed, these three towers are parallel but shifted. Here there is sand-erosion data available and you see here the comparison between on the left-hand side the wind tunnel test for sand erosion. On the right-hand side the CFD simulations. Both indicate amplification factors defined in the same way, and on the right-hand side what you see in white is actually amplification factors slightly above two. And you see that although there are some discrepancies between those flow fields, the experimental one and the numerical one, that the magnitude and also the extent of the areas with the highest amplification factors, about 1.8 to 2, is quite good. However, you also see that if you look at the blue areas that CFD predicts amplification factors of about 0.2-0.4 while in the experiments we have 0.8. And this is actually a characteristic feature of the comparison of steady RANS CFD simulations with wind tunnel experiments; steady RANS severely underestimates wind speed amplification factors at positions where these are rather low. But in terms of wind comfort and wind danger we're focusing on the high amplification factors. So it's important that we predict those areas accurately. This validation step of course has been repeated for other wind directions. Of course, it was also performed in a quantitative way; I only show you the qualitative comparison here. And here you see actually that we get, with steady RANS and the realizable k-epsilon model, a surprisingly close agreement of the high wind speed ratios between both results. And indeed, this is actually almost too good to be true so definitely here there will be some errors that have compensated each other. And then this is another configuration and also here the agreement that is almost too good to be true. But this is indeed what came out of the simulations and also here, different errors will have compensated each other, will have balanced each other out. So, based on the sub-configuration validation, we actually had quite some confidence in this simulation approach. However, it's possible to criticize sub-configuration validation and indeed this has been done, but if no wind-tunnel data is available and the buildings are yet to be realized, this is really the only alternative. And this is really a minimum requirement for quality assurance in CFD, that in some way a validation study is very carefully performed. So, let's go back to the simulations then for the complex case study. Here you see the three buildings with some surrounding buildings. We inserted them into a computational domain, then a computational grid was generated based on a grid-sensitivity analysis which was performed for the sub-configuration cases. You see here the grid resolution being much higher in the passage and around the canopy. This was a grid actually of 2.9 million cells around those buildings. And what you see here are the contours of the amplification factor in a horizontal plane, at 1.75 meters height, so at pedestrian height for the wind direction indicated. And you see indeed on the color bar on the left that the amplification factors range up to a value of three, and this actually happens in this through-passage. An amplification factor of three is excessively large. This actually has indeed to do with the pressure short-circuiting. What you see here on the right-hand side are static pressures, gauge pressures. And then indicated for the same wind direction as the flow field, the mean wind-speed on the left-hand side. And here, indeed, you see the overpressure and underpressure indicated on the right-hand side and then the resulting strong jet in the through-passage. So if we then apply this discomfort criterion, which is not the one from the Dutch standard because the Dutch standard did not yet exist at the time that we performed this study. Then with this maximum limit of 10% of exceedance of the threshold, actually what we found here were percentages that were much larger. So these are discomfort percentages much larger than the allowed 10%, so definitely something had to be done here; wind comfort would be extremely bad. So, then we need to turn to remedial measures. So in focusing on remedial measures it's good to look at what is causing the problem. And the problem is caused by overpressure, underpressure and then pressure short-circuiting. So shown here in this graph is again static pressure. I left out the color bar here, with overpressure indicated with the plus sign, underpressure with the minus sign for this particular tower. One thing you could do of course is closing the passages, but given the fact that this was the contest-winning feature of the design, this was not possible. Another option, a theoretical one, would be to actually reduce the pressure difference over this passage by extending the passage. For example, with air-tight, transparent tubes, but apart from the fact that you cannot have them end and start outside the overpressure and underpressure area, well you can imagine what happens with graffity on these tubes. So, this would certainly not be a good option here. Another option could be, a theoretical one again, screens in the passages. Screens could be added to actually increase the flow resistance and decrease the flow rate. Apart from the fact of course, that the fire department would never approve of this, another problem was that from the simulations we found that actually the local wind-speed gradients due to the screens in the passage became even larger. At this point, we were kind of running out of ideas. And then we came up with the idea of a four-wing revolving door in the passage. If you have a four-wing revolving door, this means that the passage at any moment is closed. However, this idea was not very well received by the other partners in the project, because a four-wing revolving door indeed has round shapes. These round shapes, even if they are transparent they would reflect the light and actually do not allow a clear sight through the passage. And this was a requirement by the policemen on the beat. So finally, almost completely running out of ideas, we came up with the idea to install sliding doors in the passage. So that then if a high wind speed would be present, always one of the doors would be closed, and the other one could be opened. Of course then when a person wants to enter the building from the side where the door is closed, he has to push a button, he or she has to push a button. The other door has to be closed. Then this door can be opened and the person can go inside. And then, strictly, also, always, at least the passage is closed on one side so there's no wind flow, no net wind flow through the passage. So this was the idea that was finally withheld. Of course it's not always needed to close one of these doors. If wind speed is low on a nice, sunny, non-windy day, for example, both doors could be kept open. But in order to know when to close the doors, you could measure the wind speed in the passage. However, if you close one of the doors, you don't measure anything anymore. Well, you measure zero wind speed and that was the point actually when this upper passage proved to be very useful, because based on the CFD simulations, we could demonstrate that there is a very strong correlation between the wind speed in the lower passage and the wind speed in the upper passage. And surprisingly, when you close the lower passage, that that had almost no influence on the wind speed in the upper passage. And that's why, in this upper passage, we devised an automatic control system based on an anemometer, so a wind speed measuring device. And based on these measurements, the doors, at least one of the doors, would be closed or would be opened. And then indeed, wind discomfort, ideally, if the system is well adjusted, well controlled, wind discomfort is zero percent. And indeed it was implemented, in this way, actually to substantial satisfaction of all partners. So what you see here is a view of one of the realized towers. You see the canopy, which is actually quite a massive canopy. Here are the sliding doors in the lower passage. Then if you look closely then you see the upper passage already. If you zoom in here you see the wind speed measuring device located and then on the door is this sign which is in Dutch. It says: keep the wind outside, and it asks for some patience of the people actually wanting to enter the building, because they first have to push the button and wait until the other door closes and then this door would open. So, let's turn back to the module question. If you have a building like this one here with a through-passage, what kind of remedial measures would be most effective? Well in this case, it's definitely the four-wing revolving door and the sliding doors. The canopy above a through-passage, even if this canopy would be very large and very wide, has a surprisingly small effect. And also screens in the passage, apart from the fact indeed that the fire department would never allow this, also have quite a limited effect. In this module, we have learned about how the assessment of wind comfort and wind danger is performed based on CFD in a complex case study. And what remedial measures can be applied in building projects when you want to improve wind comfort and wind safety. In the next module, we're going to focus on how the assessment of wind comfort and wind danger is performed in another case study, also with CFD. And what other types of remedial measures can be applied if we want to improve comfort and safety. Thank you again for watching, and I hope to see you again in the next module.
