Welcome back to Sports & Building Aerodynamics in the week on building aerodynamics. In this third module on pedestrian-level wind, we start again with a module question. Consider the group of buildings indicated in the figure here. Where in the middle we have a covered square and the openings of that square, the entrances and the exits are indicated with the arrows and with the red color. What could be the main cause of high wind speed on the square, so below this roof, for the wind direction indicated? Is that the downflow from the high-rise building on top, indicated with the letters Wh? Is that pressure short-circuiting between the entrance and the exit of the square? Or is that the approach flow at ground level that approaches over this Green Boulevard indicated in green. 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 in a complex case study in a densely built urban environment. And you will understand what remedial measures can be applied in this practical case study to improve wind comfort and wind safety. So this is a study that was performed in 2009-2010 and was actually a study for our own university campus here in Eindhoven in the Netherlands. And the study is reported in this scientific article. And also in this article a general framework for wind comfort and wind safety studies is presented. This is a so-called high-level framework which is no indication of the fact whether it's high quality or not, no it's an indication that this framework is based on the very important existing Best Practice Guidelines that were treated in week three of this MOOC. So in this framework a distinction is made between three cases in which you can, or should apply wind comfort and wind safety studies. First, you have new developments in an existing configuration, and then on-site measurements can be done for the existing configuration which is often very similar to the new one. Or it are new developments for which no on-site measurements are available. Or we have a third situation, which is the development of a completely new urban configuration, which is either completely new, or substantially different from the existing one, and then of course, there are no on-site measurements available because this is a design stage. This is an overview of the framework, and I will briefly take you through parts of the framework, so we're going to zoom in here. We start with the question: "Are these developments in an existing urban configuration? If that is the case, yes. And the question is: "Are there on-site measurements available that you can use for validation?" If that is the case then we go to the validation module, which is a separate module that you see here on the right-hand side. And in this module we're going to perform then CFD simulations according to best practice guidelines, of course. This we do for the existing configuration, we perform a grid-sensitivity analysis with at least three grids, which is of course very important. We compare with the experiments, and if then the validation is successful, if not, we have to go through this loop again, if the validation is successful, we go back to the general module, then we perform CFD simulations for the twelve wind directions. Then we apply also the procedure for wind comfort assessment. And then we have a look at whether this new design involves major design changes compared to the existing situation. If that is not the case well then we can perform the simulations for the new design with the same set of parameters and the same kind of grid resolution we have used in the previous study, the study for the existing case. So again following the procedure for wind comfort assessment, then we check if the design is comfortable and safe, is this yes, then we can approve the design, is it no, then we have to start communicating with the urban planners or the building designers as the case may be, and come up with a new configuration, a new design, which might be substantially different. If it is substantially different, we'll have to go to sub-configuration validation. This is a separate module indicated here, where also again the same procedure is applied. We of course need to satisfy best practice guidelines, we have to make a grid-sensitivity analysis for sub-configuration cases. And after that, based on successful validation for these cases, we can apply the same settings for the new configuration. So this way you can run through this framework several times until finally a satisfactory solution is obtained. And the framework takes into account that in all the steps, the necessary best practice guidelines are satisfied. So let's look at what this means in practice. This is the building actually under investigation as it was before. And this is the new design, where actually a partially high-rise construction is erected on top of this building, where actually some of the side walls are taken out and are becoming entrances and exits for a large roof-covered square, which should of course have a very attractive wind climate. So the research mission here was; first assess wind comfort and wind safety in the present and in the future situation, because of course we want to improve wind comfort, we do no want to make it worse. And if necessary suggest remedial measures to have a satisfactory wind climate. So let me very briefly take you through a part of our campus. This is the top view with an indication of all the buildings and their heights. But let's look at this aerial photograph and take some of the highest buildings out of this plan here. This is the Vertigo building. It's the building of the Department of the Built Environment, about 55 meters in height. This is the Main Building, 45 meters high. And this is the Potentiaal building of 55 meters high. So these are the three highest buildings. Then we generate a computational domain, generate also a high-resolution grid. Well actually we created three grids to perform the grid-sensitivity analysis. This was a grid made by Wendy Janssen, a PhD student who did a fantastic job here in making a high-quality high-resolution grid. With only prismatic cells to limit numerical diffusion but also to allow convergence, smooth convergence with the requested higher-order discretization schemes, second order in this case. So here no tetrahedral cells, no pyramidal cells, not a single one, all prismatic cells and hexahedral cells. This is another view of the model that was made of the existing situation. Here you see on the left side a figure of the Main Building, which is a building that's actually standing partly on columns with an open through-passage indeed. So this is a situation where certainly high wind speeds might occur. And on the right side in this image you see the Vertigo building. And then the right image indicates the corresponding computational grid. Then we look at the boundary conditions. For this, we have to estimate the aerodynamic roughness length based on the land-use in a circle, an area of a distance of ten kilometers around the location of interest. So the white rectangle here is our computational domain, and around this area we estimate as good as possible, as good as we can, the aerodynamic roughness length from the Davenport-Wieringa roughness classification. And then the inlet profiles of mean wind speed, turbulent kinetic energy, and turbulence dissipation rate are indicated on this slide. Then the wall boundary conditions are very important here. We take into account the relationship, in this case for the ANSYS code, between the sand-grain roughness height and aerodynamic roughness length. Apply appropriate sand-grain roughness heights and roughness constants. Then we apply steady RANS with the realizable k-epsilon model, standard wall functions, modified for roughness indeed with the sand-grain roughness parameters. And these are some other computational parameters. And then we performed a grid-sensitivity analysis; this was the coarsest grid. And what we did in refining the grid was actually comparing the finer grid with the coarse grid at several positions on the campus terrain. And these positions are indicated here in the right figure with the red dots. So, this is the basic grid. So, we refined the grid substantially and then we compare the wind speed ratios between the basic grid and the fine grid on this graph on the right side, and you see the one-to-one line there. So ideally, both grids give identically the same outcome; all dots should be on this line. This is definitely not the case here. So the coarse grid was clearly not fine enough, so we refine it even further. And then we see that the differences actually between the basic grid and the fine grid, certainly for the higher wind speed areas, are very limited, which indicates that here we can use the basic grid with an acceptable level of error. And then of course validation is very important. We had a period of about six months, where the wind speed was measured at a few fixed positions but also many mobile positions across the campus, at which the measurements were made after working hours, so when cars and most people have disappeared from the campus terrain. So we can measure in an almost empty situation, so the worst-case scenario you might say. And then we can compare the wind speed amplification factors that we have at, for example, the fixed measurement points. This is the corner of the Vertigo building, where you see in black the measured values, and in orange the simulated values, and apart from the west wind direction you see actually a very close agreement. And the differences actually in the west wind direction are due to the fact that in this case the measurement position is exactly in the shear layer, which is a region with very high wind-speed gradients and a very small shift in wind direction already gives quite a substantially different value in CFD. So this discrepancy had to be expected. This is for an other, more open measurement position close to the Main Building with a through-passage. And here you see again a very good agreement between the steady RANS CFD simulations and the corresponding measurements, actually measurements averaged over quite a long time, so you also see the standard deviations of the measurements indicated in these graphs. We can also compare wind directions. Also here we see quite a good agreement except for the particular wind direction where the shear layer again is kind of disturbing our measurements or the measurement is disturbing the shear layer you might say. And we also found a generally good agreement for the other position. So this provides quite some confidence in the computational grid that we selected, also in the procedure, the computational settings and parameters, so we can apply this also for the new situation. Let's first look at what the Dutch Wind Nuisance Standard gives us for the present situation. Here we see a top view. On the left side, you see the exceedance probabilities ranging from zero to about 22%, and the threshold generally is 10%. And you see some areas highlighted in yellow here, and these are areas indeed with a higher amplification factor, so higher exceedance of the five meters per second threshold. And on the right side, the right figure, you see the quality classes by the Dutch Wind Nuisance Standard, where A is very good for all activities while D actually is poor for some activities such as sitting. And indeed you see some particular features being highlighted. The through-passage of course, which is known to be, as mentioned before in previous modules, the most important issue of amplifying wind speeds, so we have that below the Main Building there. We also have the corner of the Vertigo building where we have the corner stream, which is quite important and then actually there is also a jet here between two buildings, and this jet is present because well generally, southwest is the main wind direction in the Netherlands, and that's what you see indicated here. This jet actually reaches up to the new building. So if we then continue to focus on the new building, and that is what you see in the figures at the bottom. There you see actually that if we have the opening as intended, that this jet actually blows straight into this opening, and this will not give rise to a comfortable wind climate. Then of course there were also for architectural and urban planning reasons to have this entrance actually directly aligned with the passage between the other buildings. Of course from a wind comfort point of view we should have shifted the passage. We also evaluated that, that was a very successful and efficient measure. But this could not be withheld based on architectural reasons. So then, the best thing we could achieve together, with taking into account the architectural requirements, was to install sliding doors there in this position. And with the condition that these doors should be closed on windy days, and when the square is intensively used. Finally, to conclude these three modules on pedestrian-level wind, some very brief tips for avoiding wind discomfort and wind danger. Generally, as also mentioned before, there are three items, either one of them or a combination of them that cause the problems. It's the frontal vortex, or horseshoe vortex, corner streams, and pressure short-circuiting. It's important to take into account wind discomfort and wind danger because it can be life threatening at worst. And at least, very harmful for the reputation of your building and for you as a potential designer. The Dutch standard says that wind nuisance and wind danger must be included at least in any building design where building height is larger than 15 meters, or if you have surrounding buildings when the height is larger than two times the height of the low-rise buildings. However, this does not mean that you cannot have wind comfort and wind danger problems for lower buildings. If you have buildings with a through-passage then a height of five meters might already be enough to give you quite some problems. Of course, it's important that you do not position building entrances, parks, playgrounds and shopping streets, and so on, at these positions where you get high wind speeds. You cannot walk with children there, of course, unless you want to lose them. So, be careful in planning these activities. And if you then need to resort to remedial measures, well, measures that are often quoted are screens and fences. But you have to imagine that you have an atmospheric boundary layer approaching, towards let's say a wide high-rise building, and if you put this fence there, as indicated here, which in this case can be a solid fence, this is the gray one. Well there's a very large mass of air coming down at high speed and this is not something that you can just stop with a small fence, and not even with a large fence. So the air will always find its way around, so here the pedestrians will not be very satisfied. On the other hand, you can apply canopies. But you have to imagine that then this canopy should not be a small canopy, this should be a very large canopy. A canopy with dimensions in the order of the building dimensions, and then indeed of course if you did not take this into account as a designer, this will dramatically alter the appearance of your building. But in this case, you can actually catch the standing vortex by the canopy, keep it above pedestrian level, deviate it from the canopy at large height, and then the air will actually spread out and then your pedestrians might experience a comfortable climate. Another option is to have podium-shaped extensions, again with dimensions in the order of magnitude of the building. So this is a very substantial change to the original design. Also here your pedestrians will be happy, of course provided that you do not allow them to walk on the podium, which could be extremely dangerous. And then if all of these measures are not satisfactory, or not withheld for any reason, well the last resort often is vegetation. But really, if you have a standing vortex coming down along this building facade, which has actually not just a few meters, but maybe several tens of meters, effect away from the building. A very large mass of air again, with a very high wind speed, this is not something you can stop with a row of trees. And also the fact of course that the stem of the trees will not have a very substantial effect. So, you should put a lot of trees before this building, and this is often, of course an impossible and an unacceptable situation. So the most important aspect and most important issue is, take wind discomfort and wind danger into account as early as possible in the design stage. So back to the module question. This was the case studied here, so what is actually the reason for causing potential wind discomfort or potential high wind speed on this square? Well it's not downflow from the building because the building that is on top is not that high. It is actually also shielded from wind by the Main Building, indicated with HG here. But it is actually the pressure short-circuiting. That is one of the three main reasons, as mentioned before, standing vortex, corner streams, pressure short-circuiting, that is to be held responsible for wind comfort issues. So in this module we've learned about how assessment of wind comfort and wind danger is performed, based on CFD in a complex case study, a case study of the Eindhoven University campus. And what remedial measures can be applied in this practical case to improve wind comfort and wind safety. In the next module we're going to focus on a different topic: natural ventilation of buildings. And we're going to see how CFD can be used to substantially improve the natural ventilation of a very complex building. Thank you again for watching, and we hope to see you again in the next module.
