Welcome back to Sports & Building Aerodynamics. This is week four on Building aerodynamics. At the end of this week you will understand wind-flow patterns around buildings. You will understand why these patterns are so complex and why they are often misinterpreted. You will understand how wind can create wind nuisance or even wind danger around buildings for pedestrians. You will understand the important aerodynamic processes for buildings, such as natural ventilation and wind-driven rain. And you will understand why successful integration of wind energy in the built environment requires detailed knowledge of building aerodynamics. As any week we also start this week with a quote. This time from Socrates saying "The only true wisdom is in knowing you know nothing." These are the contents of week four. We will start with two modules on wind flow around buildings. Then three modules on pedestrian-level wind conditions around buildings after which we'll focus on natural ventilation of buildings. Then there is again two modules, this time on wind-driven rain on building facades and then we conclude with two modules on wind energy in the built environment. So let's start with module one. Wind flow around buildings. We start again with a module question. You see a high-rise building indicated here, and you also see the orange arrow indicating that quite a large amount of the air flowing towards this building is deviated downwards to pedestrian level, to ground level. And then the question is, as this high-rise building indeed creates amplified wind speed at ground level. What do you think will happen to the wind speed at ground level if the building height is increased by a factor four? Do we think that it will remain the same? Do you think that the wind speed will increase or that, C, it will decrease? Please hang on to your answer and we'll come back to this question later on in this module. At the end of this module, you will understand how wind flows around an isolated building. You will understand the different components of the wind-flow pattern. And you will understand the unsteady character of this wind-flow pattern. So let's start by having a look at the wind-flow pattern around an isolated building. So let's first start form a flat, uniformly rough terrain. Where if you have neutrally stratified conditions, we have an atmospheric boundary layer approaching, so this is a logarithmic mean wind-speed profile and then if we add the building here then this flow pattern will become much more complex than the simple logarithmic law. Indeed then this is a nice indication of what this flow pattern looks like. And I will briefly run through the different components of this flow pattern with you. So first, and indicated with number 1, we have the flow that goes over the building. Then especially at lower altitudes we have the flow that is actually oncoming and then flows around the building and then number 2 actually turns into number 9. At about 70 to 75% of the height of the building, we have a so-called stagnation point. And from this point on actually the flow separates in different directions. So we have a 4 that indicates the flow that separates sideways and goes around the vertical edges of the building. But we also have downflow from the stagnation point, which is actually a quite important and large mass of air that is coming down. Then actually, that mass of air generates a so-called standing vortex or horseshoe vortex, at the base of the building. And this is often a position where high wind speed is experienced. There actually air is flowing in the opposite direction than the oncoming flow, and at some point these collide, creating a new stagnation point, indicated with number 7. Then you have this standing vortex that actually wraps around the corners of the building. And these regions are called the corner streams and actually they unite with the flow indicated by number 9. And actually together they form the corner streams, which often are also areas with quite high wind speeds, or at least strongly amplified wind speeds. Then behind the building we also have a recirculation flow that is actually characteristic of the complexity of the wake. Then at point 11 actually, where we have the division of the flow in either the normal flow direction, or the opposite direction, we have again a stagnation region at ground level. Then beyond that, downstream of that point, we have the restored flow direction, and then it still takes quite a while for the atmospheric boundary layer profile to develop again, so the wake is quite long, quite extensive. In this wake closer to the building, we have large vortical structures behind the building. And in the shear layer between these large vortical structures and the corner streams we have small vortices in this region of very high wind-speed gradients. We can look at this flow pattern from different points of view and let's look at it from the top, if we focus for example on results from sand-erosion experiments indicated here on the right side. And here actually you see the same flow features indicated with the same numbers as you see in the figure on the left-hand side and let's look at a few of them in more detail. The corner streams, which are also indicated in dark gray or black in the right figure. You also see the standing vortex indicated there, which again also creates quite a high wind speed. So earlier sand-erosion in wind tunnels are darker colors in these photographs. And you also see the stagnation region here, which is a lighter color in the right figure, so postponed erosion of sand grains and this is indeed a region where two flows collide and where the mean wind speed is rather low or at least the amplification is very low. You can also look at this flow pattern with the oil-streak technique and then you see the result on the right-hand side. Also here we have the similarities; you see the standing vortex indicated there where the streaks indeed are created by flow, flowing away from the building in the upstream direction. You also see the stagnation region here where no clear streaks are drawn because there is no clear direction of the mean flow. And you also see the wake vortices very nicely indicated here. So what you also see in these graphs from the sand-erosion technique, provided by Beranek and Van Koten, is the numbers that are indicated. And these numbers are amplification factors, so they are actually the wind speed that occurs at a given point in the flow divided by the wind speed that you would have at the same location without the building present. This means that if there's no building everything would be equal to one, no amplification of the wind, also no deceleration, no decrease of the wind speed, but you see here that due to the presence of this particular building, and the (building) dimensions will follow later, that the wind speed in the corner stream is amplified with a factor of two, which is actually very substantial, and that the wind speed in the standing vortex is amplified with a factor of 1.4 to 1.6, which is also very pronounced. You see in the stagnation region actually that wind speed is reduced by about 20%. If you look at different wind directions or buildings oriented in a different way, you will get different extents or different areas of these flow regions. Here you see that in this schematic drawing a small standing vortex is present but you don't see that clearly in the sand-erosion results, however we do see some very small, nevertheless, powerful corner streams. So again here the values reach up to 1.8-2 in amplification of wind speed. And this is a same result, similar graphs for a lower building, but a wide building, and then also here you see the standing vortex indicated clearly. And also the corner streams which here are actually very large areas extending to quite large distances downstream and these are also areas where the wind flow is amplified or wind speed is amplified quite substantially. So let's turn back to the module question. A high-rise building creates amplified wind speed at ground level and this indeed is due to the fact that a large mass of air, actually for 70% to 75% of the building height, actually that mass of air is deviated downwards. This is not only a large mass of air, it is also air at high speed because in the atmospheric boundary layer wind speed increases with height. Then this actually comes down to ground level, and what will happen if you increase the building height by a factor four, in general, is that more air flowing against the facade is deviated downwards and that you have a larger mass of air, but again, also air at higher speed at this pedestrian level. So, the wind speed at ground level will substantially increase. And an example is shown here. What you see here is sand-erosion contours, results for a building with a length of 80 meters and a height of 25 meters. If you increase the height to 100 meters, so four times larger, you see indeed that these corner streams become much larger. And that in general also the values of wind speed in the standing vortex and in the corner streams will also increase. Let's look again at a wind-flow pattern around an isolated building but now from a different point of view. We take a vertical cross-section, we have again the upstream undisturbed wind-speed profile that is approaching, then we have a stagnation region, if the building is lower, this might be at a lower height than the 70% to 75% that we find with high-rise buildings. So we have the standing vortex indicated here. Then there is also part of the flow that goes upward that then separates, so it detaches from the surface. This is something that we also talked about in week one. And then you might wonder why does flow separation with sharp-edged buildings always occur at these sharp edges? Well let's have a brief intermezzo here and focus on a point, or fluid molecule, an air molecule, that flows along a given path at a point that is at position P and that has a velocity vector v. Then we can also look at the acceleration vector in this point, at this position. Which is of course at the rate at which the velocity vector changes with time, and this can be decomposed into the tangential acceleration and a normal acceleration. However, if you're looking at a sharp edge like this, and you have the air molecule flowing in this way, well actually what occurs at point P is that you have a normal acceleration that is infinite, which is not possible. Which means the flow has to separate, and has to detach and does not follow this 90 degree angle. So this is what happens here: flow separation, then a complex recirculation forms on the roof, which is actually very unsteady. And I will show that to you later on in this module. Then reattachment can occur, if the building depth is sufficiently long. Then there is the so-called free shear layer, where vary large flow gradients are present and small vortical structures. And then in the wake there are different large-scale vortical structures interacting with each other in the so-called recirculation, and this wake actually extends to quite a long distance downstream, so this is the near wake, and then the extension of this free shear layer is then called the far wake. This is yet another graph, indication, of the same flow features. But it's a bit more illustrative because it indicates the swirling motion in the standing vortex. So we see here this horseshoe vortex indeed swirling, and wrapping and rolling around the edges, you see the separation lines indicated in this graph quite nicely. You see also reattachment lines on the roofs and on the sides, which indeed occur again if the building depth is sufficiently large. Then you have this lateral edge and elevated vortex pair indicated here. And the cavity zone which is the near wake. Then you also have the reattachment and also the stagnation region there, that we discussed earlier. And then further on, further downstream you have the turbulent wake with again substantially reduced mean wind speed values compared to the approach flow. What I would like to show you with this small movie is a Large Eddy Simulation, so this is a vertical cross-section for a very simple cubic building. And what you see here are contours of the mean wind speed, where the high wind speed values are indicated in orange and yellow and the low ones in blue to green. And what you see here at the inlet, which is the left side, since the flow is going from left to right, is the approach-flow turbulence. So this is a turbulent atmospheric boundary layer that indeed has quite substantial turbulence intensity embedded in it. But you also see downstream of the building that the building itself generates turbulence and this is so-called building-generated turbulence. What you also see nicely in this graph is that the separation bubble, which is the blue area on top of the roof, that this actually forms, that it grows, that it shrinks and at some point, that it also collapses. So the separation bubble is a very dynamic, very transient area in the flow field, just as the wake, which you can also see in this movie. So, in this module we have learned about wind flow around an isolated building. About different components of the wind-flow pattern and the unsteady character of this wind-flow pattern. In the next module, we are going to focus on wind flow around building groups and on the so-called Venturi effect around buildings. Thank you for watching and we hope to see you again in the next module.
