Welcome back to Sports & Building Aerodynamics in the week on building aerodynamics. In this module we're going to continue our focus on wind flow around buildings, and we start again with the module question. This is a top view of two high-rise buildings; left in a so-called converging and right in a so-called diverging arrangement. The wind direction is as indicated. Which configuration yields the highest wind speed in the passage? Is it A) The converging arrangement, on the left side. B) The diverging arrangement on the right side. Or C) You'll get the same wind speed in A and B. Please hang on to your answer and we'll come back to this later in this module. At the end of this module you will understand wind flow around building groups. And you will understand the so-called Venturi effect around buildings. Let's first look at three typical problem configurations. The first one is, well you could say the most problematic one, the one that will give you the highest wind-speed amplification that you can achieve in a built environment, that is a passage through a building. And you see here a front view, and also a top view. So what happens when the wind direction is perpendicular to this wide facade, is that you get an overpressure area in front of the building, an underpressure area behind it. And what you do actually by creating this through-passage is creating a very efficient shortcut, or pressure shortcut, which will give rise to a very strongly amplified wind speed in this passage. Actually this overpressure-underpressure area is also partly what drives the corner streams. So you can reason in terms of building aerodynamics from the perspective of pressures or the perspective of velocities and arrive at the same flow fields. So this is actually indeed corner streams being explained from the pressure point of view, but what we will focus on here is the passage through a building which will give a very strong wind speed. And this is an example of sand-erosion tests for a building, which is quite a long building, it's a scale model of course, but the full-scale dimensions are 160 meters in width, 25 meters in height and there is a passage with a width of 10 meters. And then you can see indeed the strongly amplified wind speed in this passage. And on the right side you see a wider passage, 20 meters and also again strongly amplified wind here. So, pressure short-circuiting between windward and leeward facades. Then we have a second problem configuration, which is a passage between parallel buildings that is indicated here; parallel buildings standing side-by-side. Again an overpressure area, underpressure area, causing pressure short-circuiting and of course, this is a configuration that you cannot avoid. Every city is full of buildings that are standing next to each other, so these are also areas where wind speed is substantially amplified. Not far as much as in the previous case, the passage through a building, but still these are areas where you don't want to put entrances, for example, opening windows, doors, playgrounds for children, because this is a place where you will you might lose all of these components. So that's another problem area. Here again, sand-erosion contours around a building, two buildings actually, each with a length of 80 meters. And you see on the left-hand side a passage width of 20 meters, and on the right-hand side, a passage width of 10 meters, and in the passage and also downstream of the passage actually, clearly indicated that the passage jet width amplified wind speed. And then there's a third problematic configuration. These are again parallel buildings, and I only show a top view here. But parallel shifted buildings. So again, overpressure area, underpressure area, and then a very broad occasion for pressure short-circuiting. And these are again, sand-erosion contour plots and you can see here, that especially on the right-hand side, that you have very large areas with very strongly amplified wind speeds, large areas with amplification factors up to two. So these are definitely also not areas where you would like to place playgrounds for children or terraces and so on because these will be most likely very windy locations. Let's then turn to the so-called Venturi effect between buildings. Is that a fact or is it fiction? Well I give you this quite provocative question mark because it is quite an important issue, and there's quite some misinterpretations here. If you would like to have more information about that I would like to refer you to these three articles here. But I'll give you a short summary in the next slides. And this actually also boils down to the question, the module question that I asked you before. Because this is what could be called a so-called Venturi configuration, which could yield the Venturi effect, but is this true? Because the Venturi effect refers to an increase of fluid speed due to a decrease of the flow section. As indicated in this very simple drawing on the left-hand side where, of course, you see a drawing that corresponds to the slip condition. Of course, there will be a very narrow boundary layer, because a no-slip condition holds but overall this is an indication indeed that due to a decrease of the cross-section, the fluid speed will increase. However, this is a confined flow. Flow around buildings is an open flow and of course there is no law in physics that says that all the oncoming flow in the atmospheric boundary layer has to go through that narrow passage because the flow could also go around and over the buildings. So the question actually is, is the Venturi effect present in the so-called non-confined flows in urban physics and in wind engineering, because the Venturi effect is a term that is really very often used in architectural engineering, in building engineering and even in wind engineering. So that's what we set out to investigate. Of course, when we want to talk about the Venturi effect, we should know what Giovanni Battista Venturi actually studied, what he actually wrote. So this is on the right-hand side a picture of Venturi and on the left-hand side, you see the front page of his book and I have to thank Sandra Johnson here and her colleagues from the Niels Bohr Library because they were so kind to copy this very fragile book for me. And after reading it indeed, it turned out that Venturi only looked at confined flows in this investigation. Then let's go back to passages between buildings. There are different types, and here you see top views; parallel side-by-side, parallel shifted, and then you could say that the perpendicular type, could be called a Venturi configuration. And indeed there are articles in the scientific literature that state the conditions that you have to satisfy if you want to generate the Venturi effect. Often of course, in pedestrian wind comfort you want to avoid it. But for example, for wind energy between buildings, you might want to use this effect, if at all present. So this article for example, indicates that the height of the building should be larger than 50 meters. The sum of the two lengths of the building should be larger than 100 meters. And you should have a so-called exposed site, meaning not any other high-rise buildings that might obstruct the flow. And then this article states that you get a maximum flow through the passage when the passage width is two or three times the height. So this seems very intriguing and this is also what therefore we set out to investigate. So we chose these building models, buildings at an angle of 90 degrees. The length of each of them is 100 meters. And the width is 10 meters. The height ranges from 30 to 60 meters. So these are conditions needed to get this Venturi effect indeed. And then we investigated a wide range of passage widths, from 10 meters to 180 meters, to be sure that if there is a Venturi effect, we would find it. These configurations were also tested first in a wind tunnel, because indeed, validation of CFD simulations, and that is the tool we used here, is crucial and essential if accuracy and reliability should be given. So this is a view of the experimental setup in the Concordia University atmospheric boundary layer wind tunnel. You see actually the building models are in this case very simple models. But nevertheless, the results were quite surprising. Then CFD simulations were made for these models, and the different configurations with the ANSYS CFD code using the steady RANS approach with the realizable k-epsilon model. Standard wall functions with sand-grain roughness-based modifications, fitted to the roughness that we had in the wind tunnel. We used the appropriate relationship between the sand-grain roughness height and the aerodynamic roughness length. Second-order discretization schemes as they should be. At least second order, that is. And more details can be found in the reference below this list. And let's look at some results. What I'm showing you here are amplification factors in a horizontal plane at a height of about two meters in full scale above ground level. So this is about pedestrian height, maybe for a tall pedestrian. Nevertheless, what you see here is indeed that the wind speed in the passage is amplified more in the diverging arrangement, which is actually opposite to what one would assume. And if you look at different passage widths, we actually always find the same conclusion. In a diverging arrangement, you get the highest wind speed, and not in a converging one. So what is going on here? Well, this is definitely a counter-intuitive result. And what you see here actually, is that in this V-shape, in the converging arrangement you get a large mass of almost stagnant air. There's also a pressure build-up there and the oncoming air actually will not completely go its difficult way through this overpressure area and then through the narrow passage. But it will find yeah, and take the easiest way. And the easiest way in this case is to a large extent, around the buildings and over the buildings and not through the narrow passage. On the right-hand side, you see that this low wind-speed area or overpressure area is much smaller and therefore you get two corner steams in the passage that join together and that give the important strong passage jet. So actually what is happening here is again there is a subsonic upstream disturbance, meaning that the flow actually in subsonic regime, has an upstream disturbance. And this is indeed what you see here. The building configuration translates an overpressure upstream, and this overpressure actually acts to divert the oncoming flow. These were steady RANS simulations, and these are Large Eddy Simulations of the same configuration and a perspective view in this case. And here indeed you see on the left side the low wind-speed areas, so almost stagnant air, very slowly moving, but rather turbulent. And you see that the high wind-speed areas, indicated in yellow, orange, and red are not in the passage but around the buildings. While on the right-hand side, in the diverging arrangement, you see a much smaller low wind-speed area, a much smaller blue area. But you see that the highest wind speed actually here occurs in the passage and not around the buildings. So this is also briefly indicated here. Well, you might argue, the Venturi effect is about fluxes and flow rates through cross-sections or through planes and not about amplification factors in the horizontal plane. And indeed, that's correct. So, let's have a look at how fluxes through planes, vertical planes, are influenced by the presence of the buildings. If you would have the Venturi effect and you would assume that the flux Fp, which is the flux through the passage, would be substantially larger than the flux through an equivalent plane in free-field conditions. However, for all the configurations that we tested, and that was quite a large number, we found the exact opposite. The flux through the passage is always less than the free-field flux. And this due to the fact that the buildings are actually a resistance to the flow and they act as to divert the flow in horizontal and vertical directions away from the passage. And this is a schematic drawing that we made based on the CFD simulations, that actually indeed shows that the flow, when approaching the passage, due to the overpressure area, lifts off, flows over the passage and that, although indeed there is a small downflow close to the facades indicated in light grey here, but overall the reduction in flow rates is certainly substantial compared to the free-field conditions. So let's turn back to the module question then. Well, surprisingly, indeed the answer here is the diverging arrangement gives the highest wind speed in the passage. So conclusions. There is no Venturi effect in the passage between the buildings that we investigated, because it's an open flow. It's not a confined flow and Giovanni Battista Venturi studied confined flows. Wind tunnel measurements and CFD simulations indeed here show the opposite effect being present, so you could say that in a converging arrangement, the flow in the passage chokes to some extent. But this is really a very deeply rooted misunderstanding about wind flow between buildings, and if you want to know how deeply this is rooted, keep watching in this module because we will illustrate some important design flaws based on the misunderstanding of the Venturi effect. In this module, we've learned about wind flow around building groups. And about the so-called Venturi effect around buildings. In the next module, we're going to focus on wind effects on people, but also wind discomfort and wind danger. How buildings can create wind discomfort and wind danger, and how we can assess wind discomfort and wind danger. Thank you for watching, and we hope to see you again in the next module.
