Welcome back to Sports & Building Aerodynamics in the week on building aerodynamics. In this module, we're going to focus on pedestrian-level wind. And we start again with the module question. Which quantity do you think would be best to describe wind danger around buildings? So note here that the capital U is the mean wind speed and sigma U is the RMS value. So the standard deviation of the turbulent fluctuations. Is that A) the mean wind speed, B) the mean wind speed plus one standard deviation, or C) the mean wind speed plus three standard deviations, or yet D) none of the above. 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 the difference between wind effects, wind discomfort, and wind danger. You will understand how high-rise buildings can cause wind effects, wind discomfort, and wind danger. And you will understand how wind comfort and wind danger can be assessed. So if you put high-rise buildings in the atmospheric boundary layer wind-flow pattern, indeed they tend to bring high wind speed down to pedestrial level. And this is, as also mentioned before in the first module, a large mass of air and also at high speed, because wind speed increases with height. And this is indicated schematically by the arrow in orange here. So we get amplified wind speed at pedestrian level. I would like to show you two short movies to indicate what kind of effects this can have on pedestrians. So what you see here is the first movie, which is a movie taken from an office in a certain high-rise building, a high-rise building that is not surrounded by any other high-rise buildings, and you see the effect of the amplified wind on the pedestrian or in this case the cyclist around this building. So, this is indeed a rather windy day, but amplification of wind speed around this building in the corner streams is very pronounced. And, this is a movie that goes on for quite some minutes, and then actually you also see the situation getting worse; more people, more cyclists are actually stepping off their bike. But then, even not being able to walk besides their bike in a regular fashion. I'll move towards the second movie. I also show the reference here. This is another type of pedestrians and they walk around the corner of a building and then they're actually literally blown off their feet by the strong corner stream around this building. So this is probably their first experience with wind discomfort or wind danger, as you could call it. But, indeed you can see the effects, the clear effects of amplified wind speed on pedestrians. Especially, because, when you walk into a corner stream, this also has a surprising effect. And, therefore, the effect will often also be larger than if it would be a steady zero-gradient flow. Is it important? Well it is important. I don't have numbers from many countries, but research by the Flemish Institute for Healthcare, for example, has indicated that falling is a major cause of death for people older than 75 years. And then, often this causes injuries and most often this is hip fracture which can be lethal. Another illustration that it is important is this article from 1975 from Lawson and Penwarden indicating that two old ladies died after being blown over near a high-rise building; they died by skull fracture. It can be less dramatic, but still very serious, for example, when shops at the bottom of high-rise buildings are left untenanted because of the windy environment. So shoppers are just discouraged, they don't want to go there, it's too uncomfortable, too windy, and they cannot rent the shops. So often these uncomfortable wind conditions can be really detrimental to the success of new buildings. And that's why for high-rise buildings, many urban authorities world-wide now require wind comfort studies to be done. Either based on wind tunnel measurements or based on Computational Fluid Dynamics, and only then a building permit can be obtained. So let's have look at wind effects first to compare it against wind discomfort and wind danger. Well this the extended so-called Land Beaufort Scale with indication of wind effects. And you see here for different Beaufort numbers and associated wind speeds at a height of 1.75 meters, different wind effects ranging from no noticeable wind to actually people being blown over. These are wind effects. If we want to move towards wind discomfort and wind danger we have to look at generally both a threshold wind speed and a maximum allowed exceedance probability of this threshold. And this an example of a comfort criterion that has been used in the past. You see first the threshold wind speed. So we use here the effective wind speed, which is the mean wind speed plus one standard deviation. And the threshold wind speed is six meters per second. And the six meters per second can only be exceeded for 10% of the time. If it is exceeded more, it is called wind discomfort. For wind danger, then the surprising effect that gustiness has on the wind, the turbulence, is much more important, and then we use a factor of three for the standard deviation. And we use two thresholds, 15 meters per second for elderly people and 20 meters per second for younger people. And here wind danger of course is much more severe than wind discomfort, so we have an exceedance, maximum allowed exceedance probability of only 1%. What are the causes? Well as we also partially explained before, it's building aerodynamics and the type of building aerodynamics, indeed, that generates high wind speed near ground level. And often this is, expressed in terms of the corner streams and the standing vortex being the most problematic areas around a building. But also pressure short-circuiting. So if you are dealing with wind discomfort or wind danger, it is generally one of those three, either the standing vortex or the corner streams or pressure short-circuiting or a combination of these three phenomena. If you want to assess wind discomfort or wind danger, well then this is the methodology that can be used. Imagine that you have a certain urban environment where you want to put a new high-rise building, you want to assess the wind comfort at this position. And therefore you want to know the wind statistics at this position. However, you have to be extremely lucky to also have a meteorological station in this city. Usually that station is quite far away; 10, 20, maybe 100 km. But there you will have wind speed statistics. And then you need to translate those wind speed statistics, to the building site. And then the comfort criterion can be applied. So we need indeed the wind statistics. That's the first step. Then the aerodynamic information to translate them to the building site, and then a suitable comfort criterion. And I put suitable in bold and in italics here because many comfort criteria are not based on experiments, not based actually on psychological research as it should be. So different comfort criteria sometimes give very different results. So let's briefly look at the different components. Statistical meteorological data should preferably be data covering 30 years or more. It's often potential wind speed and wind direction, which means that it corresponds to a grass-covered surface, uniformly rough. And if it doesn't, it should be corrected to this condition. Because of course, the surroundings influence the wind speed quite a lot. And then the statistics can be described by a Weibull distribution, which is given here for one particular wind direction. And this gives the probability that for a given wind direction a threshold is exceeded by the wind speed. You see the three Weibull parameters here that for every wind direction can be determined based on the meteorological data, and then with this distribution the next stages in the process can be started. Then we need the aerodynamic information. This is often done in terms of a wind speed amplification factor. That is actually the ratio of the wind speed at the building site and that at the meteorological site. Which is generally split up into two parts, a so-called design-related contribution, which is influenced by the local surroundings of your building. Which is something that as a designer theoretically you could influence, and the terrain-related contribution, which is the influence due to the fact that between the meteorological station and let's say the border of your city or urban area or neighborhood you have probably a wide variety of different terrain categories, where you also have a large superposition of internal boundary layers, which is actually very complex. And often actually this terrain-related contribution is estimated in a quite simplified way. The design-related contribution is what you can assess by wind-tunnel testing or by CFD. The terrain-related contribution on the other hand has different methodologies. The simplest one actually is applying the simple logarithmic law for the mean wind speed at the meteorological site, and at the location of the city, and then relate those two equations at the given height of 10 meters. And then for the ratio of the friction velocities, there is this expression that was provided by Simiu and Scanlan. Then we need a suitable comfort criterion. Well, many criteria exist, they generally don't have a strong experimental basis and that also leads to the fact that if you apply different criteria for the same case one criterion might indicate that the situation is perfectly comfortable, while the other might indicate that's it's definitely not comfortable. So this is a problem. There has been in the past a very good and extensive comparison work in 1993 by Bottema, who actually came up with these criteria. So for wind comfort, the threshold wind speed of six meters per second and the maximum allowed exceedance probability of 10%. And for wind danger the different values as also shown before on one of the slides. I have to mention here that there is now, since 2006, the Dutch standard for wind comfort and wind danger in the built environment and because I have not been involved, I can mention that this is an excellent piece of work. It's also the first standard on this topic in the world. And there were specific reasons actually to make this standard. Actually, I have to mention also that this is based on extensive research work, both concerning comfort criteria, but also concerning terrain-related contributions. And the cause of this standard was actually fourfold. First, to have a uniform procedure for assessment of wind comfort and wind danger in the whole of the Netherlands because different laboratories used different comfort criteria, different procedures for terrain-related contributions. And then it would depend very much on which laboratory you would go to what the outcome would be. Uniform criteria for wind comfort and wind danger have been set. The uniform transformation of wind statistics, and also quality assurance, was put in this standard. And what is very important and unique in this standard, is that the standard explicitly allows you to use CFD instead of wind-tunnel testing. And this is also an indication of the increased potential of CFD and also the increased accuracy and reliability of course, if best practice guidelines are satisfied. This is the comfort criterion in the Dutch standard. I'll briefly show you a few components. What you see here is that the threshold wind speed is five meters per second. That is for the mean wind speed. We don't consider turbulence here. Then here are the exceedance probability ranges. And depending on this range and depending on the type of activity, whether it's traversing or strolling or sitting, you see that you get different judgments or qualifications of the wind comfort ranging from good to moderate to poor. And an important step also here is that the degree 'Moderate' has been added, because if you put a separation between good and poor at one specific value, then of course if you are close to that value round-off errors might determine the actual outcome, which of course is unwanted. And then this is for wind danger. There we use 15 meters per second as a threshold. Here you see two ranges of exceedence probabilities. And again the outcome depends on the type of activity. So let's go back to the module question now. Which quantity would be best to describe wind danger around buildings? Well, wind danger around buildings is actually associated often with the surprising effect and of the gustiness of wind. And then indeed it's definitely the third one, where we take into account the gustiness, the standard deviation with a factor three. In this module, we've learned about the difference between wind effects, wind discomfort, and wind danger. We've seen how high-rise buildings can cause wind effects, discomfort and danger. And we've also had a look at a procedure to assess wind discomfort and wind danger. In the next module, we're going to focus on how assessment of wind comfort and wind danger is performed based on CFD in a complex case study. And we'll also have a look at what type of remedial measures can be used in building projects if you need to improve the pedestrian-level wind comfort and wind safety. So thank you again for watching, and we hope to see you again in the next module.
