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Welcome back to Sports & Building Aerodynamics, in the week on wind-tunnel testing. In this module we're going to focus on measurements and flow visualization. And we start again with a module question. Which of these measurement techniques is or are most closely related to Computational Fluid Dynamics in terms of their visual character? And several right answers are possible. Is it A, hot-wire and hot-film anemometry; B, the Irwin probe; C, laser-Doppler anemometry; D, the sand-erosion technique or E, particle image velocimetry? Again 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 some common measurement and flow visualization techniques and the main advantages and disadvantages of these techniques. The scope here will be that we only focus on a limited number of measurement and flow visualization techniques, mainly focused on so-called environmental wind engineering rather than structural wind engineering. And these are, as also listed in the module question, hot-wire and hot-film anemometry, Irwin probe, LDA, sand erosion, and PIV. Let's start with hot-wire anemometry. It actually consists or focuses on a very fine wire, 1 to 10 micron in diameter with a length of 0.5 to 3 millimeters and with a high temperature coefficient of resistance. And this wire is then electrically heated and the flow over this wire or along this wire exerts a cooling effect on it. And there are three types, depending on which type of property you want to keep constant. So there's constant current anemometry, constant voltage anemometry, and constant temperature anemometry. Hot-film anemometry then, has actually a similar principle to hot-wire anemometry. But here we don't have a wire. Actually we use a 1 to 5 micron thick conducting film on some kind of substrate. There are some particular advantages and disadvantages associated with these techniques. The advantages definitely are the very high frequency response, larger than 10 kilohertz and the high spatial resolution that you get due to the small dimensions of the wire or the film. Disadvantages, however, are that it is an intrusive technique because you put the equipment inside the wind tunnel, you actually disturb the flow to some extent. It has a natural directional insensitivity, this means that whether the air is flowing from this direction or from this direction, it will have an equal cooling effect on the wire if the wind speed is equally high and the turbulence intensity is equally high. So we cannot measure wind direction. So it's not suitable also for high turbulence levels that include flow reversal, because then errors occur in the measurement of both mean wind speed and turbulence intensity. Another device is the Irwin probe. And this is a photograph of such a probe. It was originally developed by Peter Irwin. And specifically for wind-tunnel studies of pedestrian-level wind and later it was called by others the Irwin probe or the Irwin sensor. And here you have a vertical cross-section of this probe so it actually consists of a hole of a given diameter in the model street surface. And in in the center of that hole you have a tube that is actually protruding out with a slightly less external diameter. And what you actually measure with this probe is the pressure difference, and from that, based on earlier calibration steps, you can derive then an estimate of the wind speed at this pedestrian-level height. The probe has some specific advantages. It is low cost, it allows fast measurements at a large number of locations and certainly important, is much less fragile than hot-wire or hot-film anemometry. There are some disadvantages; like hot-wire and hot-film anemometry it's an intrusive technique. It has also the natural directional insensitivity. It's therefore also not suitable for high turbulence levels. And if you derive a wind speed from the measured pressure difference, well that is based on the assumption that the logarithmic law applies at this height in the boundary layer. Another technique is laser-Doppler anemometry, and this technique actually uses the Doppler shift in a laser beam to measure flow velocity. So what is established is two crossing beams of collimated monochromatic and coherent laser light. They generate a set of fringes, as indicated here. And then when the particles in the flow, because these type of measurements require seeding. The particles in the flow that pass through these fringes, they will scatter light and the oscillation of that light has a frequency that is related to the velocity of these particles. And this way, this velocity can be measured. Some specific advantages are that it's non-intrusive apart actually from the seeding because that you have to insert into the flow. It has a high spatial resolution. It has directional sensitivity, so it can measure high-turbulence intensity flow and flow reversal. On the other hand it has a relatively high cost. You need to seed the flow. You need to carefully align the beams. And sometimes temporal resolution is limited to some extent by the inertia of the seeding particles. Then there's the sand-erosion technique. In this technique, the wind-tunnel turntable is first covered with a thin layer of sand. That's the first calibration stage, at this stage there are no models yet in the wind tunnel. Then the wind speed is increased in steps until at a certain wind speed, the sand will erode, will be blown away. This is then the so-called free-field erosion speed. Then in a second stage, the building model is placed and here you see a top view of a building model with the flow coming from the left, and then also the wind-tunnel speed is increased in steps. And then gradually also the sand is blown away, of course, first at the positions where the wind speed is amplified the most by the building and later on also by the others. And the wind tunnel is allowed to operate until a certain so-called steady-state scouring stage, or an erosion stage has set in. And then photographs are taken, which afterwards are converted into an estimate of the wind speed, the mean wind speed. Also, here some specific advantages and disadvantages. It is low cost. It allows a fast indication of the high-wind speed areas. It is a so-called whole-area technique, because it gives you information over the entire surface where pedestrian-level winds occur. And it has a strong visual character, which certainly helps in communicating results to building designers, urban planners and architects. Disadvantages are however that also this is an intrusive technique. It also has this natural directional insensitivity because if the flow is coming from either left or right, it will erode sand grains at the same threshold velocity. And it is rather difficult to obtain reliable quantitative information. Although recently some very successful attempts have been made here. Finally, there's particle image velocimetry. Here we measure the movement of seeding particles between two laser light pulses. So we illuminate the flow field. We illuminate actually the seeding that we bring into the flow. These are the pulses, then the images are cross-correlated and a velocity vector map is actually obtained for the different interrogation areas. This is an example of a PIV measurement in a vertical center plane through a cubic building with the flow going from left to right. And you see that the type of image is actually quite similar to what is obtained from CFD simulations. This is another illustration. Here you see the flow, the turbulent flow over a street canyon, and how this flow actually interacts with the flow inside the canyon. So this is not a CFD result, these are results from time-resolved particle image velocimetry. Also this technique has advantages and disadvantages. It's non-intrusive again, apart from the seeding. Has a high spatial resolution, it has directional sensitivity, so it can be useful for high-turbulence intensity flow measurements. And it is also a whole-area technique. On the other hand, it has a relatively high cost, certainly the time-resolved PIV. You need to seed the flow. You need to carefully align laser and camera. And often or sometimes the temporal resolution can also suffer from the inertia of the seeding particles. And then there is smoke visualization which is not really a measurement technique but a more visualization technique. And well, this is illustrated here for the airflow around an airfoil. So back to the module question, which of these measurement techniques are most closely related to CFD in terms of visual character? And that's definitely the sand-erosion technique and the particle image velocimetry technique because they, to some extent, provide whole-flow field data. In this module, we have learned about some common measurement and flow visualization techniques. And about the main advantages and disadvantages of these techniques. In the next module, we're going to focus on the importance of similarity in wind-tunnel testing, on the most relevant dimensionless numbers and their meaning, on some difficult issues in matching dimensionless numbers in a wind tunnel, and on very basic items of flow quality. Thank you for watching. And we hope to see you again next time. [BLANK_AUDIO]
