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Assignment: Photographic Essay 5 • Photographic essay on fluid motion and/or particle phenomena in a living and working world - Assume you are a journalist writing an popular science article for an engineering magazine - Write on an example of fluid/particle mechanics research ✴ Practical (industrial/real/environmental) systems ✴ Living (biological) systems - Topic must describe flow and/or particle physics and must be one of current active research, i.e., it must be ✴ Novel (new science), or, ✴ Provide new solutions to existing problems - Non-specialist audience ✴ Can assume audience have some general familiarity with technical terminology (e.g., they know what surface tension is) ✴ But not necessarily have a specific background in fluid or particle mechanics ✴ Thus, any technical jargon needs to be explained (e.g., need to explain what a boundary layer is) - Photographic piece - use many colourful and visually stimulating images (credit given to creativity, originality and aesthetics) - Proper citation (i.e., attribution) for figures and text - No more than 5 A4 pages and 1000 words (use photographs, animations and visuals to help get your point across) . 20% novelty/interesting idea, 40% image quality, 20% writing, 10% proper citation, 10% grammar and word count. HOW HIGH CAN WE BUILD? Descending through the clouds, I see nothing but desert. The beating sun reflects off the sand to pierce my naked eye. The plane banks right and begins its holding pattern, waiting for its turn to land. As we circle around, I see a massive spire; piercing the sky with tapered edges from its base to its peak. The reflective windows shine like a jewel in the otherwise flat and empty land. It’s the Burj Khalifa. A testament to the will of man and his technological prowess. It may be hard to imagine now but back in 18851, the world's first building to be called a skyscraper was only a meagre 10 stories high. It was considered gargantuan in size back then but is now barely even worth a second look. However, it was this building (The Home Insurance Building in Chicago)1 that is widely known as the father of the skyscraper. Fast forward 120 years of technological development and inno- vation and we get the world we live in today. The most devel- oped cities flaunt their wealth and power by building higher and higher into the sky. However, as we reach for the heavens, new problems begin to emerge. At these great heights, the design of a building needs to be aer- odynamically robust as to avoid certain situations that can cause the building to come crashing down. One such phenome- non that can cause these situations is called vortex shedding. On a windy day, lamp posts and many other similar structures suffer from vortex induced vibrations (VIV); where the structure will oscillate back and forth in the wind, potentially causing it to fail. This phenomenon is also responsible for overhead power lines ‘singing in the wind’2. When vortex shedding occurs, vortices are created in an oscillating pattern on either side of the object (Background 1). These vortices cause the structure to oscillate slightly from side to side due to the change in pressure. If the frequency of these oscillations matches up with the structure’s resonant frequency*, the amplitude of the oscillation can be increased dramatically, sometimes continuing to increase until the inevitable fail of the structure2. Background 1—Vortex Shedding caused by a mountain There have been many studies4, 5, 6 over the years to try and mitigate the effects of vortex shedding and many solutions. Perhaps the most interesting ones are the use of helical strakes7 on chimney stacks and using tuned mass dampeners8. Chimney stacks are quite notorious for suffering from vortex shedding. However, using a helical strake, the effect can be mitigated7. The corkscrew like structure is mounted around the exterior of the chimney and introduces turbulence (Figure 1). This turbulence never allows the frequency of oscillation of the vortex shedding to come close to the resonant frequency of the structure. It also has the bonus of forcing wind up the sides of the stack and blowing the exhaust higher into the sky. This idea of disrupting the flow around the structure has led to the design of many other fins and attachments that perform similarly. Helical strakes are also used in a variety of underwater applications10. However, they require differ- ent dimensions than those used in wind as to be more effective in producing turbulence. In under- water applications, fairings also provide a much more stable option10. Fairings have a fin that can rotate in order to orient itself with the flow direction and disperse the vortices created by vortex shedding. A mass damper is essentially a pendulum that hangs in the centre of a building and whenever the building moves, the mass damper counteracts this force by oscillating in the opposite direction8 (Figure 2). This is extremely effective at targeting certain frequencies of oscillation such as those close to the resonant frequency of the building. However other frequencies can be amplified9. Therefore, the mass damper needs to be tuned to a certain frequency, like a guitar string, so that it is not detrimental to the safety of the structure. The largest mass damper in operation today belongs to the Taipei 10111 (Figure 3). A building in China’s Xinyi district. This structure was classed as the world’s tallest building from 2004 until 2010 and as such, needed a massive tuned mass damper, 728 ton11 to be exact. To put that into perspective a typical car weighs around 1.5 ton. This massive damper even has its own mas- cot, the damper baby. In 2010 the Taipei 101 lost its crown as the world’s tallest building to the Burj Khalifa. A building that does not have tuned mass damper. So how did such a building surpass the Taipei 101 by 322m13? Figure 1—Effect of Helical Strakes vs solid cylinder on air flow. Figure 3—Taipei 101’s mass damper and its mascot, Damper Baby. Background 2—Taipei 101 and the surrounding city Figure 2—How a mass damper works. The Burj Khalifa takes the elements of the helical strake and applies them on a massive scale never seen before. The main concept used is: if the shape of the structure varies with height, there isn’t a constant oscil- lating frequency that affects the tower. It creates its own turbulence to disrupt the formation of vortices and thus prevent any VIV’s. The tower only sways around 1.5m at its tallest point12. In comparison, the tallest building in America sways the same amount and is 400m shorter17. Such a design had never been used anywhere in the world and the results speak for themselves, the second tallest tower in the world is dwarfed by around 200m and no other project, bar one, will come close for some time. The only planned project that will potentially pass the Burj Khalifa, is the Jeddah Tower. It’s planned to reach a height of 1000m and uses a similar stepping pattern to reduce the diameter of the tower as it ap- proaches the top14. It appears this is the only design for the time being that will allow a building to reach such great heights. But perhaps dampening VIV’s is not the entire story here. There have been various studies that confirm that the energy produced by vortex induced vibrations can be harnessed to provide a new method of supplying clean energy to the world15, 16. Rather than an active damper system, like a mass damper, the Burj Khalifa opts to use a much simpler approach. The Burj Khalifa has a 3-pronged base with each prong stepping back towards the centre of the tower at certain heights. The design comes from Is- lamic architecture and has a total of 27 steps around the exteri- or12. The shape looks very familiar; almost like a corkscrew (Figure 4). Figure 4—Projection of the Burj Khalifa design. Background 3—Burj Khalifa and the surrounding city. References (Superscripts refer here) 1 Guinness World Records. (2019). First skyscraper. [online] Available at: http://www.guinnessworldrecords.com/world-records/first-skyscraper/ 2 Olenick, R., Apostol, T., Goodstein, D., Frautschi, S. and French, A. (1987). The Mechanical Universe: Mechanics and Heat (Advanced Edition). 3 Hyperphysics.phy-astr.gsu.edu. (2019). Resonance. [online] Available at: http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/reson.html 4 Strykowski, P. and Sreenivasan, K. (1990). On the formation and suppression of vortex ‘shedding’ at low Reynolds numbers. Journal of Fluid Mechanics 5 Piccirillo, P. and Van Atta, C. (1993). An experimental study of vortex shedding behind linearly tapered cylinders at low Reynolds number. Journal of Fluid Mechanics 6 Huang, X. (1995). Suppression of Vortex Shedding from a Circular Cylinder by Internal Acoustic Excitation. Journal of Fluids and Structures 7 Zhou, T., Razali, S., Hao, Z. and Cheng, L. (2011). On the study of vortex-induced vibration of a cylinder with helical strakes. Journal of Fluids and Structures 8 Strommen, E. and Hjorth-Hansen, E. (2001). On the use of tuned mass dampers to suppress vortex shedding induced vibrations. Wind and Structures 9 Huang, K. and Chou, T. (1995). Use of active mass dampers for wind and seismic control on super-high-rise buildings. The Structural Design of Tall Buildings 10 Rigzone.com. (2019). How Do Vortex-Induced Vibration Suppression Devices Work?. [online] Available at: https://www.rigzone.com/training/insight.asp? insight_id=359&c_id= 11 Amusing Planet. (2019). The 728-Ton Tuned Mass Damper of Taipei 101. [online] Available at: https://www.amusingplanet.com/2014/08/the-728-ton-tuned-mass-damper- of-taipei.html 12 En.wikipedia.org. (2019). Burj Khalifa. [online] Available at: https://en.wikipedia.org/wiki/Burj_Khalifa 13 Burjkhalifa.ae. (2019). Home | Burj Khalifa. [online] Available at: https://www.burjkhalifa.ae/en/ 14 En.wikipedia.org. (2019). Jeddah Tower. [online] Available at: https://en.wikipedia.org/wiki/Jeddah_Tower 15 Vortex Induced Vibration for Aquatic Clean Energy. (2011). Applied Mechanics and Materials 16 Narendran, K., Murali, K. and Sundar, V. (2016). Investigations into efficiency of vortex induced vibration hydro-kinetic energy device. Energy 17 Elderkin, B. and Liszewski, A. (2019)