Like a hurricane

A novel flow-reversing valve, designed in the UK, could play a crucial role in the design of hurricane-resistant buildings. The valve, created by engineering consultancy Cambridge Consultants, can switch the direction of air flow seven times per second. It will be used in a special test house – at the University of Western Ontario in Canada – to apply fluctuating pressure to a series of pressurised hollow ‘tiles’ mounted around the outside of the house. By mimicking the conditions of a hurricane around the house, the Ontario team hopes to understand which parts of the building are most in need of reinforcement. “They want to measure the effects of these pressure variations on buildings,” says Gary Kemp, who led the team at Cambridge Consultants. “They want to see where there is scope to improve the strength of buildings – and where they might relax specifications, so that the overall cost is similar.” This method has been adopted because it would be impractical to build a wind tunnel for a full-size house. “Rather than have a massive wind tunnel – which would need to be 10 times bigger than the house – they reproduce the measured pressure from a scale model,” says Kemp. “Because it’s turbulent flow, that pressure keeps changing.” The scale model is tested in a small wind tunnel. The model includes a series of air holes, allowing pressure to be measured at various points of the house. This allows a pressure profile to be created – which is then mimicked on the full-size test house. The pressure profile at a flat surface will remain fairly constant, while the profile ‘under the eaves’ is likely to fluctuate. This is where the valve’s ability to switch quickly comes into play. He says that many flow-reversing valves have been developed in the past, but none have the ability to switch as quickly as this one – and with such repeatability. “You can run the same experiment many times and know that you have given it the same input,” says Kemp. “The key to achieving the performance is the flow-reversing valve.” Large, flat surfaces such as walls are covered by large tiles (up to 8 feet square), while sharper corners – which are likely to see more pressure variation – will be covered with tiles as small as 1 foot square. All the tiles are 1 foot deep, and sit in the space between the house and a surrounding structural ‘cage’. Every tile is pressurised by air from an air actuator – which consists of a fan and the valve – to deliver pressures between +5 and -20 kPa. “The fan runs at constant speed, but the valve determines how much air gets into the tile,” says Kemp. “It can switch from full negative to full positive up to seven times per second.” The control system is clever enough to measure the pressure in each tile, compare this with the desired pressure, then make an adjustment in real time. It can also compensate for changes in conditions – such as if the wall breaks during the test and extra pressure is needed. In the real world, a broken wall would not lead to a reduction in the hurricane’s power – so the test house needs to reflect this. At its simplest, pressure is controlled by the position of a bowtie-shaped disk – which separates the ‘positive’ and ‘negative’ chambers. It was important to create a linear relationship between the position of the disk and the pressure it created. “Getting this right was a key part of the design,” says Kemp. “We used a lot of fluid flow modelling using CFD [Computational Fluid Dynamics].” One element of this was to taper part of the opening between the chambers – so that when it opened for the first time, there was a smooth build-up in pressure, and not a sudden ‘rush’. CFD was critical for getting the flow behaviour in the valve chamber absolutely precise – which ensured that the linear relation between valve position and pressure was as accurate as possible. Wind control Cambridge Consultants also developed the control system. In all, 140 tiles are attached to the outside of the house – each served by an air actuator. All 140 actuators are controlled by a single Linux-enabled server – but only just. Geoffrey Robson, who developed the control system, says that the server’s ability to handle the information is right at the edge of its capability. Information is transferred across a gigabit Ethernet link. Ethernet is readily available, robust and reliable; on the downside, it is relatively slow. “You often have microsecond delays on Ethernet,” says Robson. “Because the system works very rapidly, this can introduce complications.” The software overcomes this time lag – or latency – by predicting the position of the disk that controls the pressure. It can determine an approximate position in advance, so the disk moves most of the way before a ‘fine tuning’ delivers the exact pressure required. “When we started, we did not think we could do this over Ethernet,” says Robson. “This is fast control over a slow network.” Blowing in the wind The ‘Three Little Pigs’ facility – as the test house has been dubbed – grew out of a need to understand how hurricanes affect low rise buildings. “There’s been a lot of work into devising high-rise structures that survive hurricanes, but not so much for low-rise buildings,” says Gary Kemp. Scientists at the University of Western Ontario, led by Michael Bartlett, expect to bring the house “to the point of collapse” over the next two years. The results should lead to ways of building more resistant low-rise houses. The Institute for Catastrophic Loss Reduction (ICLR) in Canada estimates that the likely cost of reinforcing a house – based on the eventual results from this study – might add 1% to its cost. The university has already commissioned 10 actuators – which will be delivered this month – to test building panels. Pointers A flow-reversing valve that can switch its airflow direction seven times per second delivers fluctuating pressure to a set of ‘pressure boxes’ – mimicking hurricane conditions around a low-rise building Around 130 air actuators can be controlled by a single Linux-enabled server. Clever software overcomes the relatively slow Ethernet communications link