EXPLORE THE POSSIBILITIES
Windy buildings and melting cars
Using simulation to stop skyscrapers affecting the local environment
By Luke Morris
There’s a huge amount of work that goes into the design and building of a skyscraper.
Safety is paramount, naturally. Architects must ensure that they can withstand wind, rain, and any other elements thrown at them. Heating and cooling systems must be incorporated into the design, along with staircases and elevators offering easy access and emergency exits.
But have you ever thought about external safety and comfort factors?
At some point, you’ve probably experienced the ‘wind tunnel’ effect in the vicinity of tall buildings. This has become more of a problem as cities such as London have seen a rapid growth of skyscrapers in the 21st century. When the 20 Fenchurch Street Building, better known as the Walkie Talkie, was built, there were complaints about the strong gusts at the base of the tower. One person even said they nearly got blown over as they were walking past. And the tallest building in Leeds, 32-storey Bridgwater Place, was responsible for a death in 2011 when strong winds toppled a lorry parked nearby.
So, what is it that causes this phenomenon and what can be done to prevent it?
Downdraught, gusts, and channelling
High winds are created around tall buildings by what’s known as the ‘downdraught effect’. This occurs when wind hits a building and, with nowhere else to go, is pushed up, down, and around the sides of the structure. The air forced downwards then increases wind speed at street level. Completely square cornered buildings create a further acceleration of wind around the sides that can be dangerous for pedestrians. And several towers standing together cause the air to be squeezed through a narrow space in an effect known as ‘channelling’ - this is a form of the Venturi effect, named after 18th-19th Century Italian scientist Giovanni Battista Venturi.
The strongest gusts are created when the wall it hits is facing the prevailing wind – in Britain, this is from the southwest. And London is more susceptible to channelling than cities such as New York as much of its layout is based on medieval street patterns. These narrow roads are much more likely to trap the wind than the wider streets of more modern cities. Combine all these factors and you have the potential for the extreme winds capable of blowing vehicles and pedestrians over, leading to injury and even death.
One solution is to build more rounded buildings, such as London’s Gherkin. As there is less flat surface the downdraught effect is reduced and there are no sharp corners for the air to accelerate around. But the customers that commission these skyscrapers don’t want to be forced into major design decisions like this. Aesthetics are crucial to architects winning projects, so they need to find the right balance between looks and safety.
As concerns over the dangers of wind tunnels have grown, the City of London has introduced stricter rules on new skyscrapers. These require developers of towers more than double the height of surround buildings to carry out both wind tunnel testing and computational fluid dynamics (CFD) assessment. If necessary, experienced wind engineers must be consulted to confirm the findings of the tests and provide assurance that there will be no dangerous effects created.
In 2019, Joe Osman, Robin Stanfield, and Alex Turpitt founded ArcAero, a specialist wind engineering consultancy. Having worked together in wind engineering services for over a decade they contributed to projects including the London 2012 Olympic Park, Manhattan Loft Gardens in London, Mercedes Benz Stadium in Atlanta, and London’s Gatwick Airport. With their vast experience they wanted to provide building developers with a solution to all their wind engineering challenges.
ArcAero’s goal is to enable clients to design and deliver architectural structures that are aesthetically pleasing, comfortable and safe. They knew that wind tunnel testing, whilst essential, is very costly and time consuming so from the start they planned to use more CFD testing.
Having worked in the industry for so long, they were clear on what they needed from a CFD solution. They chose Simcenter STAR-CCM+ due to its capabilities for high fidelity, fast turnaround, and powerful workflow automation. As Joe Osman puts it, “Workflow automation is key for us to create a robust and repeatable process with minimal scope for human error.”
To begin the CFD process, the geometry first must be prepared for pre-processing. Due to this coming from clients in a wide range of CAD formats it can take several days or weeks to clean up and repair the data to extract enough detail to model the development and surrounding area. ArcAero uses the built-in surface repair tool in Simcenter STAR-CCM+ to reduce this to a single day or less. It can all be done with just a few clicks, allowing them to focus on the rest of the project whilst the geometry preparation is automated.
And the automation doesn’t stop there. ArcAero has developed a process using Java so that all the remaining simulation setups and pre-processing are done automatically, meaning most models are ready to run with 10 minutes once the CAD data has been prepared. “We spend minimal engineering time on the interface,” Osman explains. “This allows us to focus on results rather than CFD and the process behind it.”
Take Lower Essex Square in Birmingham as an example. A CFD model was built and run quickly using this automated process to predict wind hotspots that were likely to be created. They then verified these finding in the wind tunnel at Imperial College in London, leveraging the CFD simulation to visualise problematic wind patterns and explore design strategies to alleviate the hotspots. Finally, the engineers used their automated process to simulate a number of different design options in succession. These iterations continued until the optimum solution was found and this was then tested in the wind tunnel for final validation.
Power from the Cloud
The only real limit to the speed of the wind engineering process is the available computing power. ArcAero chose to use cloud computing instead of purchasing their own high-performance computer as it gave them the most flexibility whilst maximizing performance and minimizing cost.
“With varying demands in project workloads, cloud computing offers a cost-effective and flexible option in comparison to on-premise hardware,” Osman explains. This was another reason for choosing Simcenter STAR-CCM+ as it can be used on the Gompute cloud platform. Osman estimates that this allows them to get results within 30 minutes compared to two days if they were using a local computer. The Gompute platform enables seamless customization and integration of STAR-CCM+ scripts to tie in with the entire automation process. This gives ArcAero a higher level of control and delivers a robust, repeatable process.
Cloud computing is perfect for a growing business like ArcAero as it avoids large capital expenditure on hardware and allows them to flex up or down as required. “Cloud enables our company to handle large fluctuations in capacity as we scale up and pay for what we use, which perfectly suits our highly variable throughput,” says Robin Stanfield. He goes on to explain how parallelization and flexible licensing make this the ideal solution. “Cloud hardware allows us to get the most of our licensing costs. We can run virtually unlimited simulations simultaneously. This reduces the turnaround time and allows us to do more thorough exploration of the design space and help our customers to a better solution.”
Protecting against all elements
Wind isn’t the only problem skyscraper developers face.
Did you hear the one about the building that melted cars?
Yep, it was the Walkie Talkie again.
On August 29th, 2013, when the building was nearing completion, Martin Lindsay parked his Jaguar on Eastcheap in the City of London. Two hours later he came back and found part of the car including the wing mirror and badge had melted. On the windscreen was a note from the construction company saying, “your car’s buckled, could you give us a call?” The developers accepted responsibility and paid the repair costs of just under £1000.
But how did this happen?
Dr Svetlana Shtilkind, Dr Andrey Ivanov, and Maxim Popov from Siemens Digital Industries Software used Simcenter FLOEFD to investigate the phenomenon.
They began by creating a full-scale CAD model of the building and surrounding landscape, taking area topology data from Google Maps. The solar radiation parameters (location and time) were specified using Simcenter FLOEFD and automatically included in the simulation.
They estimated the ray-exposed area dynamics as a result of the sun’s position on August 29th and determined that the most heated area at the time of the incident was the section of Eastcheap where the car was parked. The sunlight focus area was then placed in the CAD model and a more exact car position was defined by the focus trajectory analysis.
By defining the relevant areas of the building and the car they were able to simulate the melting that occurred. This showed that the solar influence on the car’s bonnet (hood) and wing mirror peaked at 12:40 at around 1,300 W/m2. This is around 1.5 times higher than the average solar radiation flux value relative to the location, day, and time. The solar influence on the wing mirror lasted just ten minutes, but due to the extreme temperature and the hollow construction with a thin plastic outer shell this was enough for it to melt.
The investigation found that it was in some ways fortunate that the damage was relatively minimal. It compared the results of the reflection from the parabolic glass of the Walkie Talkie with more reflective material. In the case of total or mirror reflection, the Simcenter FLOEFD simulation reported a maximum flux value of 6,000 W/m2. An extreme scenario that would undoubtedly have much more serious consequences.
The future of skyscrapers
Clearly, skyscrapers aren’t going anywhere. Except up.
Inherent in human nature is the desire to build bigger and better. To push the limits of architectural imagination.
But as larger and more elaborate structures are built, more phenomena caused by the local environment will be uncovered.
With the costs involved in designing and constructing skyscrapers, developers can’t afford the prospect of being forced into an inordinately expensive redesign or even the complete removal of a building.
Simulation is an essential component of the skyscrapers of the future. The technology available now enables engineers to predict problems early in the design process and work with architects to find the ideal solution. Combined with wind tunnel testing to validate simulation results, we can all be assured that as well as looking impressive, modern skyscrapers will pose no danger to people and property in their vicinity.
Simulation is an essential component of the skyscrapers of the future. The technology available now enables engineers to predict problems early in the design process and work with architects to find the ideal solution.