If Governor Arthur Phillip had tilted George Street and other Sydney streets 20 degrees counter-clockwise – to protect them from the wind – it would have reduced the prevailing winds that can turn these streets into wind canyons.
“Approximately 20 degrees counter-clockwise from current orientation would have been better to minimize channeling of prevailing winds. So George Street has geared up to run more SSE-NNW,” said Dr. Matt Glanville, the founder of CPP Wind in Australia.
A World Masters cycling champion with a PhD in wind engineering, Glanville has been taming the winds of Sydney and beyond for nearly 30 years. CPP’s lab in St Peters contains a scale model of the Sydney CBD that includes buildings that his company has tested in its wind tunnel.
His company also advised the City of Sydney on changes to wind standards introduced last December.
Glanville got a taste of the wind as a BMX rider. When he saw the movie breaking away He was enthusiastic and started street racing.
“It gives me a guilty pleasure to head into the national park away from traffic in strong synoptic tailwinds and attack Strava [a cycling app] Records on the open road,” he said.
Developing a sixth sense for wind and aerodynamics was a definite advantage in cycling; especially when it comes to “slipstreaming,” riding close behind another cyclist, he said.
“To further intensify our challenge, the faster we go, the less friendly the power-to-speed ratio becomes,” he wrote Cycling Australia.
The wind tunnel effect and downwind effect result from many tall buildings in close proximity to each other. This creates low-pressure regions that cause the wind to move faster and more erratically near the ground, causing the same problems that slow down cyclists.
To mitigate the effects of wind on residents and office workers, the City of Sydney introduced wind standards for towers with more than 15 commercial floors in central Sydney last December. These are designed to ensure that public areas are safe and someone can sit or stand comfortably in an outdoor cafe or pub, or go to a meeting.
As our cities become denser and more skyscrapers are built, the science of wind engineering is now being used to test its impact on almost everything.
In the company’s wind tunnel, CPP also tested the viability of plants on eco-friendly buildings such as One Central Park on Broadway, the effects of wind on proposed public art such as the Cloud Arch, which was potted by the council, and cladding and windows on high-rise buildings across Sydney.
Because the $2.4 billion Central Park Sydney Precinct’s reputation rests on its green credentials, various plants have been exposed to 45 mph winds from CPP.
The idea was to find “the triffids,” the plants that would survive intense heat and winds in the most exposed parts of the building, said Sacha Coles, founder of Aspect Studios. It worked with project partners Oculus and Junglefy on the landscaping and design of Central Park.
Some plants were crushed. Coles said only the “toughest, roughest boots and all plants” – Parthenocissus tricuspidata (Boston ivy) and Pandorea jasminoides (beauty bower) – survived.
Mick Caddey, project manager at Frasers Property Australia, which developed One Central Park, said wind is an important and complex part of the project.
“Wind is a really fine science, but sometimes it’s really difficult to work around and solve. Some of it is counterintuitive: Wind doesn’t always hit the building and go around, it goes up and down,” he said.
“When you walk through the city, you come across these uncomfortably windy spots. Ten minutes later you are in a quiet leeward square. It’s really mysterious.”
Caddey said the biggest problem isn’t the windward side of the building. It was the suction effect, where the wind goes around a building “like water moving around a rock to create a whirlpool”.
He said the wind created a massive area of low pressure on the sides and backs of buildings, causing suction on the facade.
To protect the residents and the gardens, One Central Park used awnings, trees and glass panels up to 1.8 meters high on balconies.
Greenfield developments are now planned to protect pedestrians from the wind and avoid creating wind tunnels, Glanville said.
But less is possible on old roads like George, Kent and Sussex, which date back to the early days of colonial settlement. “When you buy a property on Sussex Street it gets windy and there’s less we can do and we focus on using local treatments like awnings,” he said.
The western edge of Sydney’s CBD near Barangaroo and the Rocks is one of the windiest spots in the city. Gas Lane at Millers Point, for example, captures winds from the south, and a report by Glanville’s company for the council found that wind flow accelerated down the lane from Observatory Hill.
As a cyclist, Glanville can feel and see the wind speed. An average wind speed of four to five meters per second (about 15 km/h) makes someone’s hair fly up or a small flag stretched out.
“At six to eight meters per second [about 30km/h], your hairdo is ruined. It’s too windy for stationary activities like al fresco dining and window shopping,” he said.
When the wind roars at 20 meters per second, or 45 mph, to what the industry standard — the Lawson Comfort Scale — describes as an emergency level, it can knock over an infirm person or cyclist and send garbage cans and patio furniture flying.
Buildings can act as windbreaks, sometimes protecting pedestrians and workers at Martin Place. But they also channel winds through gaps, with air moving faster as space narrows.
“Imagine sand falling through an hourglass, with the sand moving fastest at the narrow neck. The wind flowing around a building is similar, with the flow being squeezed around the sides and accelerating to the highest velocities near the side corners,” Glanville said.
When winds hit the facade of a building, they accelerate as they move around the sides, blowing out windows forward and sucking them out the sides and back.
This so-called Bernoulli effect caused about a quarter of the windows in John Hancock’s 1970s Boston skyscraper to fall down, some being blown out while the others were sucked out.
In the same way that cyclists like Glanville minimize drag by tucking their bodies from handlebars to seat in a low, streamlined shape on the bike, architects and wind engineers are now refining building shapes.
According to the Council on Tall Buildings and Urban Habitat to the City of Sydney, a square building has three times the air resistance of a round one.
The new Sydney standards also recommend that developers of tall buildings with more than 40 buildings taper the shape, which also confuses the wind, and incorporate a podium.
As skyscrapers around the world have tripled in height, they have changed shape from squares and rectangles to jagged asymmetrical towers designed to break the wind.
The tallest building in the world, the Burj Khalifa in Dubai, is 830 meters high and is said to “confound the winds”.
It is three times taller than Sydney’s tallest building, the controversial 271-metre-high Crown Sydney (Sydney Tower is 309 meters tall but is classified as a structure and not a building). The tapering tower on a base likened to a massive pencil sharpener serves to deflect the winds more than a vertical square block.
At the top, the Burj sways about two meters back and forth. To reduce sway, many high-rise buildings are equipped with tuned mass dampers. Glanville said these dissipate energy much like shock absorbers in cars.
CPP helped develop tuned mass dampers, about the size of two cars but constructed of solid steel, on the top of the new AMP Quay Quarters Tower at Circular Quay.
“The reason they’re called ‘tuned’ mass dampers is because they rock back and forth, vibrating at the same frequency as the tower. What you’re trying to do is direct the energy into the damper mass, rocking it like a cradle or a rocking chair. From there, the ‘shock absorbers’ absorb the energy of the oscillating mass.”
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https://www.smh.com.au/national/nsw/how-the-sydney-cbd-s-wind-tunnels-came-to-be-20220920-p5bjjl.html?ref=rss&utm_medium=rss&utm_source=rss_national_nsw How the Sydney CBD wind tunnels were formed