EXPLORE THE POSSIBILITIES
Trains, planes and waterfalls
A new terminal in Singapore's Changi Jewell airport designed to challenge the standard expectations of airport terminals.
By Stephen Ferguson
The Rain Vortex at the Changi Jewel airport terminal (Image courtesy of Supanut Arunoprayote.)
Modern airport terminals are usually functional buildings, with climate control settings carefully designed to accommodate passengers dressed for their destination rather than local surroundings.
If you travel often, it becomes difficult to tell airport terminals apart. The only real difference between them is the variety of retail experiences designed to extract the maximum cash from passengers as they wait for often delayed flights.
Not so in Singapore, where the newly opened Changi Jewel terminal has become an iconic leisure destination. At its centerpiece is the world’s largest and tallest indoor waterfall: the seven-story-high “Rain Vortex” through which 10,000 gallons of harvested rainwater plummet every minute. Located inside a giant greenhouse atrium, surrounded by a terraced forest, the cascade of water is often accompanied by a spectacular display of choreographed lights and music.
However, in the hot and humid tropical climate of Singapore, managing the thermal comfort of occupants is always a challenge, even more so while trying to minimize energy consumption and greenhouse gas emissions.
Building the world’s largest indoor waterfall inside a giant glass greenhouse in a tropical country while keeping the space comfortable for occupants was obviously a considerable engineering challenge. To complicate matters further, the atrium is penetrated by a passenger train service that has the potential to suck in warm air from the outside and expel cooled air back into it.
To address these complicated challenges, the architects responsible for designing the terminal appointed Atelier Ten Ltd to provide strategic environmental design, analytical consultancy and conceptual services engineering. Balancing the competing demands of abundant heat and light needed for plants and superior passenger thermal comfort for people was one of the project’s key challenges. Atelier Ten produced detailed models of the thermal environment in the atrium, including using ray tracing technology to model the light entering through each triangular glass panes on the roof.
I spoke with Atelier Ten’s Nikolai Artmann and Henry Woon about the challenges of designing the thermal environment of the Changi Jewel airport terminal.
Waterfall engineering
Their focus soon turned to the waterfall. Although one might naively assume that the waterfall would have a cooling effect, there was some concern that the vast mass of moving water might disrupt the thermal environment in the atrium.
“Although the architects originally hoped that there might be some evaporative cooling benefit, the real concern was the potential for de-stratifying the thermal environment inside the building,” explained Henry Woon.
We all know that hot air rises. This thermal buoyancy effect means warmer air accumulates at the top of the atrium, from where it is extracted for air conditioning. Atelier Ten were keen to make sure that the warmer air at the top of the atrium wasn’t entrained by the waterfall and dragged down to the lower occupied levels.
“The intention was to only expend energy in air-conditioning the occupied lower areas of the building, and the risk was that the waterfall would entrain warm air from further up and drag it down to the lower levels,” said Woon. “We were concerned that the waterfall mixing up the air volume would harm the thermal comfort of occupants of the atrium.”
A secondary concern was that moisture from the waterfall would add to the humidity of the air in the occupied space, adding load to the air conditioning system.
Engineers started by building models in a spreadsheet to try to understand the entrainment and evaporative cooling effect of the waterfall but soon concluded that more detailed engineering was required.
“One issue was that there was no literature on waterfall simulations that we could reference, apart from a few simulations of naturally occurring outdoor waterfalls that weren’t very useful,” said Nicolai Artmann, who led the indoor climate modeling on the project.
To answer these questions, Artmann built a detailed numerical model of the waterfall using Simcenter™ STAR-CCM+™ software.
“We needed to work out how to segregate this warm air and reduce the impact to the core displacement ventilated condition space at low level,” said Artmann. “But also, to calculate the evaporation rate of the waterfall and how much latent load it actually added into the system.”
However, simulating a waterfall on this scale, including all the essential physics, provided its own challenge.
“These types of multi-phase simulations are computationally costly, so we exploited the rotational symmetry of the waterfall and simulated a single meter-wide slice at high resolution,” explained Artmann.
“We looked at the water droplet traveling down through this air-conditioned space and worked out the amount of water moisture released,” said Artmann. “But we also discussed whether there are some ways to estimate the amount of moisture released from droplets when they hit the bottom of the vortex. And how much water will turn into vapor, and then add onto the latent load into the air conditioning system.”
Having characterized the waterfall using detailed simulation, Atelier Ten engineers used that data to include source term models in their comprehensive model of thermal comfort in the atrium:
“We injected momentum sources to represent the air circulation created by the waterfall, focusing on how we disrupt our displacement ventilation, air conditioning strategies,” explained Woon. “I think the model, the software itself, was reasonably accurate in predicting the track of the waterfall. And then in terms of how it affects our air conditioning load as well.”
Of course, the real proof in any simulation is whether the predicted outcome is realized in the actual building once it is operational. Atelier Ten conducted significant post-implementation validation testing to check the veracity of their model.
“I would think that is reasonably well represented, based on my experience,” said Woon. “And we have been carrying out equipment and taking measurements in many spaces inside the dome. And multiple times as well, and we would always carry out site visits with our equipment, measuring temperature, et cetera. So, our simulation is generally quite aligned with site measured data I would say”.
If you visit the Changi Jewel (or look at photographs of it), you will see a one-meter-high balustrade around the Rain Vortex waterfall. This was not included in the original design and was introduced because of the simulation results.
“Early simulations showed air being dragged by the waterfall from the occupied areas into the pit in which the water is collected, the air will then rush upwards and carry quite a lot of water vapor with it” explained Artmann. “To prevent this water vapor entering air conditioned zones, we designed a one-meter-high solid glass balustrade that deflects the air upwards instead of sideways which would affect the visitors’ experience. This was a successful strategy based on our observation, that we would not have employed if not for the computational fluid dynamics simulation.”
Driving trains at walls
“In Singapore, it is summer all year round. That requires constant air-conditioning, and so there are stringent building codes concerning sustainability,” says Henry Woon. “Building regulations strictly prohibit the leaking of air-conditioning into the outside environment. Any opening without an automatic means of closing is deemed to be not complying with the regulations. You need to have effective automatic shutters, sliding doors or air curtains to be compliant.”
The issue here is that a train runs through the air-conditioned garden of the atrium.
Thanks to the engineering innovation of Atelier Ten, passengers have an uninterrupted view of the Rain Vortex waterfall as they pass through the air-conditioned atrium. Photo credit:Marvin Chandiary
“The original plan was to enclose the train line in a transparent glass tube, but the embedded carbon cost of building the tubes out of concrete, glass and steel began to look prohibitive,” says Woon. “They would also have blocked the view and been difficult to keep clean.”
“The first idea was to use a series of massive air curtains to create the vestibule,” continued Woon. “However, early simulations showed that it would require about a hundred cubic meters of air per second to create a useful curtain, which would cost too much in terms of fan energy. Then we looked at water-curtains before eventually settling on the idea of fast action doors that would open in front of the train and close behind it.”
To overcome objections from the regulator, Atelier Ten engineers had to demonstrate exactly how much air would leak during the train's passage through the tunnel.
“The Jewel project is of national interest; it’s the gateway to Singapore and the first thing that many international visitors will see, and so the regulators intensely scrutinized our design solutions,” said Woon. We submitted five times before we finally managed to persuade them - using simulation - that we could prevent significant leakage into the outside world.”
Atelier Ten solved this problem and optimized the performance of the system, using Simcenter™ STAR-CCM+™ to model the passage of the train moving into, though, and out of the atrium. This involved simulating the entire space of the massive atrium over the two minutes period which would take for the train to pass through it.
“We had to collaborate with Siemens for this, both in engineering expertise and supercomputing resources,” said Nicolai Artmann. “The action time on the doors is about 0.2 seconds, but the simulation had to run for over 500 times in order to cover that 2 mins period, and it took weeks of computing.”
“One thing we had to consider was the piston effect of trains entering the space, sometimes two trains heading in opposite directions, and causing a piston effect that would force air out of pedestrian entrances on arrival, and suck it back out on departure,” said Artman. “The CFD simulation results convinced us that we had to reduce the speed of the trains in order to prevent piston effect.”
Using extensive simulation, Atelier Ten were able to convince regulators that their scheme would prevent the leakage of air-conditioned air, but had further work to do to convince them that the scheme was safe and would not impede train operations in the event of the failure of the moving door mechanisms.
“We had some fun installing a full-scale mock-up of the fast action doors in the car park, and then driving a forklift into them a full speed,” said Woon. “They are actually very lightweight and as you push against them, they automatically roll up, so there is no danger to passengers. In the event of power failure, the doors would automatically just roll up.”
Thanks to Simcenter, and their own engineering ingenuity, Atelier Ten were able to design a solution that not only reduced the carbon footprint of the building, but also offered more utility for users of the space.
The good news is that the system is working as designed. Air temperature monitors inside the tunnel confirm that air-conditioned air is not leaking into it. Passengers can enjoy an uninterrupted view of the atrium gardens and the Rain Vortex waterfall.
