Using simulation to reduce the carbon footprint and provide more utility for users

Managing thermal comfort

Modern airport terminals usually feature climate control settings carefully designed to accommodate passengers dressed for their destination rather than local surroundings.

That is not the case in Singapore, where the Changi Jewel terminal has become an iconic leisure destination. Its centrepiece is the world’s tallest waterfall: the seven-story “Rain Vortex” through which 10,000 gallons of harvested rainwater plummet every minute. Located inside a giant greenhouse atrium and 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,” says Henry Woon, director of design consultants, Atelier Ten. “To complicate matters further, passenger train service passes through the atrium, which has the
potential to suck in warm air from the outside and expel cooled air back into it.

“We solved these challenges and satisfied the strict environmental standards imposed by the Singapore planning authorities by performing extensive simulation using Simcenter STAR-CCM+,” he reports. “This enabled us to understand the complicated physical interactions caused by the world’s largest indoor waterfall and the train system that passes through the atrium.”

Simcenter is part of the Siemens Xcelerator business platform of software, hardware and services.

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. (image courtesy of Marvin Chandiary)

Waterfall engineering

Although one might think the waterfall would have a cooling effect, there was concern the vast mass of moving water might disrupt the thermal environment in the atrium.

“Although the architects originally hoped there might be some evaporative cooling benefit, the real concern was the potential for destratification of the thermal environment inside the building,” says Woon.

Warm air rises so the thermal buoyancy effect means warmer air accumulates at the top of the atrium from where it is extracted for air conditioning. Atelier Ten wanted to make sure the warmer air wasn’t entrained by the waterfall and dragged down to the lower occupied levels.

“The intention was to only expend energy for air conditioning in the occupied areas of the building, and the risk was the waterfall would entrain warm air from further up and drag it down to the lower levels,” says Woon. “We were concerned the waterfall would mix up the air volume and 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, increasing the load on 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 more detailed engineering was required.

“One issue was 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,” explains Nikolai Artmann, an environmental associate at Atelier Ten, who led the indoor climate modeling on the project.

To answer these questions, Artmann used Siemens Digital Industries Software’s Simcenter™ STAR-CCM+™ software to build a detailed numerical model of the waterfall.

“We needed to work out how to segregate this warm air and reduce the impact on the core displacement ventilated condition space at low level, but also to calculate the evaporation rate of the waterfall and how much latent load it added into the system,” says Artmann.

However, simulating a waterfall on this scale, including all the essential physics, provided its own challenge.

“These types of multiphase simulations are computationally costly, so we exploited the rotational symmetry of the waterfall and simulated a single meter-wide slice at high resolution,” explains Artmann.

“We looked at the water droplet traveling down through this air-conditioned space and worked out the amount of water moisture released. But we also discussed whether there were ways to estimate the amount of moisture released from droplets when they hit the bottom of the vortex and how much water would turn into vapor, and then added on the latent load on 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 and air conditioning strategies,” says Woon. “I think the model, the software itself, was accurate in predicting the track of the waterfall and 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 once the building is operational. Atelier Ten conducted significant post-implementation validation testing to check the veracity of their model.

“We have taken measurements in many spaces inside the dome multiple times such as temperature, etc.,” says Woon. ”The simulations we conducted were generally aligned with site measured data.”

The Changi Jewel has 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,” says Artmann. “The air will then rush upwards and carry quite a lot of water vapor with it. To prevent water vapor from entering air-conditioned zones, we designed a one-meter-high solid glass balustrade that deflects the air upwards instead of sideway, which will affect the visitor’s experience. This was a successful strategy that was made possible by using Simcenter STAR-CCM+ for the computational fluid dynamics simulation.”

Driving trains at walls

“Singapore is a summer-all-year kind of place that requires constant air conditioning so there are stringent building codes concerning sustainability,” says Woon. “Building regulations prohibit the leaking of air conditioning into the outside environment. Any opening without an automatic means of closing is deemed out of compliance with regulations. So you either need to have effective automatic shutters or sliding doors or air curtains can be used to comply with this air conditioning retention requirement.”

The issue here is a train runs through the air-conditioned garden of the atrium.

“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. However, early simulations showed it would require about 100 cubic meters of air per second to create a useful curtain, which would cost too much in fan energy. Then we looked at water curtains before eventually settling on 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 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,” says Woon. “We submitted five times before we finally managed to persuade them by using our Simcenter STAR-CCM+ simulations that we could prevent significant leakage into the outside world.”

Atelier Ten used Simcenter STAR-CCM+ to model the passage of the train moving into, through and out of the atrium. This meant simulating the entire space of the massive atrium over the two minutes it would take for the train to pass through it.

“We had to collaborate with Siemens in engineering expertise and supercomputing resources,” says Artmann. “The action time on the doors is about two-tenths of a second, but the simulation had to run for over 500 times to cover that two-minute period, and it took weeks of computing.

“One thing we had to consider was the 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. The Simcenter STAR-CCM+ simulation results convinced us we had to reduce the speed of the trains to prevent piston effect.”

Atelier Ten used Simcenter STAR-CCM+ to conduct extensive simulation to convince regulators their scheme would prevent the leakage of cooled air. But they still needed to convince the regulators the scheme was safe and would not impede train operations if the moving door mechanisms failed.

Thanks to Simcenter and Atelier Ten’s engineering ingenuity, the firm was 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 system continues to work as designed. Air temperature monitors inside the tunnel confirm that cooled air is not leaking into it so passengers can enjoy an uninterrupted view of the atrium gardens and the Rain Vortex waterfall.