Fugaku is currently the most powerful publicly ranked supercomputer in the world – but we weren’t supposed to have it yet. The supercomputer, situated at Japan’s Riken scientific research institute, was scheduled to come online in 2021. When the pandemic struck, Riken decided to launch Fugaku almost a year ahead of schedule. Since then, Riken and Fugaku have found a particular niche in the crowded COVID-19 research landscape, conducting a swath of intensive simulations focused on how viral droplets proliferate through masks, face shields, train cars and more – and in the process, drawing a very direct line between supercomputing and the general public. HPCwire spoke with Dr. Makoto Tsubokura, head of the Complex Phenomena Unified Simulation Research Team at Riken, to learn more about the past, present and future of Riken’s policy-targeted coronavirus research.
Over the last several months, Riken has consistently hit the news with its COVID-19 research on Fugaku. Early research showed how viral droplets spread through train cars, demonstrating that simply opening the windows could dramatically increase ventilation and decrease the risk of infection. From there, Riken moved on to facial coverings, discovering that face shields were largely ineffective at stopping viral spread, with nearly 100 percent of certain droplets escaping – but that cloth masks, especially non-woven cloth masks, were effective at stopping most droplets. Since then, Riken has simulated offices and auditoriums – and the institute even partnered with a liquor company to develop effective facial coverings for eating and drinking in restaurants and bars.
How Riken approaches coronavirus droplet simulation
At work behind these simulations: Fugaku’s record 415 Linpack petaflops of supercomputing power, delivered by nearly 160,000 nodes (another record) equipped with Fujitsu A64FX Arm CPUs – and Riken is putting all of it to work on COVID-19.
“We have used a total of three million node hours since May,” Tsubokura said. These hours go toward meticulously simulating thousands and thousands of droplets moving through a space, including a medley of inhabitants, obstacles and airflows.
“For masks and face shields, infection risk analyses are done in situations where people are in close proximity with each other. In the analyses, tens of thousands of droplets of different sizes (diameters) are modeled,” Tsubokura said. “The size and number of the droplets vary depending on the activities, such as coughing, singing, and speaking, and the airflow is taken into consideration for coupled analyses. Various conditions, such as evaporation from the surface of the droplets, adhesion to the wall, and repulsion, are also modeled and incorporated into the calculations.”
For these interpersonal simulations, Tsubokura said, his team typically used around a hundred of Fugaku’s nodes for fifty hours per case. For the simulations of larger spaces (like auditoriums), the team uses similar methods, applying the results of the interpersonal simulations as boundary conditions. These larger simulations use correspondingly more resources: over 500 nodes at a hundred hours per case. And, of course, a single case doesn’t cut it. “Although each simulation requires a modest amount of computational resources,” Tsubokura said, “we need to analyze numerous cases in order to meet various needs and draw meaningful conclusions.”
Riken’s humidity research should have you sweating
Among the most recent research from Riken is some upsetting simulation of viral droplets’ interactions with humidity. To investigate the link between humidity and COVID-19 spread, the researchers simulated the aerosolization of particles at a variety of humidity levels. They found that air with humidity less than 30 percent resulted in more than twice the aerosolized particles than air with humidity of 60 percent. This research has grave implications for the winter months in the Northern Hemisphere, as colder air doesn’t retain as much moisture as warm air.
The below clip from the simulation, courtesy of Riken, illustrates humidity’s dramatic effect.
“After being emitted by coughing and speaking, droplets larger than tens of micrometers will fall and land on the surface within one square meter,” Tsubokura explained. “In contrast, droplets smaller than several micrometers will become airborne as aerosols, and float in the air for a long time (tens of minutes). Aerosolized droplets will leak out of the gaps between the mask and face, and, therefore, ventilation is necessary as an effective defense against infection.”
“Depending on the humidity of the air,” he continued, “droplets smaller than tens of micrometers are rapidly dehydrated and shrink. The effect of humidity below 50 percent is especially remarkable; in the few seconds the droplets travel to the surface of a desk or something, they can be aerosolized and stay in the air for a prolonged period.”
“Thus, in the dry indoors in winter, it is important to take a two-pronged approach,” he concluded. “We need to add humidity to minimize droplet aerosolization and ensure proper ventilation at the same time.”
A direct link to policy
Riken understands the necessity of connecting this research to reality. Its simulation projects have been conducted in coordination with a wide range of entities: private businesses like construction companies, automobile manufacturers, air conditioner manufacturers, airlines, mask manufacturers; and government bodies like Japan’s Ministry of Education, Culture, Sports, Science and Technology, its Ministry of Land, Infrastructure, Transport and Tourism, and the City of Kobe. Tsubokura sits on a special committee in Japan’s Cabinet Office, where he shares the simulation results for use “as scientific bases for guidelines for reopening events and other activities in Japan.”
What’s going on now – and what’s next?
Just like the pandemic, Riken and Fugaku show no signs of slowing down.
“In response to the requests from the governments and industry mentioned [above], we are currently performing simulations of situations in public transportation (taxis, buses, airline cabins, ambulances), public places (restaurants, bars, hospital rooms, classrooms, live houses, multi-purpose theaters), and also various outdoor events,” Tsubokura said. “In the future, we would like to provide data to help build new lifestyles, building designs, etc. that would allow for sustainable prosperity in the age of coexistence with coronavirus.”