ALBUQUERQUE, NM – Two groups of researchers at Sandia National Laboratories have published papers on liquid droplets sprayed by coughing or sneezing and distance traveled under different conditions.
Both teams used Sandia’s decades of experience with advanced computer simulations to study how liquids and gases move for its nuclear inventory management mission.
Their findings reinforce the importance of wearing masks, maintaining social distancing, avoiding poorly ventilated indoor spaces, and frequent hand washing, especially with the emergence of new, more transmissible variants of SARS-CoV-2. , the virus that causes COVID-19.
One study used high-performance computer simulation tools developed by Sandia to model cough with and without breeze and with and without protective barriers. This work was recently published in the scientific journal Atomization and sprays.
Stefan Domino, the senior computer scientist on the paper, said his team has found that while protective barriers, such as plexiglass partitions in grocery stores, provide protection against larger droplets, very small particles can linger. in the air for an extended period and travel a certain distance. depending on environmental conditions.
Separate computer modeling research at Sandia looked at what happens to small aerosol droplets under different conditions, including when a person wears a face mask. This study showed that face masks and shields prevent even small cough droplets from dispersing over great distances, said researcher Cliff Ho, who is leading the effort. This work was published in the journal Applied Mathematical Modeling on February 24.
Cough simulation shows persistent particles
In simulations performed by the Domino team through Sandia’s high-performance computers, larger droplets from a cough with no crosswind and no facial cover fell to no more than about three meters, or about nine feet from distance. They also found that the dry ‘droplet cores’, or aerosols, left behind after liquid evaporates from a droplet traveled roughly the same distance but remained in the air for the two minutes they had modeled. .
Add a plexiglass partition to the mix, and their computer simulations showed that larger droplets cling to the barrier, which lessens the risk of direct transmission, but the smaller droplet nuclei persist in the air, a said Domino.
When they added a 10 meter per second breeze from behind to the barrier-free simulation, the larger droplets traveled up to 11 1/2 feet and the droplet nuclei traveled further.
This study does not challenge the 6-foot social distancing standard recommended by the Centers for Disease Control and Prevention, designed to prevent direct contact of the majority of large droplets. In a typical cough from an infected person, about 35% of the droplets may contain the virus, but models of how much SARS-CoV-2 and its variants are needed to infect another person, Domino said.
“A recent review article on SARS-CoV-2 transmission in the Annals of Internal Medicine suggests that respiratory transmission is the primary route of transmission. As such, we believe that establishing a credible modeling and simulation tool to model the transport of pathogen-containing droplets emanating from coughs and how they persist in the public spaces we all inhabit. are an essential part of the required science, ”he said. Partitions, masks, social distancing, staying home when unwell, and getting vaccinated are still important to help reduce transmission, especially with the newer, more transmissible variants.
Domino also performed computer modeling of outdoor open spaces and found that standing people exposed to a kneeling cough had a relatively low risk of exposure compared to sitting people. It was because of the way droplets and aerosols interact with the complex breezes that move around people. This work was published in the International Journal of Computational Fluid Dynamics on April 1. Domino’s simulations used over four million hours of computer processing and were run on many computer processors at the same time.
Simulations support social distancing, masks
Ho used a commercially available computer model of fluid dynamics to simulate various events that expel moist fluids, such as coughs, sneezes, speech, and even breathing, to understand how they affect the transport and transmission of fluid. airborne pathogens. He surmised that viral pathogens were aerosolized in tiny droplets and that the distribution and concentration of pathogens could be represented by the concentration of the simulated exhaled vapor.
“I introduced spatial and temporal concentrations into the modeling to develop quantified exposure risks based on separation distance, duration of exposure and environmental conditions, such as airflow and facial coatings. Ho said. “I could then determine the likelihood of infection based on spatial and temporal aerosol concentrations, viral load, rate of infectivity, viral viability, likelihood of pulmonary deposition, and inhalation rate. “
The model also confirmed that wearing a face shield or face shield significantly reduced the forward stroke of exhaled vapor and the risk of exposure by about ten times. However, vapor concentrations near the face persisted longer than without a facial coating.
Overall, the model showed that social distancing significantly reduced the risk of aerosol exposure by at least tenfold and left time for dilution and dispersion of the exhaled viral plume. Other models quantified the degree to which being upwind or sideways from the source of the cough reduced the risk of exposure, and the degree to which being directly downwind of the cough increased the risk of exposure. ‘exposure.
The risks of exposure decreased with increasing distance, but the greatest increase in benefit was at three feet. Ho’s models also quantified the extent to which wearing a mask reduces the risk of exposure at different distances.
In short, computer modeling has confirmed the importance of social distancing and wearing masks. Plus, staying upwind and increasing the ventilation of fresh air in places like grocery stores, restaurants, and schools can help reduce the risk of exposure.
Ho also performed computer modeling of school buses and found that opening school bus windows increased ventilation and reduced the risk of exposure. Specifically, to achieve sufficient ventilation, at least two sets of windows must be open, one near the front of the bus and the other near the rear of the bus.
Sandia Inventory Management Work Aid Simulations
Sandia researchers were able to apply many of the same computational tools used in their nuclear inventory management mission to simulate cough and sneeze droplets, as well as Sandia’s advanced high-performance computing resources. For the nuclear deterrent mission, these tools study such things as the reaction of turbulent jets, plumes and propellant fires under different conditions.
“We can deploy our simulation tool capability on other applications,” Domino said. “If you look at the physics of a cough or a sneeze, that includes attributes of that physics that we normally study at Sandia. We can simulate the trajectory of droplets and how they interact in the environment.”
These environmental conditions can include variables such as temperature, humidity, launch path, and crosswind strength and direction. They can also include natural and man-made barriers.
In addition to studies by others on the cough spray, Sandia’s computer simulation capabilities add the value of seeing how droplets from a cough will react to different conditions. Sandia’s simulation tools combine the mass, momentum and energy of droplets to capture the detailed physics of evaporation that helps distinguish between droplets that settle and those that persist in the environment.
The research projects were funded by Sandia’s Laboratory Directed Research and Development Rapid Response, the Department of Energy’s Science Office through the National Virtual Biotechnology Laboratory, a consortium of DOE national laboratories focused on responding to COVID- 19, with the support of the Coronavirus CARES Act.
Sandia National Laboratories is a multi-mission laboratory operated by National Technology and Engineering Solutions of Sandia LLC, a wholly-owned subsidiary of Honeywell International Inc., for the National Nuclear Security Administration of the US Department of Energy. Sandia Labs has significant research and development responsibilities in the areas of nuclear deterrence, global security, defense, energy technology, and economic competitiveness, with primary facilities in Albuquerque, New Mexico, and in Livermore, California.