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Amid the COVID-19 pandemic, determining how to safely reopen schools, colleges, and universities has been a primary focus. A number of studies conducted this past year have investigated airflow and ventilation in classrooms, airborne pathogen transport, and how masks affect pathogen transmission. The consensus of these studies is that wearing masks and social distancing in a well-ventilated room decrease the risks of transmitting COVID-19. However, implementing these practices can be tricky in certain circumstances, in particular in music schools.
Music schools often have small classrooms where students and instructors meet for lessons and practice sessions. In a small space, social distancing can be difficult or impossible, and wearing masks is often not an option for students who sing or play wind instruments. In addition, singing and playing wind instruments increases the rate at which potentially virus-laden particles are introduced into the environment.
Given these factors, how can we make music classrooms safer for music students and instructors? One possible solution is portable air purifiers, which have the potential to improve ventilation and filter out viral aerosols. However, the World Health Organization (WHO) and the Centers for Disease Control don’t currently have guidelines on how best to use air purifiers or exactly how much safer they make a classroom.
To fill in these gaps, Sai Ranjeet Narayanan, a graduate researcher in the Department of Mechanical Engineering at the University of Minnesota, and his advisor, Dr. Suo Yang, teamed up with the University of Minnesota’s School of Music to investigate the potential of portable air purifiers to make music rooms safer. While their study focused specifically on music classrooms, the implications of the research are much broader.
“We started this project right in the middle of the pandemic,” Sai said, “and we could see straight away that the outcomes of this project could significantly help not only the music school, but any enclosed space, such as other types of classrooms, offices, or hospitals.”
Computational fluid dynamics (CFD) was Sai’s tool of choice for this study. To ensure the simulation results were as representative and applicable as possible, Sai modeled his geometry on a standard classroom at the School of Music that is used for one-on-one tutoring sessions or solo practice sessions (Figure 1).
CONVERGE is designed so that you can run a simulation with exactly as much detail as your analysis requires (geometric complexity, spatial and temporal resolution, etc.). For this simulation, Sai took advantage of CONVERGE’s meshing capabilities to embed a fine mesh in certain parts of the domain, such as near the inlets, outlets, and the region in front of the aerosol emitter (i.e., the student). In addition, Sai used CONVERGE’s Adaptive Mesh Refinement to add cells when and where they were needed to capture the important flow phenomena in the room.
Sai simulated a variety of scenarios common to the music classroom, including a student singing, a student playing a wind instrument, and a student playing piano. He investigated three different parameters: (1) the effect of the air purifier on the room’s ventilation rate, (2) the best location to place the air purifier in the classroom, and (3) the effect of the aerosol injection rate on the aerosol airborne suspension rate and surface deposition rate.
To study the effect of the air purifier on aerosol removal and ventilation rate, Sai looked at a case in which a student alone in a classroom sings for 10 minutes and then leaves the room for 25 minutes.
Figures 2 and 3 show the effect of the air purifier on the airflow in the room. In the case without an air purifier (Figure 2), the airflow is driven by the building’s HVAC system. In Figure 3, you can see how the streamlines deviate once the air purifier is introduced and drives the airflow.
To quantify the effect of the air purifier, Sai calculated the number of aerosols removed with the air purifier and compared it to the number removed without an air purifier (Figure 4). In the case with an air purifier, the number of aerosols removed is two orders of magnitude higher than the baseline case.
To decrease chances of COVID transmission in a room, the WHO recommends a ventilation rate of at least 288 m3/h per person. Without an air purifier, the ventilation rate in the room due to the HVAC system is about 166 m3, significantly less than the WHO’s recommendation. With an air purifier, however, the overall ventilation rate increases to approximately 488 m3/h, far exceeding the WHO’s recommended value.
Finally, Sai found that with an air purifier, 97% of airborne aerosols are removed 25 minutes after injection stops (i.e., when the student leaves the room). This suggests that enforcing a break of 25 minutes between uses will make the music classroom much safer for the next student.
In order to achieve the maximum impact of the air purifier, you need to determine the best location to place it inside the classroom. To investigate this, Sai studied a scenario in which a student is playing a wind instrument and an instructor is standing on the opposite side of the room. Figure 5 shows the different air purifier placements that were tested (no air purifier, elevated left purifier, ground purifier, elevated right purifier) and the deposition trends for each position. As you can see, both the elevated left purifier and the ground purifier show similar trends to the case with no purifier, although the ground purifier shows an overall reduction in deposition. The elevated right purifier, however, shows a very different pattern, indicating the air purifier significantly affects the airflow streamlines in this position.
Next, Sai quantified the number of airborne aerosols for all four purifier locations. In Figure 6, you can see that the ground purifier case results in the lowest number of suspended aerosols. The elevated right purifier case actually increases the number of aerosols compared to the baseline case, because it disrupts the natural airflow from the HVAC system. This demonstrates how important the placement of the air purifier is—if placed in the wrong location, the air purifier can make the room more dangerous. Overall, Sai determined that the best location for the air purifier is on the ground near the injection source.
The video below shows the difference in aerosol cloud profiles between the case without an air purifier and the case when the air purifier is in the optimal location.
Finally, Sai investigated the effect of injection rate on both airborne aerosols and surface deposition. He considered three scenarios that exhibit different injection rates: (1) a student playing a wind instrument, (2) a student singing while wearing a surgical face mask, and (3) a student playing piano while wearing a cloth face mask. No air purifiers were included in these cases.
The effects of injection rate on the deposited and airborne aerosols are shown in Figure 7. Sai found that both the airborne suspension rate and the surface deposition rate increased linearly with the injection rate.
“Discovering these trends were linear is important because it means we can predict the aerosol suspension and deposition rates for different injection rates without having to conduct a full simulation,” Sai said. “This can also be extended to any geometry, so it’s not limited to just this scenario.”
Sai’s studies produced highly practical, applicable results. He found that an air purifier can significantly help with ventilation in enclosed spaces, and determined the amount of time needed between sessions in the music room for it to be safe to use again. In addition, he determined the optimal location to place the air purifier for maximum benefit, and discovered a linear correlation between aerosol injection rate and aerosol suspension and deposition rates. While Sai looked at a specific music room, this same case setup can be used for different geometries.
“Working on a project such as this really does feel like you’re contributing to the community,” Sai said. “You’re helping the music school make decisions about their safety guidelines, and the results could be extended beyond the music school to other scenarios. It was a very rewarding project.”
Currently, Sai is working on applying this technique to simulate an orchestra in an orchestra hall. With multiple musicians performing on stage with different kinds of wind instruments, he is investigating how the aerosols circulate on stage to determine which students will be at risk.
Overall, CFD is a great tool to help make our communities safer as we work on reopening society and navigating the post-pandemic world. If you’re interested in learning more about Sai’s research, you can check out his paper here.
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Narayanan, S.R. and Yang, S., “Airborne Transmission of Virus-Laden Aerosols Inside a Music Classroom: Effects of Portable Purifiers and Aerosol Injection Rates,” Physics of Fluids, 33, 2021. DOI: 10.1063/5.0042474
