COVID has infiltrated every sphere of our daily lives. Posterity would remember the last two years as historical. COVID has also affected researchers and their research. COVID has opened new avenues of research in virology, epidemiology, and seemingly unconnected fields like Aerospace Engineering and Fluid mechanics. We shall look at one such research project conducted by researchers from IIT Kharagpur. Prof. Sandeep Saha, along with Arghyanir Giri and Neelakash Biswas from the Department of Aerospace engineering, IIT Kharagpur, in collaboration with Princeton University, USA, Imperial College London and CNRS, France, has performed a detailed analysis of the situation, which helped assess the risk of transferring pathogens from an infected person to a susceptible person.


The study led to some interesting conclusions. The Paramshakti supercomputer present at IIT Kharagpur performed a significant role in the visualisation and numerical simulations of colliding respiratory jets by creating a virtual environment. The air exhaled by humans during breathing, coughing and speaking release a jet-like airflow called respiratory jets. The numerical results were independently verified by the experimental results of the other overseas groups. Prof. Saha believes that coding is integral to every field these days. It has become an essential tool, and every student must endeavour to master it. However, it is still a tool at the end of the day, and core concepts are indispensable. Students must not forget core concepts in their quest to become great coders. They had initially got open source code to work on but had to tweak it themselves to deal with the project appropriately.

The research mainly focused on the collision of respiratory jets when both individuals are speaking, highlighting the Blocking Effect concept. According to professor Saha, when both the individuals speak, the respiratory jets coming out of their mouths collide, temporarily shielding the susceptible speaker from the pathogen-carrying jet. This temporary shielding is the Blocking Effect.

The splitting of the jet from the infected person means that it spreads out laterally and does not effectively reach the susceptible person.

The analysis revealed that vertical offset(the height difference between the mouths of the persons involved in the conversation) plays a crucial role in making the blocking effect effective. According to the professor, the blocking effect is practical at small offsets due to the collision of opposing jets, which has a substantial influence on limiting airborne transmission at early times. An infected speaker's aerosols may enhance indirect airborne transmission if the face-to-face conversation occurs at a table with multiple individuals.

When both the speakers are standing, the offset height could be significant, and the blocking effect ceases, as the jets do not interact. The strength of breathing and speaking signals may differ, and its consequences on the collision dynamics are as yet unknown.

But if the offset is intermediate, i.e. about (8-10 cm for 1 m separation), jet entrainment and the inhaled breath assist the transport of the pathogen-loaded saliva droplets towards the susceptible speaker's mouth, the risk of infection increases. Jet entrainment is an advanced concept but it can be summarised thus- interacting with the susceptible person's jet actually aids the infected jet to reach the susceptible person.

While the blocking effect is supposed to be lesser at d = 0.95 than d = 0.85, we see that the jet reaches earlier in the latter case. This is due to jet entrainment and inhalation from the mouth.

So, there are two factors at play here- jet entrainment and blocking effect. The former increases the chance of infection while the latter decreases it. Optimal distance for infection (we know it sounds bad!) is an intermediate offset.

A graph indicating the risk of infection for different offset heights represented through different colours.


Professor Saha further added that there is a broad scope for future research, and the team intends to add more complexity to the project. In future, the team aims to cover the limitations by collaborating with epidemiologists and virologists and finding more precise and accurate results by including various factors related to human behaviour and the human body and the conditions for individuals to get infected. They would need to discuss some fluid mechanics concepts with the virologists. Prof. Saha believes that collaboration is all about exchanging ideas by going to the level of your collaborator.

As we all know, it is complicated to mimic the exact conditions of real life in computer simulations and labs, so they made some assumptions during the research to overcome these difficulties. For a deep understanding of the work, an enthusiast also needs to know the assumptions.

Prof. Saha briefly described a few things they had assumed in the course of the project to make it simpler, and this is just an initial foray into the topic.

To show the blocking effect, they had assumed that both persons were ejecting respiratory jets at the same time. However, this rarely happens in real life. Instead, it is considered rude to speak while another person is speaking. On a lighter note, 'better be rude than sorry,' remarked Prof. Saha. They had only simulated the first 30 seconds of a conversation. That is, they studied the behaviour of the respiratory jets for the first 30 seconds since they were ejected. However, the typical conversation is usually much longer.

They had assumed a highly simplified model wherein there were two orifices, and respiratory jets were ejected by both continuously. However, in reality, we open and close our mouths while speaking. The behaviour of the jets could also be a function of how we move our mouths, that is, the loudness and the phonetics we use. The way we move our mouths is usually culturally variant. Thus, there is also a scope of linguistics in the research.

Many environmental factors such as air temperature, pressure, humidity, air density etc., vary in every place on Earth. During the research, they assumed a standard temperature valid for most sites on Earth. Still, they would have to make specific changes if the temperatures are too extreme and very different from the body temperature. The respiratory jets would have the same temperature as the body temperature, and on coming in contact with outside air, it may move up or down depending on whether the outside air is colder or warmer than the body. This would affect the transfer of pathogens.

The presented problem is complex, but Prof. Saha is optimistic about its potential. Prof. Saha is also confident about the future of Indian research. His two students who participated in the study gave him hope that Indian research is in good hands. The study also highlights the rising need for interdisciplinary research to tackle socially relevant problems.


1.The original paper

2. A video containing the simulation