Written by Krishnendu Sinha
As the COVID-19 pandemic unfolds in India, the focus has been towards un-locking and opening up the economy. A harsh reality has begun to set in that until vaccines or new therapies are developed, we will have to learn to live with the pandemic. Apart from changes in social behaviour like physical distancing and mask usage, retrofitting of workplaces and communal spaces, like neighbourhood stores, office buildings and classrooms, will play an important role in reducing the risk of transmission and spread of the coronavirus.
It is accepted that the spread of the SARS-CoV-2 virus is primarily person-to-person, through the droplets and aerosols generated by a person coughing, sneezing, or even talking. A droplet, usually sub-millimetre in size or larger, rapidly falls to the ground after it is emitted due to gravity. Aerosol particles are much smaller (as small as a thousandth of a millimetre) and can stay suspended in the air, potentially carrying infectious virus particles with them, and can be breathed in by people. Therefore, understanding air flow and ventilation, especially in closed spaces where aerosols tend to persist, is a key aspect of controlling the infection spread.
The question now is: How does one reduce the risk of contracting COVID-19 in enclosed spaces, like a neighbourhood store or a barber shop? A barber shop has some unique challenges. Customers at the shop stay seated in one place for the duration of a haircut, typically 10-15 minutes. What influences the risk of infection during this time? Certainly, mask etiquette followed by the barber and the other customers is important, as is physical distancing between customers. Equally important is the question: Is there adequate ventilation to disperse airborne droplets and aerosols? Perhaps, the barber himself has many of the same questions on his mind. After all, he spends much more time than the customers in that enclosed space.
We consider the barber shop in the IIT-Bombay campus, frequented by faculty and students (most of whom are continuing their education online, from home, at this time). The campus barber’s shop has a layout that is perhaps similar to what one would encounter in most urban settings in the country. The shop typically comprises a closed room (in some cases air conditioned — this is the case for the campus shop) and is typically not well ventilated. It has more than a handful of people at any given time. There are customers coming in and going out, and maintaining social distancing is challenging. In such a scenario, can managing the air flow be used to enhance ventilation and reduce the risk of air-borne transmission?
In technical terms, the study of air flow is called fluid mechanics and it is important in many fields from aerospace and mechanical engineering to astrophysics and environment. Fluid mechanics teaches us how the air flows in passages and around obstacles. The mathematical equations of fluid mechanics are well known, but solving them is very difficult. Highly sophisticated computer simulations are required to predict the flow of air, and this constitutes a field of study called computational fluid dynamics. CFD can tell us how the air moves around in the barber’s shop and how the infectious droplets are carried around by the flows. It can also calculate how the air flow can be altered by fans, windows, air-conditioning and a host of other factors.
CFD has already been used extensively to study air flow in closed spaces such as railway compartments, office spaces and shopping malls. Several reported studies concern themselves with improved human comfort and reducing power usage, for example, by better design of air conditioning systems. In these times, dominated as our lives are by COVID-19, CFD studies have focused on the effect of masks (showing how masks cut down the number of droplets emitted and the distance that they travel) and of air flows and aerosol transmission in super markets. A report from a group in Hong Kong has shown how the infectious droplets from a single source can spread to the entire compartment of a high-speed railway.
Specifically, in the context of the barber shop, CFD can be used to design retrofit solutions to minimise air-borne infection spread. Simple measures like installing exhaust fans or even a simple pedestal fan and opening the rear door (fortunately, we have one at the IITB barber shop) can make a big difference. The idea is to quickly disperse droplets from a potential source inside the room to the outside to minimise the risk to the barber and other customers. Simple calculations can already tell us the size and power required by exhaust fans. For example, assuming a typical size for a barber shop of 10 ft by 15 ft by 10 ft, if we wish to replace the air in the room every minute, it will require fans that can suck out 1500 cubic foot of air per minute, or 1500 cfm as it is given in exhaust fan specifications. To do it twice as fast, we can either install two such fans, or a single exhaust fan of 3000 cfm.
While simple calculations are a good starting point, one needs detailed CFD for rigorous analysis. Exhaust fans can either augment the natural air circulation in the room or they can obstruct it. Knowing the precise flow patterns created by the exhaust fan therefore becomes critical. For example, there can be secondary air flows, sometimes known as dead zones, where pockets of air can stay trapped for a long time. Naturally, this does not vent the air out of the room and a person sitting in such a dead zone is more prone to breathing in persistent aerosols. The placement of fans and vents to avoid such dead zones in the barber shop is critical to the air flow management. We need CFD models to identify dead zones in a room and also to give an estimate of how long the air lingers around in such pockets. Using CFD, we can arrange fans and vents in a computer model of the room to arrive at the best possible solution.
Scenarios where the air flow changes as a customer comes in or leaves through the door, or when there is a sudden draft of warm air from outside can also be calculated. CFD can also predict how things change with weather or geographical locations. However, the equations of fluid mechanics do not yield so readily to even modern-day computers as the problems start getting increasingly complex. For example, answering a simple-minded question like “what happens to the millions of hair pieces on the floor?” can stretch the limits of super computers. Knowing that will really help as no one wants them to start floating around.
Retrofit solutions designed using CFD can be extended to other closed spaces like the corner grocery shops, small eateries and doctor’s clinics. Each can have their own way of managing the air flow to reduce the infection. CFD can help work out retrofit solutions tailored to their respective needs. For bigger setups like lecture halls, theatres and shopping malls, we have to deal with many more people and multiple entry and exit locations. The size of the problem is bigger, but the science is the same. We will need larger and more fans and vents, and CFD can tell us where it is most effective to place them. Smart solutions, with real time data, can possibly be tuned to the number of people present at a given point in time. This can potentially be more efficient in terms of power usage and installation.
A group of Indian researchers called the Fluid Mechanics Research for COVID-19 (FMRC) have come together to address the fluid flow problems posed by the re-opening of the economy. This group involves researchers from different IITs (IIT-Bombay, IIT-Madras, IIT-Roorkee) and from industry, and aims to use advanced modelling tools to suggest retrofit solutions that will help reduce the risk of transmission of infection. Of particular interest to this group are issues relating to fluid flows in closed rooms, public transportation and, of course, classrooms.
The writer is professor of Aerospace Engineering, IIT Bombay
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