Every human being is innately different and so is our cough. 7.5 billion people. 7.5 billion different coughs. There’s been plenty of research into the human cough. Enough, in fact, to tell us what an average cough from an adult should look like. It’s this average cough that’ll help us uncover some key insights behind how cough droplets travel and move in an aircraft cabin.
The human cough has long been a source of infections, both airborne and through contact. This was 100 years ago. A warning for the Spanish Flu.
This is 100 years later. Today. A warning for Covid. Times have changed but infections and the human body haven’t.
But what we have today is a greater understanding of infection transmission through years of research and advanced technology like computer simulation to understand how infections spread.
A cough generates a high-velocity jet from the mouth up to 50 mph. Within this jet is a mixture of droplets and aerosols. Some you can see. Some of you cannot. Anything over 5-micron diameter in size is a droplet. Under 5 micron diameter is an aerosol. In this presentation, we’ll refer to both as particles. Infectious virus particles from the lungs, throat, and airways hitch a ride on mucus and saliva droplets and aerosols. Larger the particle, the more viral load it carries.
Now when an average human being coughs, how far do the particles travel?
The larger particles mostly fall to the ground within 6 ft of the coughing person due to gravity. This is why we have social distancing measures today. The recommendations come from the assumption that the larger particles are the most dangerous form of infection transmission. The smaller aerosol sized particles are lighter and float in air. The virus particle doesn’t float by itself. The virus particle lies at the center of a particle and that’s how it travels.
For us to understand infection transmission, we need to understand how the particles travel in air. This is where computer simulation comes in.
How do you design safer indoor environments that minimize infectious particle spread?
Testing is one option. You can have humans physically cough in an indoor space. You can also use jet injection techniques like the one in the picture at the bottom. But testing is time-consuming and gives only a small sample size. How do you test for 100s of representative scenarios and coughs? It’s just not physical possible. This is where numerical simulation comes in. We call it computational fluid dynamics simulation or CFD.
Why do we need CFD simulation?
Because it’s faster and efficient to simulate 100s of scenarios and designs on a computer than to test them physically. You can model the cough particles coming out of the mouth and the airflow in the room and really track where the cough particles go. You can really understand every single particle’s behavior if you need to. This holds true for understanding cough particle transport as well.
What affects the droplets and where they go?
An average human cough has around 3000 droplets. A cough from a patient suffering from severe respiratory illness could have upto 10000 droplets. In comparison, we exhale around 600 droplets per minute when talking and around 40,000 droplets when sneezing.
The jet of air from a cough can travel as fast as a 80 km/h and is critical in transporting the cough droplets What are some factors that affect where these droplets go? Is the person indoor or outdoor? How much space do we have for the droplets to travel? Temperature and humidity affect how fast the droplets evaporate. What is the airflow around them? Is there an air-conditioner? A fan? Maybe they are outdoors and it’s windy. How are they coughing? How big is the mouth opening during cough? And more variables. How many more?
Everything from head position when coughing, whether the person is coughing into their elbow, how sick they are, what obstacles are in front of them, size of droplets all affect where they go. But there’s more-Evaporation, body heat, no of droplets in the cough and so many more variables affect where the droplets go.
Truth is there are far too many variables, each with a different level of impact on the droplet transport.
The challenges in simulating a cough with CFD
We have put a lot of effort into accounting for as much of the cough physics as possible to get the most realistic representation of the cough.
Right approach: First, you have to choose the right modeling approach – You need to simulate the exact geometry and include as much relevant physics as you need. Thanks to the Siemens NX Human software, we were also able to get representative human models for the simulation.
Droplet modeling: You need to get the right distribution of droplets. The different diameters involved, how many droplets of each diameter are present, what angle the cough comes out, average mouth opening area for a cough all need to be considered.
Air flow modeling: What about the air flow? While the larger droplets are influenced by gravity, the smaller aerosols remain suspended and follow the air flow. You need to accurately model the airflow. For example, if you are simulating cough in an aircraft cabin, you need the highest fidelity representation and modeling of the cabin airflow.
Cough jet: Next is the cough jet which is a transient jet that changes with time. Air comes out at the highest speed at the beginning of the cough, then slows down gradually. All this happens within a second. You need to capture that.
Droplet tracking: Next is tracking the droplets. Various forces act on a droplet – lift, drag, shear force, thermophoresis. All of these need to be accounted for. Droplets evaporate too. As they shrink in size, their movement in air varies. This needs to be considered as well.
Different scenarios: But all of these variables are for one cough from one person for one scenario. What about other scenarios? Different cough strengths, different patients, protective barriers, change in environment around the patient. These need to considered as well.
It’s very easy to rush into modeling a cough without completely considering all these representative physics or choosing a poor representation of the indoor geometry.
Keys to modeling a cough in an airplane accurately
A diligent modeling framework where we include the relevant variables as is.
A Multiphysics CFD approach that accurately models these variable.
Proper visual representation of the droplets in space and analysis of the droplet transport.
Right approach to droplet tracking
Join our Webinar: High-fidelity CFD simulation of cough droplet propagation in aircraft cabin.
In this webinar, Siemens and Airbus will talk about the role of high-fidelity computational fluid dynamics (CFD) simulation in understanding the transport of particles/droplets from a human cough in aircraft cabin.
Credit: Siemens Digital Industries Software
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