# Back to School: Simulation of Flu Virus Spread in a Classroom

This guest post on Innovation Intelligence was written by Nari Yoon, CFD Engineer at Cradle North America. sc/Tetra and scSTREAM for CFD analysis, both developed by Software Cradle, are available through the Altair Partner Alliance.

Sigh—the sweetest two-month vacation just whizzed by like a bullet, and school is back in session, with all the excitement and exhilaration of new beginnings. And guess what else? Flu season is on its way!

Figure 1: Classroom model

Some might have heard that one student with the flu could infect an entire classroom of children. While this sounds scary, it appears to be true. According to the New York State Department of Health, students within three feet of the infected student can “easily be infected” by inhaling airborne droplets. How is that possible? How fast do the droplets travel and how wide do they spread over the class?

We decided to test the flu virus spread with our tool of choice, computational fluid dynamics (CFD) simulation.

The set-up starts with a normal indoor flow simulation, having supplies and return vents located in a 9*9*3m classroom. A teacher and 35 students with desks are then modeled. The first simulation was run to establish the steady-state flow.

A poor sick student was seated in the middle of the classroom, coughing and sneezing every 10 seconds. Diameters of 10 µm, 5 µm, and 1 µm droplets at a speed of 30, 45, and 100 miles per hour, respectively, were expelled.Unfortunately, the subject did not bother to cover his mouth in the simulation. Having the

Figure 2: Velocity at breathing level

previously established steady-state flow as an initial condition, transient analysis was run.

The figure below shows the estimated transmission of the cough series. The students with dark red color become very susceptible to flu, and the ones with light red become moderately susceptible. In our result, it took only five minutes for most of the students to be exposed to the germs. (Maybe he coughed too often!) If the sick student were seated at the far left, from which location the flow is actively moving to the right, the time lapse to infection would have been even shorter.

An old but important lesson learned: cover your mouth when coughing!