Articles
Professor Andy Woods, head of the BP Institute, based at the University of Cambridge
For the past 15 years the BP Institute (BPI) has been examining many complex and challenging flow problems, from measurements of surface adhesion to large-scale models of ocean mixing. The fundamental experiments we carry out at the BPI laboratories are for a diverse range of industries, from oil to printing and paint.
BPI researchers are currently working with the NHS to better understand the dispersion of particles around people in hospitals to improve the design of ventilation systems and ultimately try to prevent the spread of disease.
We have modelled a corridor with two ward rooms off to the side. A “person” moving up and down the corridor produces a turbulent wake behind them. When we inject dyed water into the corridor, we can see how the microbe particles it represents are going to spread as someone walks through it.
It is just as if a person is walking through a cloud of airborne water droplets and bacteria left by someone who has coughed, or that it is being piped into the corridor from a ventilation system.
As the researchers inject the dyed water it sweeps around the person and creates a wake that drags the particles along behind them. The key question is what is the settling time of these airborne particles compared to the distance they travel and the fall speed as a result of getting captured in somebody’s wake? With multiple people walking up and down a corridor you can imagine it spreads quite effectively.
We take a series of high-res photos and calibrate the light intensity of the dye. From the multiple pictures we take a time average and get a statistical picture about the mean spreading rate. This is all quantified to get a dispersal coefficient. We can then use that to start quantifying the data in models.
Ultimately, you might want to build a series of models to be able to understand how different frequencies of people moving along the corridor will spread airborne particulates. What we have found is that the particulates may spread up to tens of metres before they settle, depending on the size of the particles and the frequency and speed of people walking down the corridor.
The assertion is that a ventilation system can manage these airborne particles by replacing the air. However, the air changes per second will be much slower than the wake behind the person. What this means is that people moving around are going to mix up contaminants and spread them out very effectively.
It changes the way you think about where bugs are and how best to optimise your ventilation system. It also may change the protocols about how people move around and how you isolate air in different parts of a hospital. This is of particular interest in operating theatres, which have extremely high-speed ventilation systems. BPI is working with the NHS and the Royal College of Surgeons who are very interested in the impact of people movement on airborne particles in such environments.
Ventilation costs in hospitals are quite high, particularly in operating theatres, and one of the questions we will help to answer is whether the systems are truly effective in managing the spread of airborne particulates and bacteria.
We are building more complex models to understand this better, as well as working with surgeons, who could impose new protocols or evolve the design of more effective ventilation systems.