̽��ֱ�� of Cambridge - Megan Davies Wykes

No ‘safest spot’ to minimise risk of COVID-19 transmission on trains

sc604 — Wed, 22 Jun 2022 04:00:00 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge and Imperial College London, developed a mathematical model to help predict the risk of disease transmission in a train carriage, and found that in the absence of effective ventilation systems, the risk is the same along the entire length of the carriage.

̽��ֱ��model, which was validated with a controlled experiment in a real train carriage, also shows that masks are more effective than social distancing at reducing transmission, especially in trains that are not ventilated with fresh air.

̽��ֱ��results, reported in the journal Indoor Air, demonstrate how challenging it is for individuals to calculate absolute risk, and how important it is for train operators to improve their ventilation systems in order to help keep passengers safe.

Since COVID-19 is airborne, ventilation is vital in reducing transmission. And although COVID-19 restrictions have been lifted in the UK, the government continues to highlight the importance of good ventilation in reducing the risk of transmission of COVID-19, as well as other respiratory infections such as influenza.

“In order to improve ventilation systems, it’s important to understand how airborne diseases spread in certain scenarios, but most models are very basic and can’t make good predictions,” said first author Rick de Kreij, who completed the research while based at Cambridge’s Department of Applied Mathematics and Theoretical Physics. “Most simple models assume the air is fully mixed, but that’s not how it works in real life.

“There are many different factors which can affect the risk of transmission in a train – whether the people in the train are vaccinated, whether they’re wearing masks, how crowded it is, and so on. Any of these factors can change the risk level, which is why we look at relative risk, not absolute risk – it’s a toolbox that we hope will give people an idea of the types of risk for an airborne disease on public transport.”

̽��ֱ��researchers developed a one-dimensional (1D) mathematical model which illustrates how an airborne disease, such as COVID-19, can spread along the length of a train carriage. ̽��ֱ��model is based on a single train carriage with closing doors at either end, although it can be adapted to fit different types of trains, or different types of transport, such as planes or buses.

̽��ֱ��1D model considers the essential physics for transporting airborne contaminants, while still being computationally inexpensive, especially compared to 3D models.

̽��ֱ��model was validated using measurements of controlled carbon dioxide experiments conducted in a full-scale railway carriage, where CO2 levels from participants were measured at several points. ̽��ֱ��evolution of CO2 showed a high degree of overlap with the modelled concentrations.

̽��ֱ��researchers found that air movement is slowest in the middle part of a train carriage. “If an infectious person is in the middle of the carriage, then they’re more likely to infect people than if they were standing at the end of the carriage,” said de Kreij. “However, in a real scenario, people don’t know where an infectious person is, so infection risk is constant no matter where you are in the carriage.”

Many commuter trains in the UK have been manufactured to be as cheap as possible when it comes to passenger comfort – getting the maximum number of seats per carriage. In addition, most commuter trains recirculate air instead of pulling fresh air in from outside, since fresh air has to be either heated or cooled, which is more expensive.

So, if it’s impossible for passengers to know whether they’re sharing a train carriage with an infectious person, what should they do to keep themselves safe? “Space out as much as you reasonably can – physical distancing isn’t the most effective method, but it does work when capacity levels are below 50 percent,” said de Kreij. “And wear a high-quality mask, which will not only protect you from COVID-19, but other common respiratory illnesses.”

̽��ֱ��researchers are now looking to extend their 1D-model into a slightly more complex, yet still energy-efficient, zonal model, where cross-sectional flow is characterised in different zones. ̽��ֱ��model could also be extended to include thermal stratification, which would offer a better understanding of the spread of an airborne contaminant.

̽��ֱ��research was funded in part by the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI).

Reference:
Rick JB de Kreij et al. ‘Modelling disease transmission in a train carriage using a simple 1D-model.’ Indoor Air (2022). DOI: 10.1111/ina.13066

Researchers have demonstrated how airborne diseases such as COVID-19 spread along the length of a train carriage and found that there is no ‘safest spot’ for passengers to minimise the risk of transmission.

We hope this research will give people an idea of the types of risk for an airborne disease on public transport

Rick de Kreij

Seksan Mongkhonkhamsao via Getty Images

Woman wearing a mask on public transport

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Wind more effective than cold air at cooling rooms naturally

sc604 — Wed, 20 Nov 2019 07:30:00 +0000

̽��ֱ��researchers found that a temperature difference between inside and outside has a remarkably small effect on how well a room is ventilated when ventilation is primarily driven by wind. In contrast, wind can increase ventilation rates by as much as 40% above that which is driven by a temperature difference between a room and the outdoors. ̽��ֱ��exact rate of ventilation will depend on the geometry of the room.

̽��ֱ��results, reported in the journal Building and Environment, could be used to help designers and urban planners incorporate natural ventilation principles into their designs so that buildings can be kept at a comfortable temperature while using less energy

Heating and cooling account for a significant proportion of energy use in buildings: in the US, this is as high as 50 per cent. In addition, as global temperatures continue to rise, demand for air conditioning – which emits greenhouse gases – rises as well, creating a damaging feedback loop.

Natural ventilation, which controls indoor temperature without using any mechanical systems, is an alternative to traditional heating and cooling methods, which reduces energy use and greenhouse gas emissions.

“Natural ventilation is a low-energy way to keep buildings at a comfortable temperature, but in order to increase its use, we need simple, accurate models that can respond quickly to changing conditions,” said lead author Dr Megan Davies Wykes from Cambridge’s Department of Engineering.

There are two main types of natural cross-ventilation: wind-driven and buoyancy-driven. Cross-ventilation occurs in rooms that have windows on opposite sides of a room. Wind blowing on a building can result in a high pressure on the windward side and a low pressure at the leeward side, which drives flow across a room, bringing fresh air in from outside and ventilating a room. Ventilation can also be driven by temperature differences between the inside and outside of a room, as incoming air is heated by people or equipment, resulting in a buoyancy-driven flow at a window.

“We’ve all gotten used to having a well-controlled, narrow temperature range in our homes and offices,” said Davies Wykes. “Controlling natural ventilation methods is much more challenging than switching on the heat or the air conditioning, as you need to account for all the variables in a room, like the number of people, the number of computers or other heat-generating equipment, or the strength of the wind.”

In the current study, the researchers used a miniature model room placed inside a flume to recreate the movements of air inside a room when windows are opened in different temperature and wind conditions.

Using the results from lab-based experiments, Davies Wykes and her colleagues built mathematical models to predict how temperature difference between inside and outside affects how well a room is ventilated.

̽��ֱ��researchers found that the rate of ventilation depends less on temperature and more on wind. Anyone who has tried to cool down on a hot night by opening the window will no doubt be familiar with how ineffective this is when there is no wind.

This is because in many rooms, windows are positioned halfway up the wall, and when they are opened, the warm air near the ceiling can’t easily escape. Without the ‘mixing’ effect provided by the wind, the warm air will stay at the ceiling, unless there is another way for it to escape at the top of the room.

“It was surprising that although temperature differences do not have a strong effect on the flow of air through a window, even small temperature differences can matter when trying to ventilate a room,” said Davies Wykes. “If there are no openings near the ceiling of a room, warm indoor air can become trapped near the ceiling and wind is not effective at removing the trapped air.”

̽��ֱ��next steps will be to incorporate the results into building design, making it easier to create well ventilated, low energy buildings.

̽��ֱ��study was part of the MAGIC (Managing Air for Green Inner Cities) project, which is developing computer models for natural ventilation, so that designers can incorporate natural ventilation into city design, reducing demand for energy. ̽��ֱ��MAGIC project is funded by the Engineering and Physical Sciences Research Council (EPSRC).

Reference:
M.S. Davies Wykes et al. ‘ ̽��ֱ��effect of an indoor-outdoor temperature difference on transient cross-ventilation.’ Building and Environment (2019). DOI: 10.1016/j.buildenv.2019.106447

̽��ֱ��effectiveness of non-mechanical, low-energy methods for moderating temperature and humidity has been evaluated in a series of experiments by researchers from the ̽��ֱ�� of Cambridge.

Natural ventilation is a low-energy way to keep buildings at a comfortable temperature, but in order to increase its use, we need simple, accurate models that can respond quickly to changing conditions

Megan Davies Wykes

Natural ventilation experiment

Megan Davies Wykes and El Khansaa Chahour

A laboratory experiment of a cross-ventilated room (side view)

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How landscapes and landforms ‘remember’ or ‘forget’ their initial formations

sc604 — Fri, 27 Apr 2018 12:52:44 +0000

Crescent dunes and meandering rivers can ‘forget’ their initial shapes as they are carved and reshaped by wind and water while other landforms keep a memory of their past shape, suggests new research.

“Asking how these natural sculptures come to be is more than mere curiosity because locked in their shapes are clues to the history of an environment,” said Leif Ristroph from New York ̽��ֱ�� and the senior author of the paper, which is published in the journal Physical Review Fluids. “We found that some shapes keep a ‘memory’ of their starting conditions as they develop while others ‘forget’ the past entirely and take on new forms.” This understanding is important in geological dating and in understanding how landscapes form.

Shape ‘memory’ and its ‘loss’—or the retention of or departure from earlier formations—are key issues in geomorphology, the field of study that tries to explain landforms and the developing face of the Earth and other celestial surfaces. ̽��ֱ��morphology, or shape of a landscape, is the first and most direct clue into its history and serves as a scientific window for a range of questions—such as inferring flowing water on Mars in the past as well as present-day erosion channels and river islands.

“ ̽��ֱ��answer to the question ‘What’s in a shape?’ hinges on this memory property,” said first author Dr Megan Davies Wykes, a postdoctoral researcher in Cambridge’s Department of Applied Mathematics and Theoretical Physics, who completed the work while she was based at NYU.

To shed light on these phenomena, the researchers replicated nature’s dissolvable minerals—such as limestone—with a ready-made stand-in: pieces of hard candy. Specifically, they sought to understand how the candy dissolved to take different forms when placed in water.

To mimic different environmental conditions, they cast the candy into different initial shapes, which led to different flow conditions as the surface dissolved. Their results showed that when the candy dissolved most strongly from its lower surface, it tended to retain its overall shape—reflecting a near-perfect memory. By contrast, when dissolved from its upper surface, the candy tended to erase or ‘forget’ any given initial shape and form an upward spike structure.

̽��ֱ��key difference, the team found, is the type of water flow that ‘licks’ and reshapes the candy. Turbulent flows on the underside tend to dissolve the candy at a uniform rate and thus preserve the shape. ̽��ֱ��smooth flow on an upper surface, however, carries the dissolved material from one location to the next, which changes the dissolving rate and leads to changes in shape.

“Candy in water may seem like a far cry from geology, but there are in fact whole landscapes carved from minerals dissolving in water, their shapes revealed later when the water table recedes,” said Ristroph. “Caves, sinkholes, stone pillars and other types of craggy terrain are born this way.”

Reference:
Megan S. Davies Wykes et al. ‘Self-sculpting of a dissolvable body due to gravitational convection.’ Physical Review Fluids (2018). DOI: 10.1103/PhysRevFluids.3.043801

Adapted from an NYU press release.

Video: Side-view photograph of candy body (initially a sphere). ̽��ֱ��upper surface remains smooth while the undersurface becomes pitted and dissolves several times faster.

Laboratory findings point to what affects the development of nature’s shapes.

̽��ֱ��answer to the question ‘What’s in a shape?’ hinges on this memory property.

Megan Davies Wykes

Dissolving candy

Chiara Ferroni on Unsplash

Sea of dunes

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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