We use computational fluid dynamics to illustrate the areas in your workplace that are being ventilated less effectively (shown in blue in the image above), and which can be improved by desk or floor fans placed in specific locations.

Computational fluid dynamics also allows us to show the paths the air takes within the workplace. This allows bespoke distributed seating plans to be designed which minimise the amount of air paths between seated desks.

## Our Process:

#### Preliminary Analysis

###### 1. Discuss

We listen to your motivation for understanding and improving your workplace ventilation and confirm that you understand the benefits of other transmission mitigations.

###### 2. Visit

We visit your premises to measure the air flow from air conditioning and windows, take 3D scans of the workspaces, and confirm our understanding of your requirements.

###### 3. Report

We calculate and report ACH values for the workplaces, provide initial guidance for each workplace, and recommendations for each workplace for further computational fluid dynamics analysis.

#### Computational Fluid Dynamics Analysis

###### 4. Simulate

We build computational fluid dynamics models for the workplaces chosen for further analysis from the preliminary analysis to show the ventilation effectiveness, air trajectories and the effects of any office modifications you may be considering, such as fans, desk separators and air cleaning/filtration devices.

We use the computational fluid dynamics model results to advise on the performance of workplace modifications you are considering and recommend changes to air conditioning, location of fans and desk separators, and distanced desk seating plans.

## The Science:

#### Aerosols and Airborne Transmission

Liquid particles of saliva and other biological substances are emitted from the mouth and nose during breathing, talking, singing, coughing, sneezing and other activities that involve the emission of gas. These particles have a distribution of sizes, with larger particles more prominent in the more violent coughing and sneezing events. The coronavirus may be present in both large and small particles. The time it takes for these particles to settle out of still air is strongly dependant on their size: larger particles of 100 microns diameter (a human hair is around 100 microns thick, on average) can settle to the ground within 2 metres in a few seconds, whereas particles smaller than 5 microns in diameter can take hours to settle to the ground. These particles smaller than 5 microns form an aerosol and can remain suspended in the surrounding air for a significant amount of time, due to the gravitational force on their mass being smaller than or equivalent to the drag force exerted when they move through the air.

The suspension and subsequent inhalation or deposition on the face and eyes of these aerosols constitutes an airborne transmission route for the coronavirus. This airborne route has been gaining prominence as a significant contributor to the spread of the coronavirus [1, 2, 3, 4]. This route is of particular importance in enclosed spaces, where the aerosol may remain for a significant amount of time, sufficient to interact with people in the space. The ventilation of enclosed spaces, including offices and other workplaces, is an important aspect of reducing the risk of coronavirus transmission.

#### Ventilation

Ventilating enclosed spaces with fresh air is a UK government recommendation for reducing the risk of coronavirus transmission in the workplace. The key aspect is the removal of stagnant air which may contain virus containing aerosol particles with fresh uncontaminated air. In order to quantify the level of such ventilation, the number of times the volume of the air in the space is replaced, or changed, is used. This “air changes per hour” or ACH describers how many times per hour the volume of air is replaced. For example, in a 120 cubic metre office space with air conditioning units which supply fresh air at a rate of 0.1 cubic metres per second, the volume of air in the room will be replaced every 1200 seconds, or three times every hour, giving an ACH of 3.

The guidelines for hospitals in the UK is to have and ACH of between 6 and 12 depending on the setting. Aircraft have and ACH of between 12 and 20, and Eurostar trains have an ACH of around 4. Increasing the ACH of your workplace is a good target for transmission risk reduction.

#### Computational Fluid Dynamics Modelling

The simple calculations of ACH for the whole space assume that the supply of fresh air, often a mechanical air conditioning unit or system, is able to distribute the fresh air uniformly throughout the volume of your workplace. In reality there will regions of the workplace that receive less fresh air than others, and so the ACH value will vary with location in the workplace. Computational fluid dynamics can be used to show how the ventilation effectiveness varies within a workplace. A key variable is the age of the air within the workplace, which is the amount of time taken for the air to reach each position in the space.

The age of air at all points in the workplace, $$\tau_i$$, can be compared to the workplace average age, $$\tau_n$$, which is related to the ACH value: $$\tau_n=3600$$ACH, where the age of air is measured in seconds. $$\tau_i$$ can be calculated from the air velocity values generated using computational fluid dynamics [4]. The ventilation effectiveness, $$\epsilon$$, can then be calculated for the whole workplace, showing the areas which are better or worse ventilated than the average:
$$\epsilon = \frac{\tau_n}{\tau_i}$$
Where $$\epsilon>1$$ the ventilation effectiveness is better than the average, and where $$\epsilon<1$$ the ventilation effectiveness is worse than the average. The image top left of this page shows the areas in blue where the ventilation effectiveness is worse than the average. These areas can be improved using local desk or floor mounted fans.

The calculation of the local age of air, $$\tau_i$$ using our computational fluid dynamics approach is compared to measured values in a simple room model below, showing the level of agreement achieved.

A simple room geometry with a single air inlet and outlet, used to make experimental measurements [5] of the age of the air along the two lines.

Comparison of the age of air from the experiment and from computational fluid dynamics along line 1.

Comparison of the age of air from the experiment and from computational fluid dynamics along line 2.

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