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Joshua C. Agar

6 Sep, 5 tweets, 2 min read

Sideview and the Topview of the previous case shown earlier of a person wearing a face shield.

Evident in the top view of the velocity field, vortices and reverse flows, typically present behind the head are now present in the face due to the intervention of face shields.

No face shield scenario:

[Close-up; Top View] With Face Shield Scenario

Due to the negative pressure due to the massive change in momentum caused by the face shield, flow reversal happens where the wind flow (along with the suspended particles) is redirected to the space bet. the face & the face shield.

The differing angles of attacks are also considered. Below is the simulation results of a cough coming from an angle of 45 degrees wrt to the front of the person not wearing a face shield. Evidently, the accumulation and deposition happened in the leeward side.

With Face Shield

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Joshua C. Agar

@JoshuaCAgar

9 Sep

Comparison of the wind flow of a person wearing a face shield (top panels) and a person not wearing face shields (bottom panels). [Tested a bit of stream tracing to determine the motion of particles]

Evidently, the particles traced experienced massive loss of momentum due to face shields (from red to blue). The loss of momentum led to increase in concentration. Also noted were the persistence of eddies (unsteady vortices).

[Velocity Field].

Note that airborne particles tend to go from regions of high flow energy to regions were less flow energy. The low-energy region initially is situated at the back of the head. Introducing the face shield situates the said region near the face.

Read 10 tweets

Joshua C. Agar

@JoshuaCAgar

8 Sep

1. The face shield (used by medical professional on top of PPEs) in that study are different from the face shields that Filipinos are using.

2. The virus and the droplets evidently still lingered in the air pointed out by this study. (Probably waiting to be sucked by the suction mechanism brought by a person moving with face shields on)

3. Bartels et al. (2021) pointed out that even though barriers and shields protect people at initial impact of the emission, the droplets will linger for some time due to the constriction brought by the barriers/shields. The study also observed slow settling velocities.

Read 8 tweets

Joshua C. Agar

@JoshuaCAgar

22 Jun

Re: face shields

As a researcher who simulate wind flows, I am puzzled with the insistence on face shields.

Face shields causes intake of air to the regions beneath the shield to induce higher negative pressures.

Higher negative pressure, higher particulate transport.

Conservation of Mass:
Flow in = Flow Out

Flow = Area x Flow Velocity

Constant flow can mean:
Area constriction due to face shields leads to higher flow velocity (intake).

Conservation of momentum:
Higher flow velocity -> Higher velocity pressure (either positive if outflow or negative if inflow).

Even without respiration, if a wind flows to the face shield, a negative pressure region will be present beneath.

Read 26 tweets

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