Thread by @JoshuaCAgar

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.

In airconditioned areas, the temperature differential between the cool air outside the face shield and the warm air beneath the face shield due to the heat radiating from our body will induce flow towards the warm region, most likely, to what's beneath the face shield.

Transport equations determine how a passive scalar (pollution, particulates, energy) is transported with a given velocity field.

Negative pressure regions will be the place where most of the passive scalar (virus) will be deposited, in essence, beneath the face shield.

I remember presenting a journal in the CE 257 class under Doc Jaime about Computational Fluid Dynamics and Transport Equations.

Slide:

drive.google.com/file/d/1wLdwHV…

Here I am trying in vain to discuss some basic principles in fluid dynamics, with the given limited materials.

Conservation of Mass

Conservation of Mass (pt.2)

Conservation of Mass (faceshield included).

Clarification: By introducing pressure to the container, I was reducing the volume of the container, resulting to outflow.

Conservation of Momentum

Conservation of Momentum (2)

Transport Equation

In bluff-body aerodynamics, the bluff body often causes flow separation, which are divided into two: strong separation and weak separation.

Strong separation is often found in the windward side. Flow separation causes negative pressure gradient, resulting to reverse flows.

On the other hand, weak separation naturally occurs at the leeward side. Both types of flow separation causes reverse flows, sometimes resulting to flow recirculation.

The advection term (acceleration across space) across the flow recirculation is relatively lower at this point. In the transport equations, since the advection term also amounts to the convection, it also indicates the accumulation of the passive scalar at these regions.

These passive scalars can either be moisture, pollution, heat, energy, and other particulates, including viruses.

Diffusion, on the other hand, is mainly controlled by the fluid's viscosity. The more viscous the fluid is, the higher the diffusion.

Transport equation in a nutshell is:

Change in amount of passive scalar = Transport due to Convection + Diffusion

Face shields encourages flow recirculation beneath it, resulting to higher tendencies for the particulates to retain and accumulate there.

Especially kapag galing sa likod ang direction ng flow ng hangin...

I just did a CFD analysis to prove the point.

The picture shows the pressure field across the domain. The blue cylinder represents the person. The face shield is also present in the model. A negative pressure region will be present beneath the face shield. Flow velocity is 1 m/s

Plotting the streamlines also reveals the existing reverse flows and recirculations beneath the face shield. There are higher tendencies for the passive scalar to accumulate and be deposited in these regions.

Reverse flows and Recirculations also occur from above and below.

Simulation Details:

Finite Volume Size: 0.1 mm^2
Number of Elements: 2.7 million
RAM allocation: 9 gb
Turbulence Model: SST k-omega model (RANS)
Flow velocity: 1 m/s

Maybe, I'll also try to simulate the same in OpenFOAM with the advantage of using the transport equations.

Demonstration of reverse flows

Demonstration: Flow recirculation

Demonstration with aerosols.

I hope na malinaw and nakikita ang pagrecirculate ng aerosol to the region behind ng face shield.