How to Start Modelling Airflow Ventilation
This is an excerpt. I’ve decided to start publishing knowledge graphs for a better experience for my readers, and mathJax, links and code seem to work easier with Obsidian Publish.
TL;DR
TECHNICAL POAST!
In literature on far UVC efficacy, experimental chambers are controlled with ventilation. Ventilation effects, along with irradiance from Far UVC, are both variables that we need to take into account for its efficacy.
Given that my house is likely to have different ventilation conditions from experimental papers - how can I model pathogen decay with far UVC given my ventilation environment?
This essay tries to answer just the very basic question, given that I only have about 1 year's worth of fluid dynamics experience : How do I get started modelling ventilation with my computer in the first place? This is, like my last post, in line with the spirit of Karnofsky, Minimal-trust investigations.
I'll cover
The point of me writing a guide document liek this
The general Navier-Stokes equations
What reasonable assumptions there are, and how we modify the equations for those assumptions
How to actually build the simulation
OpenFOAM
Construction of the environment
How can I make it easier for others to learn and use?
And some key results
From this really quick investigation, I've realised that due to some sensible assumptions, I've learnt some things about what I might want to take a closer look atL
Far UVC modellers and installers need to pay some attention to corners and walls - because non slip boundary conditions means that the velocity of air flow in proximity to a boundary is reduced, which means that:
Analysis of Far UVC on surfaces feels more relevant
Tools I built for this, and what you will need for this tutorial
Casual Physics Enjoyer, stl-to-openfoam-mesh to build your mesh
Casual Physics Enjoyer, basic-ventilation-model to do a simple laminar, incompressible flow model
Background reading
Explicit models
This is the main paper that I'll be looking to get some sensible assumptions, and a reasonable base model
A more recent modelling attempt that I would consider 'the target' in terms of understanding , and being able to replicate
My Previous Writing on this Topic
Casual Physics Enjoyer, Calculations for Practical Heuristics in Far UVC Use
Casual Physics Enjoyer, Problems Trying to Model Far UVC in a Room
Does Far UVC work?
Fluid dynamics modelling
Versteeg, H., Malalasekera, W - An Introduction to Computational Fluid Dynamics
I think that this is the most relevant book for the questions that we are trying to answer here in terms of the hard core fluid dynamics modelling.
A ground up guide in modelling Navier-Stokes in 2 dimensions.
It scratches an itch of building a toy solver in python that is readable.
I stared at these Jupyter notebooks for a while trying to get some intuition.
A cool model that I used as a reference at times.
Things that I am trying to solve by writing this
Pedagogy on CFD
Problems:
CFD is hard to learn, and probably not accessible in any meaningful way to ground-level installers
And so tools for ways to build easy, practical heuristics are probably are going to be important - see Casual Physics Enjoyer, Problems Trying to Model Far UVC in a Room, Blueprint Biosecurity, Blueprint for Far UVC
A lot of CFD tutorials are on YouTube, with all the problems that come with learning from video - like limited search functionality, going back to key points etc.
OpenFOAM tutorials don't seem to match the standard of clarity and pedagogy in the python machine learning
It feels like modelling ventilation in your own room should be 'easy' - just in terms of general interest purposes, and personal projects - just like how building simple machine learning models also feel easy.
So why do I think writing a specific guide helps?
Well, it was painful for me to figure this stuff out, so at least 1 person (future me) might find it useful.
Writing tutorials out specifically, and explicitly as possible in text so that it is fully replicable, feels like a solid end-state. The jump from theory to practice is hard, but seeing it once helps a lot to make that leap
I learnt computational chemistry with the University of Nottingham a lot quicker when a researcher in the University of Nottingham sent me a super clear, crisp, step by step walkthrough of assumptions and code, one by one
Pedagogy on fluid dynamics
Fluid dynamics is tricky to me mainly because governing equations depend on assumptions, and its really easy to get mixed up on which is which.
So I'm making this document to clarify my thinking, and also help others who are technical but not necessary in the weeds of
I thought it would be useful to explicitly list assumptions, parameters used.
In my last post, ventilation wasn't really considered
I thought it would be useful to familiarise myself with the Navier-Stokes equations in general
I wanted to model the ventilation in my room just generally
I wanted to familiarise myself with how far UVC might be sensitive to different ventilation regimes
The metric used is eACH, which is effective air changes per hour
I also wanted to figure out first principles how to toy model Navier stokes for just airflow
As a minimal trust investigation
In addition, there are the same points that I wanted to get out of minimal trust investigations, which I wrote about in the beginning of Casual Physics Enjoyer, Calculations for Practical Heuristics in Far UVC Use, and was originally mentioned in Karnofsky, Minimal-trust investigations
Specific to ventilation modelling, I wanted to learn more about
What are the actual equations that we need to use?
I had trouble finding an explicit, reasonable presentation that goes step by step
When incompressible vs compressible flow assumptions need to be used
What temperature assumptions needed to be taken into account
When turbulence assumptions need to be taken into account
Do we actually really need to rely on people using complicated closed source CFD software or can we model this in python?
How far might experiments deviate