To celebrate a new collaboration with @LukasArnoldFire @wolfris @vigne_gabriele on visibility in smoke, I would like to summarize some stuff we did learn about it so far.
🔥☁️🔥☁️🔥

The thread 👇is largely based on the ivbs workshop talk I gave one year ago in Duisburg, Germany
[2/12] If you plot the ‘visibility in smoke’ in a CFD analysis, it actually shows the local values of the parameter and not a measure of how far you can see in the smoke. The latter would be true only if the whole room was uniformly filled with the smoke of that concentration!
[3/12] In fact, the ‘visibility’ model is a clever way to plot the mass conc of soot in a way you can explain to laymen. Different fields use various models (usually based on a century-old Koschmieder’s relation), and for fire smoke, we use empirical derivation by T. Jin (1970's)
[4/12] What is kind of interesting is that this research was done way before other FSE milestones... Before zone modelling, before quantifying specific mass ext. coefficient (a critical element of that model!) and definitely before age of CFD.

I'm not sure if Jin meant that...
[5/12] So is ‘visibility in smoke’ really the distance at which you can see?

No, not really. It depends on the ambient lighting conditions and the contrast between the target and background. T.Jin has recognized that, but for some reason today it is rarely taken to account...
[6/12] A thing you must be aware, is that visibility is not linear with the density of smoke. In fact, in the interesting range (10-20 m) a tiny change in smoke concentration makes a huge change in the visibility results. You see that best by plotting scales next to each other.
[7/12] Another interesting aspect – visibility in light with different colours will differ (a lot). As we use the universal value of specific mass ext. coefficient in our calculations (at 658 nm – red light) we do not include this effect in engineering. Maybe we should?
[8/12] On a sidetone - this is one of the reasons why the sky in California turned red'ish. Smoke particles are just the right size to absorb and scatter green/blue light better than red, leading to this post-apocalyptic scenery.
[9/12] More things that matter: vis is calculated from the smoke conc, so nothing matters more than the amount of smoke you release in the model. In usual fire CFD, you control that through the heat of combustion, efficiency and most importantly – the soot yield coefficient (Y_s)
[10/12] The visibility (and ASET) change with the soot yield in a hyperbolic trend, and through experiments and simulations, with @vigne_gabriele we have found a safe value at Ys = 0.1 g/g that can be used when you don’t know exact value for your materials/circumstances
[11/12] And finally, visibility is not the ultimate measure of safety. There are cases where ‘failed’ visibility is acceptable, and ones where ‘good value’ is a failure. Smoke flow is three-dimensional and must be analysed taking the architecture and circumstances into account.
[12/12] If you did like this thread, there is this 2017 FSMF UK talk I had on this subject, covering these practical aspects in more details:


There is also the IAFSS paper with @vigne_gabriele on the impact the soot yield has:
sciencedirect.com/science/articl…
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