Smoke, Smoke, Everywhere?
This has the possibility to be my most unpopular blog ever, which I’m ok with. The topic of this blog is smoking, specifically the second hand kind. Let me start by saying I am not a smoker. I don’t care for the smell, and am happy that it is not allowed in most building, Vegas casino’s being the main exception. My mom was for a long time though, and in general it seems to me that smokers have become the easy target. They are currently a voting minority so it’s easy to impose laws that take away some of their rights, which I think is wrong.
Let me clarify, in California, a lot of municipalities have 25 ft bans on smoking near doorways or windows. Ok, I can see that, but why 25 ft? Who came up with that distance? Did anyone study how smoke diffuses in the air and figure that after 25 ft the smoke has diffused to a concentration low enough to not cause cancer? I haven’t been able to find it. But ok, I understand that if I had an office window that happened to be by the local smokers spot, I would get annoyed by the daily smell of smoke, but would be forced to stay there so I could complete my job.
Then California in 2010 tried to ban smoking in State parks and beaches. Ok, I can understand that cigarettes may lead to forest fires and litter issues, but it was veto’ed by the Governor Schwarzenegger. Currently many municipalities in California have their own bans, like in San Jose, where smoking in public parks is completely banned. This is where I thought it would be good to analyze the problem in FloEFD, Mentor Graphics general purpose CFD software.
The purpose of this study is to see what type of 2nd hand smoke concentrations an innocent bystander might be expected to experience if someone nearby was smoking. First I needed a CAD model of the park, as FloEFD is a CAD embedded analysis software. Usually as a design engineer, the CAD is the easy part, as it has already been build. For me, that is my toughest bit as I usually don’t have CAD to start with. Being CAD lazy, I found a free CAD model of a skate park on www.grabcad.com. It isn’t the trees, grass and hills I was picturing when I started this, but it is a public park so I went with it. With that, I put in a couple CAD people, one to be the smoker, and 2 to be the potential second hand smokers.
Now I needed to come up with the analysis settings. I could find breathing rates and how much air a person breathes (~ 0.5 ft3/min), so I used that as boundary conditions on their nostrils. I assumed the smoker was exhaling 100% smoke, for every breath he took. This was highly unrealistic in my opinion, but as I couldn’t find a percentage number to put on this, I had to use a conservative number. Now, I wanted to capture any buoyancy effects, so I had the exhaled smoke breath coming out at 37 degC, while my outside air temperature was 20 degC. For the mesh setup, I didn’t know where the smoke would go, but knew i needed a finer mesh in the smoke cloud to get accurate concentration results. As such, I enabled FloEFD’s solution adaptive mesher, which will periodically pause the solve, go through the results, and add mesh where it’s needed. Very handy indeed.
Lastly, I modeled the wind. I also modeled a no wind condition, but then the hotter smoke just go upwards and never did get anywhere near my other park goers. See the figure below of an isosurface showing everywhere that the smoke is 1e-6 in concentration (1 part per million), and an animation of that smoke cloud vs time when he starts smoking and puffing. Basically the smoker is covered in a cloud of stinky smoke, likely why all cloths smell like smoke when you go to a Las Vegas casino or bar. It seems like being a couple feet away from him, you would be fairly safe, well maybe not your shoes, but still 25 feet away seems very arbitrary distance considering these results.
Looking at the streamlines, you can see the contrast in forces on the smoke. Here, it’s clear that because the smoke was exhaled through his nose, it has a lot of downward momentum before any buoyancy forces can take over and bring the smoke back upwards. Had I analyzed using the mouth as my boundary condition, the results might be different. But, it the grand scheme of things, how often is the air completely still for an extended period of time. Not that likely, so I didn’t bother running a mouth breathing case.
For my wind cases, I didn’t want to have a really stiff breeze, because all that fresh wind air would dilute my smoke, yet I wanted to have enough wind to push it towards my non-smokers. I wanted to model the worst case wind, so I chose ~3 m/s. If you read my crawlspace moisture blog, you know that 1 m/s is the bare minimum a person could feel, so 3 m/s seemed like a reasonably average breeze.
Below are my angled wind results, where I directed the wind to make the one bystander right in the smoker’s wake. You can see he is right in the smoke area, but the concentration is very small. Again, I haven’t seen at what concentration of smoke causes cancer (like they provide for mercury or any other carcinogen), so I thought 1e-6 seemed like a very small amount (1e-6 volume fraction of smoke = 1 ppm, so very low). Also, again, I assumed 100% of the exhaled breath was smoke, so this is the worst possible 1 ppm smoke cloud and in reality would be smaller.
Looking at a contour plot of smoke concentration from 100 ppm to 1 ppm, you can see while he is in the smoke zone, he is in the low concentration area. How low, well I tracked the air entering his nose, and the table below shows he was getting an average of 8.9 ppm of smoke. What I see though, is the smoke zone is about 6 feet wide. I would just move out of that area to the clean air if I smelled smoke. Our other non-smoker is completely clear of any smoke. Of course, if you were “dropping in” to that bowl or doing any “ally’s” on that rail, you would be in the eye of the smoke cloud.
On the other hand, if we look at a contour plot of smoke concentration from 1000 ppm to 0 ppm, we can see that in the “smoke cloud” the concentration stays relatively high. I was surprised by this, as our skate park has a lot of ramps and things to create turbulence, which I thought would mix in a lot of fresh air and drop the concentration levels. In fact, looking at my measurement of distance from the smoker’s nose to the non-smokers nose, we can see they are about 40 feet apart, and the concentration in the smoke cloud makes it well past the non-smoker. So a 25 foot ban on smoking near doors and windows seems a little too low after looking at this (again, this is worst case with 100% of exhaled air being smoke). The smoke cloud is a local effect, but it propagates a long way down stream.
To conclude, I don’t have any specific conclusions. I just saw an issue that was perfect for CFD to look into, and FloEFD helped me answer these questions in a minimum amount of time. As is usually the case when it comes to fluid dynamics, usually I have an idea of the answer at the beginning, but the results are rarely what I anticipated, even after years of doing this. In the end I think I showed both sides of the argument, smoke does travel a fair amount of distance, but it doesn’t blanket and poison the entire park. Should it be banned from all city parks and beaches? I don’t know if my results would swing the argument one way or the other. The X factor is the amount of smoke concentration that repeated exposer to would cause cancer, which medical science needs to answer first.
Next blog will be something lighter, aerodynamics of hockey pucks