On item 4.
To be consistent with this principle, will everyone in the USA also stop driving their petroleum guzzling automobiles, turn off all air conditioning, stop using gas or fuel barbecues, and turn the house heating off unless it's guaranteed 100% from solar, wind or nuclear power ?

Because every single time each person chooses (it is a choice every time) to do any of those things they are choosing to increase the risk of health damage and death to countless numbers of people, whether from respiratory illnesses such as Athma, COPD, etc etc, before even getting to the global long term damage to those in other countries and continents due to increased drought, flooding, etc etc. And those actions are most probably harming a lot more people and are cumulative, not transitory.

Sure, but this thread is about COVID, and managing social distancing practices while cycling or running outdoors.

Discussion of how large populations tend to manage the entire spectrum of all different kinds of risk is expanding the scope of the thread considerably.

It seems like my public health authorities (I live in Ontario) are being very reasonable with respect to citizens exercising outdoors and giving us guideline on what to do when inside 6ft. They are not asking us to stop exercising outdoors and they are not asking us to wear masks while exercising outdoors. They are asking us to pass quickly regardless of form of locomotion if we have to be inside 6 ft. They are allowing this and they are allowing low risk systems in pools. I wanted to share what is going on here. People can decide what to do with that info.

Most people who came on this thread are being quite reasonable in how they seem to behaving outdoor cycling. The OP perhaps has ultra low risk tolerance and you appeared to imply that I would be letting my guard down if I followed my local public health guideline which don't ask me to exercise masked outdoors while making a pass. I am just trying to share that we have different public health guidelines around the world. There is no surge in the pandemic in India for example. Everything there is declining week over week. But the virus would spread exactly the same way outdoor cycling or jogging there as in Canada. So I get there is a full spectrum of public health guidelines around the world. I am obeying all of ours to the letter.

I recently read a study that measured the efficacy of face masks for filtering expelled droplets during speech. The N95 without vent) performed the best, followed by a surgical mask, followed by a vented N95. Multi-layer cotton or poly masks were next. They also tested a buff, and it performed worse than the control (no mask at all). Their theory was the single layer of fabric broke the large droplets into smaller droplets that still shot through the buff. And smaller droplets could potentially be worse, because they stay airborne longer. https://advances.sciencemag.org/content/6/36/eabd3083

Some of the buffs i have are double layer and some are single. If you are considering a buff to lower the chances you spread covid, you might want to make sure yours is multi level.

TLDR in ST terms:
N95 is like Silca Super Secret
Surgical Mask is Squirt
Cotton/poly multi layer masks are like R&R Gold, Finish Line Ceramic, NFS, etc
Single Layer Buff is like White Lightning Epic Ride

I find it surprising that of the dozen of aero-/ fluid-dynamics experts on this forum, only you have an opinion on the topic.

One would think that they'd have some great insight and I certainly would rest easier if they had good data.
They may not want to venture into the particles aspect of physics, but their scientific expertise on the airflow under the conditions mentioned may be of value.

There is the controversial paper published by Bert Blocken at Eindhoven University way back last year (which you are probably familiar with).
It was highly criticized for its incomplete approach (rightfully so), isn't peer reviewed yet, but afaik the data themselves were never challenged. It is also widely cited by peer reviewed work.

An easy digestible write up of the outrage and link to the complete paper can be found here:
https://www.wired.com/...-spreading-covid-19/

Anyways, just fulfilling some requests for information and found easiest to post here to close the circle on my end.
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Glad you got to play the part of a vengeful hiring manager.

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Next races on the schedule: none at the moment

Fluid mechanics has many subtopics in which people might specialize, turbulence being one of them. All fluid mechanics experts obviously know of laminar and turbulent flow, but not all have studied the theory of turbulence modeling. Also, there is not much to gain from jumping into the mud, but I like to clarify things when I feel that I can do so, particularly here since the Slowtwitch crowd is probably as mature as one can expect for an online forum (it’s not the garbage pile le***un.com is).

Sometimes, studies from non-specialists have a glaring conceptual error, such as failing to consider viral loads in this case, by their own admission. The authors have done work involving pollutants, but the questions they might have answered are likely posed more like “Where might this industrial exhaust plume affect the surroundings?” versus “What is the extent of a time-averaged boundary about a person performing an outdoor activity, such that outside the boundary, the viral concentration is less than some critical value (assuming x,y,...)?” A careful reader will catch that a time-averaged turbulent flow field never exists, in that form, in real life, particularly in the presence of nearby objects. We can only speak statistically about the probable concentration of an incorrect flow field. For example, we can’t simulate one bicycle rider and pretend that another rider in their wake is merely immersed in that flow, and does not contribute to an entirely different flow field.

Looking at the videos they released, these are clearly not high fidelity simulations, or the Stokes number of those particles is so high they don’t follow the flow field very much.

In short, they may have answered (very crudely) where air from the person breathing goes, but not the viral concentrations. If anything, one does these sorts of things before attempting a (significantly) more costly set of high-fidelity simulations, which may explain why it was not published to begin with.

(posted from phone)

I can't believe you're bringing up this infamous article. You do realize it's Exhibit A in the dangers of anti-scientific hype? This is an infamous news article and brief series of tweets (note: not a study, not peer reviewed) published in the very early witch-hunting days of the pandemic by a person who is a civil engineer (note: not a virologist) that has been demolished repeatedly by people who actually know what they're talking about (epidemiologists and virologists). This is a news article that, in the intense backlash that immediately followed, the author himself begged journalists to actually read rather than deliberately misinterpret in service of a tasty headline... advice they of course irresponsibly ignored. An article about which this was said a few days later:

"I showed Blocken's research to William Hanage, an epidemiologist at Harvard's Center for Communicable Disease Dynamics. He said that the virality of Blocken's research is harmful, and that Blocken's suggestion in the white paper that this research is a "modest contribution" toward the fight against Covid-19 "makes my blood boil."
https://www.vice.com/en/article/v74az9/the-viral-study-about-runners-spreading-coronavirus-is-not-actually-a-study


Note that this article that has disappeared from the press over the last six months because click-seeking journalists finally got it through their skulls that it's (again) NOT. A. STUDY. Despite all of that, this bullshit STILL persists because it it had some cool looking graphics and neatly fulfills some people's prejudices and fears. Stop. It.

Well this thread has gone off on its own direction, but I'll address different point.

Regardless of covid, it isn't cool to be passed with < 3' of space. If I can touch you, you're too close.

I'd just start hollering something like "more space please!"

Thank You, appreciate the great response.
Looking forward to decreased granular data on this.

I think we have the smarts and experts to figure out better what our risks are, at least when it comes to physics and chemistry.
The more we know, the better we can make decisions on how much risk we are willing to take, or knowing defined safety margins.

It’s not like each of us needs to experiment to find out under what conditions soft tissue stepping off a curb will be at huge risk of being crushed by two tons of metal.
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Exactly what I was getting ready to type. Glad I saw your post before repeating. Nicely said if I say so myself!

Anecdotally, I think the covid risk is also very low, while cycling outdoors, and the risk of something else happening (crash, etc.) is a far greater risk.

My area had most, but not all group rides stop for a couple months, and then when our area opened up in early summer, group rides started and haven't stopped. Some of these have 100 plus riders. Of the very few cyclists that I'm aware of having covid, all have traced it to something unrelated to cycling.

As it relates to the Grand Tour riders and not getting covid, I think that also shows it doesn't easily spread in a tightly spaced, but outdoor peloton. I was actually riding recently with a grand tour rider on a group ride and brought up covid. He said most of the peloton is afraid of getting *any* cold (and have v been this way well before covid) , especially the razor thin riders, so most carry hand sanitizer wherever they go. I joked they were trained well before the pandemic hit.

But then you are laid up in hospital with an immune system weakened by your crash. You will probably catch Covid-19 and die. Ride inside.

[Post sponsored by Zwift.]

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If you're riding with a true roadie double pace line it's common to be bumping elbows side by side from time to time. That's the kind of close quarters crit.riders see regularly and prepare to deal with it.

If I am out riding by myself, I haven’t signed up for a double pace line or other race-like or spreader scenario, nor do I welcome or seek this kind of thrill with others of unknown skills/health.

In fact, I may get startled and you may end up in the path of a car that pitches you at me, and either me or both of us end up hurt badly. Yes, this happens.

There are many good reasons unnecessary close and unannounced passes of solo riders are a bad idea, and now one more is called ‘COVID’.

OP was talking about solo riding and getting passed by an outside group. That’s what I’m referring to.

Inside your own group is obviously different.

FWIW we swam open water all summer. Generally speaking, no masks on the beach before starting. About 5-10 mins milling around before getting in. Warm up swim, meet back at buoy. No one from our group has contracted covid from this. Outdoors sports seems to be one of the "covid-safe" activities. Something like 80% of the spread in the states right now is from indoor social activities like restaurants and bars. I'm sure it's not IMPOSSIBLE to get covid when participating in outdoor sporting events, but at this point it seems safer than even going about daily life at the grocery store, and certainly safer than my flight training as we generally don't wear masks in the cockpit (no one can understand you on the radio, and my glasses fog up which is obviously a problem when flying).

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"The person on top of the mountain didn't fall there." - unkown

also rule 5