Great piece explaining why even cloth masks do *help* slow and prevent #COVID19.
2/ BTW, the science behind this — why even a cloth mask helps stop the spread of #coronavirus — is related to how NASA’s Viking spacecraft landed on Mars in 1975. Exactly the same fluid dynamics principle stopped the cratering of soil under the Mars landers as stops coronavirus. 
3/ These were stationary landers with a robotic arm to get soil samples. They had no wheels, so wherever they landed, that was where the science would happen. They could not drive away from the rocket exhaust blast effects of landing to a more pristine location. (Mockup photo)
4/ Misdion planners were worried the rocket thrusters for landing would disrupt the soil too much and ruin the science, so they did lots of experiments (similar these experiments I ran several years ago) to understand the cratering effect.
5/ What they discovered is they could greatly reduce the cratering by making a simple change to the rocket thruster. Basically, they put a porous “mask” over the nozzle. Instead of one big opening, the nozzle had lots of small openings, analogous to the openings in a cloth mask.
6/ The amount of gas that blew out of the thruster was *not* reduced by having many small nozzles. The speed that the gas blew out was also *not* reduced. So how could forcing the gas to go through many smaller openings make a beneficial difference?
7/ The answer: because breaking the jet into many smaller jets increases the amount of surface area on this jets. By having more surface area, they mix into the surrounding atmosphere more quickly, causing the jet to be much shorter!
8/ Here is a simulation of a jet aircraft engine’s exhaust jet mixing into the surrounding air. It has nice Mach diamonds at the exit, but turbulence breaks it apart and diffuses the momentum after a short distance. (Source: researchgate.net/profile/Juan_P…)
9/ The Viking engineers discovered “putting a mask” over the thrusters would force the gas into many tiny jets that diffuse into the atmosphere at a shorter distance, thus delivering less high speed gas down to the Martian soil. This is the same reason a mask stops coronavirus.
10/ This simulation shows how far a the organized jet flow from a cough travels (without a mask) before it mixes in the surrounding atmosphere. After that the turbulent mixing diffuses the momentum, slowing it and finally stopping it. (Source: hxgnspotlight.com/simulations-un…)
11/ The reason a simple cloth mask helps compared to no mask at all is because the one big jet is broken down into many tiny jets, each jet the diameter of the spacing between threads in the woven cloth. This increases the turbulence.(Source: Todd Simpson/flickr.com/photos/zeissmi…)
12/12 Ethan’s article includes schlieren photography of breathing & coughing behind a mask to visualize how the gas flow comes out from the mask. No big jet = no large viral load transmitted at a distance, and no big crater in Martian soil. Mask = good. forbes.com/sites/startswi…
