by
lmh » Wed Dec 18, 2019 12:29 pm
Imagine the end of the school day, when a large number of kids are released into the street simultaneously, and start to walk away.
On an average street full of bus-stops, fire-hydrants, litter-bins, lamp-posts and street-signs (an average column full of particles) some kids will, by chance, go fairly straight, while others will get caught up circling round street furniture. That's A, the eddy-diffusion bit.
As the kids walk along, they will tend to spread out along the street. The longer you leave them, the more they'll spread. If they're marched along at a fairly rapid rate, they may get quite some distance from the school while still being a coherent bunch. If you let them dawdle and drift, and take twice as long to cover a street-length on average, you'll probably find that they will spread out twice as far along the street. That's B, the straightforward diffusion bit.
If the street is lined with sweetshops and newsagents, some kids will go in and interact with the sweetshop. This will delay them relative to others who continue along the street. When a sweetshop kid comes back out, they will find themselves behind the kid that kept walking, and the faster the kids are walking, the more they'll find they've spread out. That's C, the resistance to mass transfer.