Rerynold's Numbers

. Did you know that the behavior of a tiny particle of sediment in water can be modeled by a marble in glycerin? (Glycerin is very sticky stuff. It's like syrup.) The marble falls pretty slowly. The way we can decide if totally different systems will behave similarly is this concept called the Reynold's Number. Some guy named Reynold is credited with figuring it out. You can calculate the Reynold's Number like this:

Reynold's Number= (fluid density)*(fluid velocity)*(characteristic length)/(viscosity)

The fluid velocity might actually be the velocity of the fluid say going through a pipe. However, it could also be the velocity of the marble that you just dropped in the cylinder full of glycerin. Whether it's the fluid moves of the thing in the fluid doesn't really matter. You're just interested in how they move relative to each other. The characteristic length is basiclly the length along which the two things are touching. So it's the diameter of the marble of the diameter of the pipe...
 What if the pipe was square? How would you deal with that? I have no idea. I'll have to ask the professor. What about a river bed? hmm.... I don't know there either.

The last term is the viscosity of the liquid. That you can get out of a book or the internet. People have figure them out and put them in text books and references and written papers and blah blah...
But how did they figure that out?
I don't know. Good question. Maybe I will learn that soon. Maybe I already learned it and forgot.