Zombie Motion

==Introduction==
During the scenes in World War Z where the zombies were travelling down narrow city streets, I couldn't help but think that the motion of the large masses of zombies very much resembled fluid motion. Once that realization hit me, I completely over-thought the whole thing, and this post is the result of that.
==Zombie Density==
When making engineering calculations on the flow of zombies, it is important to know which flow regime the movement falls in to: continuum, slip-flow, transitional, or free-molecular. The Knudsen Number is integral to determining the flow regime, and therefore, which equations should be used to attack the problem.
The Knudsen number relates the mean free path of the fluid (λ) to the length scale of interest (L).
:Kn \equiv \frac{\lambda}{L}
In the case of World War Z, the mean free path would be the average distance a zombie would have to travel before coliding with another zombie, rather than the average distance fluid particles must travel before colliding. The length scale is typically a tricky thing to nail down, but I would posit that a very good length scale to represent the flow of zombies would be the width of whatever corridor the zombies are chasing you in, whether that be a street or a small hallway.
A large Kn indicates that there is a large distance between the zombies, relative to the length scale, and it's flow is best approximated using free-molecular methods (Boltzmann Kinect Equation and Collisionless Boltzmann Equation). When Kn gets smaller (typically less than about 0.3) the flow can be approximated as a continuum, and the Navier-Stokes Equations become applicable. The photo at right shows a flow of zombies that could be represented as a continuum.
By treating the zombie flow as a continuum, we are ignoring the effects of discrete zombies and treating the flow as a continuous mass of zombies, hell bent on devouring everything in sight.
As with just about every fluid mechanics text book, the following sections in this post will only contain information relating to continuum zombie flow. More information about free-zombie flow can be found in any good statistical fluid mechanics text.
===Side Note: Knudsen Number and Weaponry===
In addition to helping determine which fluid regime zombie motion falls in to, the Knudsen number can also be used to determine which weapon would be adequate for fighting off a zombie attack. Very high Knudsen numbers would indicate that the zombies are spaced far apart, and that you would have a good amount of time between zombie takedowns. In these cases a bat or any other blunt object would be adequate to stop at attack. As the Knudsen number decreases though, weapons like katanas, pistols, assault rifles, grenades, and larger explosives become more and more necessary.
Shaun, Ed, and Liz are prepared to fend of a free-zombie attack, but should be weary of anything in the transition region.
Michonne has proven that she's able to handle many situations that would fall into the zombie flow transition region.
But when the zombie hoard has completely consumed your city and all hope is lost, there's really no replacement for a tactical nuclear missile.