Not everything that flows is a fluid. And when viewed from above traffic, crowds, and even herds of sheep flow in patterns like those of a fluid. In particular, these conglomerations move like compressible fluids – ones that allow substantial changes in density as they flow. From above, each sheep is just a few pixels of white, but you can see which areas of the herd have the highest density by how white an area looks. The highest density regions also tend to be the slowest moving – not surprising in a crowd.

Now watch the gates. They act like choke points in the flow and, to some extent, like a nozzle in supersonic flow. As the sheep approach the gate, they’re in a dense, slow moving clump, but as they pass through it, the sheep speed up and spread out. This is exactly what happens in a supersonic nozzle. On the upstream end, flow in the nozzle is subsonic and dense. But once the flow hits the speed of sound at the narrowest point in the nozzle, the opening on the downstream side allows the flow to spread out and speed up past Mach 1.  (Video credit: MuzMuzTV*; submitted by Trent D.)

*Editor’s Note: I do my best to credit the original producers of any media featured on FYFD, but this is especially difficult with viral videos as there can be many copies, all of which are uncredited. I’ve made my best guess on this one, but if this is your video, please let me know so that I can credit you properly. Thanks!