Waiting for an appointment, I sat in the sun next to this adorable little waterfall and looked at so many nice examples of phenomena.
What I like best: The standing waves that you see in the reflection of the tree to the right. They do move a tiny little bit back and forth, but overall stay pretty much in place. In that exact spot, the current velocity is clearly as large as the waves’ phase speed, so they can’t get away in either direction.
A close second place is how smooth the turbulent current gets right before it plunges down the waterfall (see how the turbulence upstream looks like structures are more or less as long as they are wide, and then they become really long ellipses as they are accelerated towards the waterfall and the front is going faster than the back?), and the submerged hydraulic jump (and check out the video in this post for another really cool one!). And I love how the water is boiling with turbulence below the waterfall — at least in the part in the front; in the back there is a lot less flow and a lot less turbulence. Isn’t it amazing how much there is to see in such a little bit of a stream?
Btw, do you know my awesome animations to illustrate subcritical and supercritical flow regimes? Poor figurines running up and down escalators like so: