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?
One thing I find endlessly fascinating are – you might have heard it before – standing waves. At the waterfront in Kiel I saw some the other day:
Watch the movie below and be fascinated, too! :-)
Isn’t it amazing how wave crests and troughs seem to appear out of nowhere and vanish again? When we are so used to seeing waves propagate, this is such an interesting variation of the theme! And it makes it somehow more easy to accept that waves transport energy, not mass, because if we can’t see which way they propagate, which way would they transport mass?
Looking at a creek on a Sunday stroll, and seeing lots and lots of concepts from hydrodynamics class.
For example below, you see waves radiating from each of the ducks. And you see interference of waves from all those ducks.
What happens if the ducks bring their waves closer?
At some point, all those waves from the ducks are going to hit the weir in the picture below.
And there, they are going to somehow react to the flow field caused by the changes in topography.
And you can spot so many different phenomena: Standing waves, hydraulic jumps, and lots more!
Watch the movie below to see the whole thing even better!
Btw, you might remember this spot, I have talked about standing waves from right there before. Interestingly, the wave pattern in the other post looks really different, probably due to different water levels or changes in topography (maybe someone threw in rocks or they did some construction work on the weir?). But it is still just as fascinating as last time :-)
And for those of you who like to see a “making of”:
On Monday, I showed you a movie on wave generation in Hamburg Ship Model Basin (HSVA)’s wave tank. At the end of that movie, we see that the wave energy is being dissipated by a “beach”. Well, we actually see that some of the energy is reflected in those cute little baby waves. And there is another fraction of the total energy that passes through the beach into another part of the tank. And that’s what I want to show you today.
When I’ve talked about standing waves in a tank before, that always meant the simplest form: Only one node. We have always tried to avoid higher-order modes before, partly because they are a lot more difficult to generate, at least using our method.
Improving one of the experiments run in the GEOF130 lab.
One experiment that has been run in GEOF130 forever is the “standing wave”, where a wave is excited in a long and narrow tank and then, for different water depths, the period is measured and the velocity calculated in order to compare it to the one calculated from the shallow water wave equation.
Traditionally, the standing wave is excited by lifting one end of the tank, letting the water settle down, and carefully putting the tank back down. This, however, means that someone has to lift a pretty heavy weight. So Pierre and I were quite proud of ourselves when we constructed a pulley system last year and now instead of lifting the weight up, someone could hang on a rope instead.
However, this was still hard work, and even though the picture shows a student doing the lifting, for most lab groups it was actually Pierre who did it.
But then this year, we came up with a much simpler solution and I don’t know how we didn’t see this before now. As Pierre remarked: We talk about seesawing standing waves ALL THE TIME. How did it not occur to us that the simplest setup would be a seesaw? So now we have two wooden blocks underneath the tank, one supporting it in the middle and one underneath the end where the operator is standing. So all that needs to happen now is a slight lift of the tank and then a slight downward push to bring it back in the horizontal.