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.
When a higher-order effect suddenly becomes important.
During our excursion to Hamburg Ship Model Basin (HSVA), one of the experiments we ran was on Stokes drift. You can already see in that post’s movie that there is some swimming thing moving down the tank in the direction of wave propagation, but of course we had to quantify.
“Experiment” sounds too sophisticated for what actually happened: We dropped a piece of styrofoam in the waves and took the time it took that styrofoam piece to travel two meters. The piece of styrofoam has the advantage over the other swimming thingy that it hardly sinks into the water, and therefore constitutes an almost passive tracer of the waves’ movements.
Now, we all know that Stokes drift is one of those ugly non-linear higher-order things that we ignore as much as possible. It is basically the effect of orbital movements not being closed circles, but rather spirally things. But we have all heard over and over again that the effect can be neglected, and whenever we see a bird bobbing up and down in the waves but also moving horizontally, we quickly rationalize that it must be swimming autonomously, or that there is a current superimposed on the wave field.
So, what do you think, how long will it take for that little styrofoam piece to travel 2 meter’s distance? Of course that depends on the kind of wave field, but give it a rough guess. What’s your estimate?
36 seconds! To travel 2 meters! That doesn’t sound so insignificant now, does it? I’m still trying to figure out why that happened because it seems way too fast. And according to theory it should even have travelled faster than that. So please excuse me while I put on my thinking cap…
I recently got to join a class on their excursion to Hamburg Ship Model Basin (HSVA, “Hamburgische Schiffbau-Versuchsanstalt” klingt so viel besser!). Those are amazing facilities and shipbuilding students are always excited to go there and get a glimpse at all the exciting research going on. Since they are working on the cutting edge of naval architecture, unfortunately I couldn’t take pictures of any of the model ships. But that doesn’t make this any less exciting – I still got to take pictures of the waves! :-)
Waves in the “small” towing tank (80 m in contrast to 300 m) at HSVA. Notice the student group in the back on the left? That’s how long the tank is. And they aren’t even at the far end…
Below is a movie of waves being generated in the 80 meter long towing tank. Pretty amazing!