Stokes drift

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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…

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