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January 14, 2006

The entry I actually wanted to write

I've lived in the UK for more than three years now and have confessed elsewhere already that I yet have to ride a bike on this island. More surprisingly, the apart from London, the places I can be found most of the time seem to be Coventry and, oh yes, the shiny seaside resort listening to the beautiful name of… Hull. No bold typeface. Let alone italics. Just Hull. Two visits [probably more coming] involved Rev. Two visits [probably more coming] involved my PhD.

I've got my second year report to hand in yesterday Monday and I can actually talk about some progress, most excitingly, I can explain what happened in Hull! [Which is why the extension so I could write about the experiment]

I believe as part of the Millennium projects, the city of Hull was enriched with a submarium, the Deep and Hull University managed to make a deal with them and now rent a large room with an impressive basin for experiments. So far, the basin has mainly been used by geologists for sediment experiments [they put lots of sand in it and let the water circulate through the basin and see what effect it has on the sediment, tracking the particles with a clever laser spectrometry system I can't explain - sorry, I'm hardly a mathematician, let alone a physicist], but now the professor and research fellow of the Hull fluids lab have managed to get into the basin's busy schedule and have invited us [well, mainly my supervisor, and I was sent ahead to help out setting things up and just experience what's going on. See live fluid dynamics basically!] to get data for surface waves and hopefully to see some bubbles! [The bubble thing will be explained in a bit]

The Basin

Okay, so I might have called it the fishtank in prior conversations, but it is an impressive construction [no leaks!] about 12 by 6 meters, about 2 meters high. On one side, it has six panels that can move independently and create the waves [the system turned out to have a built in maximum frequency of 1.2Hz - 6 waves in 5 seconds - which seemed justified by the enormous effort put in by the panels to push the water forward] and there is the possibility to divide the basin in 2 for instance to simulate river flow [having the water rotating around the barrier]. Some of it might sound a bit lame, and seeing the surface waves we're looking for are meant to be simulations of open sea, you wonder how much is possible. Well, with 1.2Hz with the basin only filled 70cm a wild turbulent pool was created that I'd rather not swim in [well, if it were a bit deeper] - I've got a nice movie [filmed from below the water surface - which looked really cool actually!] if you're interested. We even saw proper breaking waves! [a wave rolling over as opposed to two waves crashing into each other]

Surface Waves

If anything, it's not in my wallet of knowledge, but at least I can try and explain how they're measured. The idea is that for instance you force the waves at a certain frequency [in this case, 1 or 1.2Hz] and have some sort of sensor measure the surface elevation, which will give you a noisy wave signal. Then let a computer program analyze the signal and average over lots of signals and you'll end up with a frequency spectrum. The program will have measured the contribution of each frequency to the wave signal. In this case, you're looking to find the large contribution in the forcing region, and a noisy contribution at higher frequencies that can't be picked up by your sensor.

The sensor will be partly submerged in the water [see photos in experiment gallery. The sensor consists of two sets of wires, one set transmitting a high frequency signal and the other receiving it. The receiving wires will be a certain distance apart, which defines the highest frequency that can be picked up by the sensor, in our case 5mm implied about 12Hz]. The idea is that water has a far higher [insert electricity term here, my guess is conductance] than air, thus when the waves are higher, the signal will be stronger.

Eventually, we measured the wave spectrum a couple of times, and I believe in most cases our measurements agreed with the literature. Hurray! Only when the water was too wild [good for my research, bad for surface wave research] it seemed to deviate.


This is where it gets tricky. This week's experiment was mainly set up to study the surface water [and get used to the basin and it's configuration] and the only further experiment done involved releasing tiny glass bubbles in the water that will float and then lighting them with a laser sheet and hence showing more clearly the surface structure. I think. All on camera, I hope.

Now, all I have on camera is aforementioned film, and with a far more professional camera they took some better lit images of an area where bubbles were frequently created [nearer to the pads and the walls of the basin - mostly see through by the way]. But I also have my memory filled with lovely images of waves crashing into each other and lifting up but then crashing into the water releasing a column of bubbles that would rise but didn't bother too much about the turbulent surface of the water.

Something similar could be seen with dust particles left in the water: they would kind of go up and down in circles, but they wouldn't change their average position significantly. This is all nicely explained by exponential decay of the surface waves [obviously, a flat basin floor creates a boundary condition of zero amplitude for waves there, requiring some kind of decay from waves at the surface] but it helps to see for real. The bubbles showed this same kind of motion in addition to their buoyancy. Also, they turned out to be very small, and they stuck to the surface longer than I expected.


No. It's too late. I'd just like to comment that this is how I store science stuff in my head [or in this case, on my blog]. Don't worry about my report, it will contain far more detailed information and less yatter ["ali" -ism] - mainly coz it has to be 10 pages long, but still. More will follow some point though, as I'm bound to go back to Hull and look at more bubbles for hours in a row on days in a row and not get bored [I kid you not!]

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