Tag Archives: Isère

Why the Isère reminds me of a water jet pump

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This is what the way to and from the 13-meter-diameter rotating tank in Grenoble looks like (and you should really visit Elin & team’s blog to learn about all the exciting stuff we are doing there!!!)

And the best part is the Isère right next to the bike path:

And one thing that I find really impressive with this river (coming from a much flatter part of the world than Grenoble, where rivers aren’t typically as fast-flowing as the Isère) is how all these return flow pools form everywhere.

Watch the movie below to spot them yourself, or my annotated picture below:

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It seems really counterintuitive that a strong current would make water on it’s side flow upstream instead of flushing everything downstream or even just going downstream through stagnant water, doesn’t it? But when I thought about why that is, it reminded me of the way a water jet pump works: You flush water from a tap down through a hose, and that hose is connected with another hose through which you want to suck something (usually some gas out of some container). So there it’s the same: The fast-flowing water pulls things in from the side and takes them with it. Now for continuity reasons, the water that is entrained in the jet needs to come from somewhere, so water has to be brought upstream in order to get sucked into the jet. That’s also similar to playing with Venturi tubes where the thinner the tube, the faster the flow, the lower the pressure… Anyway, riddle solved and I can think about other stuff again ;-)

But it is a really beautiful place to be:

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I like water so much better than mountains, but mountains still have their charms, can’t deny that…

 

 

Wave watching Sunday in Grenoble

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Today I went on a wave-hunt expedition to take pictures for posts on the Froude and Reynolds number over at Elin & team’s blog (which you should totally check out if you haven’t done that yet! I am actually proof-reading my posts there and that is saying something ;-))

Anyway. Let’s look at the picture below. Do you see how there are two qualitatively different flow regimes in the Isère? Closer to the banks, you see waves that look like normal waves, happily propagating wherever they want to. And towards the middle of the river, you see that there is a lot of turbulence, but disturbances don’t propagate wherever they want, they are being flushed downstream.

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For comparison below a picture of a part of the Isère where it is turbulent all the way to the sides:

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And below a nice example of how phase velocity of waves depends on wave length. See all the small, choppy stuff being flushed downstream and then standing waves caused by some obstacle in the middle of the river? That’s because the longer the wavelength, the faster the wave propagates (assuming that we are in deep water, which I think is a safe assumption in this case). So the river is so fast that the slower waves get flushed away and only waves of the length of those created by the obstacle (or longer) can stay in one place (or even propagate against the current). I think that’s pretty cool.

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Below is one of my favourite wave-watching sights: A half slit.

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And in the picture below, we can kinda see vortices detaching behind the obstacle (or is that just me)?

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And what I really liked: see the spot below where there are all of a sudden standing waves appearing in the middle of the river? Clearly there is a sill below, but I like that you cannot see the obstacle, just deduce that it must be there from how the waves look :-)

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It’s not a hardship to be here, I can tell you ;-)

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It is quite a beautiful place! And, by the way, this is my 600th blog post on this blog. Can you believe this?

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