Wind field

Another boundary layer experience last week: On my way from work I stopped to take pictures of flags that were outside my university’s main building and that very nicely visualised the wind field (as flags tend to do).

If you just look at the flags, they look weird — they wind field was clearly not changing over time, yet the flags were at a weird angle to each other.

And in the next picture you see why: Because the air had to flow around an obstacle, so stream lines were bunching up.

The next morning, I went past there again and stopped to take more pictures, when a colleague of mine stopped next to me, looking a little puzzled that I was taking pictures of our not especially nice main building.

I explained what I was doing, and we got talking about how you see the world with completely new eyes once you have noticed, or have been shown, something tiny. Isn’t that exciting? :-)

Reading ice on a river as tracer for flow fields

Ice on Elbe river in Hamburg. By Mirjam S. Glessmer

For most of my readers it might be pretty obvious what the movement of floating ice says about the flow field “below”, but most “normal” people would probably not even notice that there is something to see. So I want to present a couple of pictures and observations today to help you talk to the people around you and maybe get them interested in observing the world around them more closely (or at least the water-covered parts of the world around them ;-)).

For example, we see exactly where the pillars of the bridge I was standing on are located in the river, just by looking at the ice:

What exactly is happening at those pillars can be seen even more clearly when looking at a different one below. You see the ice piling up on the upstream side of the pillar, and the wake in the lee. Some smaller ice floes get caught in the return flow just behind the pillar. Now imagine the same thing for a larger pillar – that’s exactly what we saw above!

And then we can also see that we are dealing with a tidal river. Looking at the direction of the current only helps half of the time only, and only if we know something about the geography to know which way the river is supposed to be going.

But look at the picture below: There we see sheets of ice propped up the rails where the rails meet the ice, and more sheets of ice all over the shore line. As the water level drops due to tides, newly formed ice falls dry and that’s all the sheets of ice you see on land.

The bigger ice floes in the picture have likely come in from the main arm of the Elbe river.

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Small port on a tiny bay on the Elbe river in Hamburg. Look at the sheets of ice on shore!

It is actually pretty cool to watch the recirculation that goes on in all those small bays (movie below picture). Wouldn’t you assume that they are pretty sheltered from the general flow?

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You saw the waterfall in my post two days ago already, when I talked about continuity. But now let’s take a closer look.

Waterfall in Lüneburg

Obviously, it’s artificial, but that makes for really pretty symmetric flow patterns: