This is really not the focus of our experiments here in Grenoble, but they are too nice not to show: Kelvin-Helmholtz instabilities!
Sheer instabilities in the flow
They showed up really nicely in our first experiment, when we only had neutrally-buoyant particles in our source water (and not yet in the ambient water). The water that shows up as the lighter green here is thus water that originally came from the source (and at this point has recirculated out of the canyon again).
Sheer instabilities in the flow
I get so fascinated with this kind of things. How can anyone possibly not be interested in fluid dynamics? :-)
Watch the movie below to see them in motion! The scanning works as explained here.
One of my favourite topics right now: Learning to “see” ocean physics wherever you go. For example here: A visit to my goddaughter in Schleswig, and this time we are practicing all she and her mom read about in MY BOOK (and if you have good ideas for a title for that book, please let me know!). So today I’m showing you pictures of phenomena similar to those in my book, but discovered on this recent visit.
For example diffraction when waves pass this pier:
In the image below, I’m showing what I mean: Waves coming in from the right have straight crests (red). As they pass the pier, they get diffracted and bent around (green).
In this spot, this phenomenon can be seen on most days. I wrote about it before, but I have more pictures from previous visits, where the same thing happens in the opposite direction, too: Waves propagating in from the left and being bent around the pier to the right.
Or we can see other wave crests, meeting a rock that breaks the water’s surface.
Those waves (shown in red in the image below) get reflected from the rock, and circular waves radiate away from the rock (green).
A similar thing can also be observed from a flag moored out in the water:
This time, incoming waves are green and the circular waves radiating off the flag are red.
Here we have the red wave crests coming in, and the green reflections.
If we look at it from a little more distance, we can also see another phenomenon: The wave crests are refracted towards the shallower shore:
Again the red crests are the original, incoming ones, and the green ones are the reflection:
And then finally, let’s look at duckies again. And on waves being created by wind:
Below you see the direction of the wind (white): One side of this little channel is shaded from the wind, so hardly any ripples there. But then on the other side, we clearly see ripples and small waves. And we see the wake the ducky made!
And one last picture: Which direction does this little channel flow in?
Yep. From the left to the right!
If you enjoy discovering this kind of stuff on your walks, or know someone who enjoys it, or want someone to learn to enjoy it, you might want to consider checking out my book. In my book, I show many pictures like those above, but I actually explain what is shown in the pictures rather than assuming (like I do on this blog) that my readers are oceanographers anyway… :-)
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.
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?
On the way to the pool I cross over the Elbe river on this pretty bridge.
Which is pretty spectacular, just because the structure itself is so amazing.
But what is even more spectacular is how every time I am there I see new things in the flow field. And the example I want to show to you today is the flow field around one of the pylons of the bridge that runs in parallel to the one I am on.
In the movie below you see a classical flow separation, similar to what might happen at an airplane’s wing. The water flowing towards you under the bridge arrives (pretty much) laminar, but then on contact with the pylon turbulence develops, eddies form and the flow separates from the boundary of the obstacle. Nice! :-)
When you throw a stick in the water and the waves don’t form circles.
Throwing something in the water usually results in waves traveling out in circles from the point of impact. But if you throw your stick into a current, the waves get distorted. Watch the movie below!
Slightly confusing that the stick drifts away, too, so that it doesn’t mark the center of the circle. But still it is clear that waves travel a lot faster downstream than upstream – at least relative to the whole system, not the water ;-)
One of the reasons I have been wanting to do the vortex street experiment I wrote about on Monday is that it is pretty difficult to visualize flow fields (especially if you neither want to pollute running water somewhere in nature, nor want to waste a lot of water by setting up the flow yourself). As a first order approximation, pulling an object through a stagnant water body is the same as the water body moving past a stationary object.
At the Thinktank Birmingham, they do have a small channel with water constantly running through, and a couple of objects that you can place in the current. Unfortunately, what you see is the wave field that is caused by the obstacle, not the current field.
Wave field developing around a body inserted into a channel
It is still pretty cool to play with it, though!
But neither of the setups (the channel discussed above or the vortex streets on a plate thing from Monday) is really optimally suited to teaching students the way a flow field will react to an obstacle. How amazing would it be if we had a flow field that could be modified to suit our needs? Stay tuned – I might have a solution for you on Friday! :-)
Wind waves on one side of the current – no waves on the other.
Recently in Bergen, I was walking to meet up with a friend at the kayak club, and I had to cross a bridge that has always fascinated me. Underneath the bridge, there is only a very narrow opening connecting basically the ocean on one side and a small bay on the other side. On this part of the Norwegian coast, the tidal range is easily of the order of a meter, so this narrow opening under the bridge makes for some pretty strong currents. In fact, when paddling through that opening, when the tide is right you can really see how the surface elevation changes from one side of the bridge to the other.
So when I was walking there recently, this is what I saw:
Strong current from the lower left to the upper right of the picture, wind blowing from the right, hence waves on the right side of the current and no waves on the left side.
This might be difficult to see on this picture, but there is a strong current going from the lower left corner of the picture towards the upper right. And on the right side of that current there are a lot of wind waves. But on the left side there are hardly any, even though there is nothing blocking the wind, just the current blocking the propagation of waves. Wind is coming from the right here.
I found it really fascinating how this current acted as a barrier to the waves and stood a couple of minutes watching. A couple of people stopped and looked, too, but didn’t find anything interesting to see and were slightly puzzled. But what I see is fetch (or that there isn’t enough of it on the left side of the current) and hydraulic jumps (or that the current is clearly going faster than the waves are). Which means that I start wondering how fast that current would have to be in order to stop waves from propagating across. Which then means I start estimating the wave lengths in oder to estimate the waves’ velocities to answer the previous question. So that’s reason enough to stand there for quite some time, just watching, right?