Speaking of wake watching, the other day I wrote about long distance wave watching in the sunset, i.e. what kind of things one can deduce on surface roughness (and its causes) from different reflections of the setting sun on the water. And then I was asked why ships’ wakes were still visible for such a long time after the ships had already sailed. So here is my attempt at an explanation:
Check out the pictures above and below. In both you see the turbulent wake of the RV Kristine Bonnevie on our recent student cruise. You clearly see where the ship’s hull has passed through the water, moved forward by the ship’s propeller, which is very clearly introducing a lot of turbulence. And you very clearly see where the ship has not been: a more or less undisturbed wave field full of small wind waves, that looks substantially different from the turbulent wake.
Now why does the turbulent wake look so clearly different from the rest of the water for such a long time, even when the ship is gone? Shouldn’t the wake be invaded by surface waves at their wave speed?
Yes, that should happen, if the wake wasn’t turbulent. As the wake is turbulent, however, there are eddies moving the water around for quite some time after the ship has passed. If the water is being moved faster than the phase speed of the waves, they can’t propagate in there, the “flow” is too fast, the waves are washed away. See where the hydraulic jump is happening in the picture below, and waves seem to be squeezed together outside of the turbulent wake?
Just because it’s fun, here my hydraulic jump animation: the wave (person) is traveling exactly as fast as the current (escalator), it therefore doesn’t move. More Froude number animations + explanations here if you wanna look at what happens if the wave is moving faster or slower than the current…
Anyway. Back to pretty pictures. Below, you see a wake that is a little older: The surface is still a lot smoother than over the rest of the fjord, because the waves still haven’t propagated into the turbulent region. And when they do, longer waves propagate in first, because their phase velocity is faster than that of short waves. But the long waves’ effect on surface roughness is smaller than that of short waves, so the wake still appears smooth for even longer.
Only when all the turbulence has died down and the water is stagnant again (Or moving with the surrounding water) will the wave field be able to grow back to look the same as everywhere else. And therefore, even if you look at water from a distance, you can see where ships have been, even when they’ve long gone themselves.
One of the instruments that was used on our recent student cruise was the so-called MSS (“MicroStructure Sonde”, sometimes also called VMP, “Vertical Microstructure Profiler”) — an instrument that is used to measure how much mixing is going on in the ocean. Those measurements can help us figure out e.g. renewal rates of bottom water in fjords, which are interesting because of the very low oxygen concentrations found there, and their impact on biogeochemistry. And of course it’s also interesting from a purely physical oceanographic curiosity :-)
In the picture below, you see the MSS being deployed: It’s a slim instrument, maybe 1.5m in length, that is attached to an orange cable that runs on a small winch.
And a big THANK YOU to cruise leader Elin (observing from the upper deck) for bringing me along on this cruise! :-)
At the end of the instrument that sticks over the railing in the picture above, you can make out little pins, protected by a metal cage. Those are the sensors for both temperature and velocity shear, both measuring at very high frequency, many many times per second. They are also very sensitve, so in the picture below you see the wooden crate that is used for storing the instrument in between stations.
Once the instrument is deployed into the water, it is not just lowered down in the way a CTD is, but it has to be free-falling through the water. In order to achive that, the person running the winch has to constantly watch the cable going into the water to make sure there is some slack on the cable.
Algot is running the MSS winch
A second way to make sure the instrument is free-falling is to constantly monitor the incoming data on a PC onboard the ship.
Elina is working the MSS “inside job” (Picture by Sonja Wahl)
While the data is being monitored, also the depth the instrument is at is being monitored, or rather its pressure. Since the instrument is free falling, it is not a simple feat to make sure it gets fairly close (approximately 10m) to the bottom, but does not hit the bottom and destroy the sondes. One way we’ve done that on the student cruise is by stopping the outgoing cable when the instrument was at 75% of the water depth and let it fall, and then once the instrument is within 20ish meters of the bottom to start hauling the cable back in (“panic” in the list below ;-))
Snapshot of the piece of paper used to keep track of what’s going on at the current station
Looking at the picture of Algot below, you know that the instrument must be on its way up. Why? Because there is clearly no slack on the cable!
Algot is bringing the MSS back to the surface
In the picture below, do you see the green fringe on the instrument, as well as the rope slung around the metal protection cage thingy for the sondes? Those are there to make sure that no eddies (and especially no trains of eddies) develop while the instrument is falling, because if the instrument was vibrating or moving in some way other than just falling freely, that would influence the data we measure.
The instrument is then brought back on board, and we are ready for the next station!
Algot and Arnt Petter bring the MSS back on board
And which spots did we measure turbulence in? In many, but especially on either side of the fjord’s sill, because that’s where we expect mixing due to tides going in and out (which we also saw in the fjord circulation tank experiment!).
Whenever I’m in a canoe or kayak, I love watching the two eddies that form behind the paddle when you pull it through the water. It looks kinda like this:
Flow around a paddle
Instead of pulling a paddle through more or less stagnant water, we could also use a stationary paddle in a flow. And that is the setup I want to discuss today: A stationary, round paddle perpendicular to an air flow.
A very cool feature of the paddle – which we know has to exist from the sketch above – is shown below: There is a point somewhere downstream of the paddle, where the direction of the air flow changes and a return flow towards the paddle starts. You can see that the threads on the stick I am placing in the return flow go partly towards, partly away from the paddle. So clearly the stick is in the right spot!
Visualizing the flow field behind a paddle with a threaded stick
Another visualization that my dad came up with below: Threads are pulled back towards the paddle in the return flow.
Visualizing the return flow behind a paddle with threads
Doesn’t it look awesome?
Visualizing the return flow behind a paddle with threads
Another way to visualize the change in flow direction is to take a rotor and move it from far downstream of the paddle towards the paddle and back.
Visualizing the change in flow direction by moving a rotor towards and away from a paddle blocking an air stream
All of this is shown in the movie:
Don’t you wish you had all this stuff to play with? :-)
(And do you now understand why I was so excited about the diving duck? :-))
And the current and the sun glint were perfect for this kind of photos:They almost look like schlieren photography images in those super old papers, don’t they? And I find it extremely fascinating how you can see the boundary layer between the flow and the stagnant water, and how wind waves don’t manage to cross that boundary. See the tiny capillary waves on the right side of the boundary? Those are locally generated because the larger waves on the top left just don’t make it over the strong shear.
You want to watch a movie? Sure!
And another thing I love on those early morning trips? Being completely alone in a pretty park, with dew on the grass and flowers in the sun :-)
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! :-)
You might think that three hours of canoe polo on a Saturday morning would be enough water for the day, but no. As when I did the experiment for the “eddies in a jar” post a while back, sometimes I just need to do some cool oceanography. So last Saturday, this is what I did: I took a plate, mixed some sugar, silvery water color, and water, pulled some stuff through the water and that was pretty much it. 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. And since it is usually pretty difficult to visualize flow around stationary objects (at least if you don’t want to pollute that little creek nor waste a lot of water). So this is really exciting.
Depending on the size of the object you pull through the water, and its speed, all kinds of different eddies develop. So fascinating! Watch the movie below to get an impression. (It’s really only an impression – it’s 2 minutes out of the 40 or so that I filmed ;-))
And for those of you who are always like “oh, I would love to, but I couldn’t possibly do this at home!”: This is what it looked like in my kitchen when I filmed the video above:
The plate I am filming is the one underneath the camera (I love my gorilla grippy). My water colors from back when I was in primary school, a paint brush, a chop stick, the plate I tried first that turned out to not have enough contrast with the silver paint, a blanket because the tiles are cold to sit on. Oh, and the flowers that I have been meaning to put into nice pots for a couple of days now. So – no big mystery here! Go try! And let me know how it went.