Balances of dyed and un-dyed waters

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Oh look, a plume of (almost) un-dyed water hitting the green lake!

I am really fascinated by the balance between green water leaking out of the pipeline and into the rain drainage, the rain falling on the lake, and the rain water coming into the lake through the rain drainage system. Right now, the water coming out of the drainage is a lot less green than the water in the lake, which is itself being diluted by rain. So much so that you can see a clear plume entering before it is mixed so much, entraining so much lake water, that you loose track of it in the green.

This makes me think about all kinds of stuff: how long between it raining on the catchment area that drains into the lake and the water actually reaching it? And how large might the catchment area be relative to the area of the lake (i.e. how large are the respective influences on the color)? So much entertainment just stemming from a little green dye :)

The first autumn storm and its impact on dye tracer and water level

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Last night it rained a lot. So the first thing to do this morning was to check what that had done to my green lake!

The dye is now a lot more diluted, but overall it still looks surprisingly green seeing that there is a lot of rain water draining into the lake. To give you an idea of how much more water is going through now than when I last showed pictures of the green stream: Look at how clearly you see the inflow into the lake in the picture above! And remember the little waterfall in the picture below? There is a lot more flow now.

Another thing that has gotten a lot easier to see now is where the dye goes into the Kiel fjord. Because the flow rate is a lot higher, so the flow itself is clearly visible, independent of the tracer, but also because … well, there isn’t a lot of water left in Kiel fjord!

This is what it looks like this morning: That little stream is water from the lake going into the fjord. Usually there is about a meter more water here!

It looks actually pretty cool to see exactly what the sea floor looks like.

Even though there are no tides in the Baltic (well, hardly any), we do have some large changes in water levels sometimes. They are due to changes in wind or pressure; in this case there was a lot of wind last night that pushed a lot of water out of the Kiel fjord into the Baltic.

What typically happens now is that this water doesn’t stay away indefinitely, but once the winds stop, forms a “seiche”, a standing wave, with a period of a little more than a day.

Of course I am going to check if there is water back by tonight, and then gone again tomorrow morning! Assuming, of course, that the winds stay calm. Otherwise that would influence where the water goes, too.

What I found really interesting, too, is that I saw a lot of herons now that I’ve hardly ever seen in this part of Kiel fjord before. It makes sense — usually there is too much water so they have nowhere to stand — but it was still weird to see five at once, and more as I walked along the fjord.

And — at last! — it was possible to see from land what those two sticks in the water are warning about: The stone in the middle! I had never actually seen that before. Now I know! And now the water can come back; wave watching is more fun when the waves have slightly shorter periods than the seiche’s 27 hours… ;-)

…Update in the afternoon…

After more rain throughout the day, we now actually see a clear plume of the rain water going through the green lake, with a little mixing on the sides as the green water is entrained!

And some water is back in Kiel fjord. Phew. So there is wave watching to be done right away:

Below, we see a really nice example of waves changing their direction as they run into shallow water, since their phase velocity depends on water depth (more about that here).

Wave reflections

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This still is one of my favourite kinds of wave field to look at. So calming. (Who am I kidding. All waves are my favourite waves, of course! I am addicted to wave watching…)

Here is a short movie for you where you see the crests coming in from the bottom right and then being reflected at the wall. Isn’t this nice? :-)

 

It’s all about the right equipment: That’s why I now own a UV lamp! I see a lot of fluorescent tracer spotting in my future!

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Before I start gushing about my awesome new UV lamp (thanks for encouraging that purchase, Uta! :-)), some other updates on the state of green in the park across the road from my house (don’t know what I am talking about? Check out previous posts on the fluorescent dye tracer).

The lake is still bright green and very well mixed, similar to what it looked like in this post. But what is a lot easier to see now is the green water coming out into the Kiel fjord. It was very hard to see on the pictures I took the other day on our fluorescent night walk, and I didn’t see any by eye the first couple of days, but for the last days it has been clearly visible:

It’s still a lot clearer by eye than on the pictures, but even in these pictures you see the plume going out of the storm drain, don’t you?

In other news: my UV lamp arrived today and I am so excited!

So here is a water sample I took out of the green stream, photographed in normal daylight and then lit by my UV lamp. Pretty cool, ey? :-)

Who wants to come fluorescent water-spotting with me? :-)

Wave watching: Refraction and diffraction of waves

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A little more wave watching, today with a focus on how waves change direction when they run into shallow water. Let’s look at this beautiful wave and see what happens when it reaches the shallow shore.

Above, you see the wake of the pilot ship, consisting of many wavelets that propagate as parallel wave crests towards the shore.

Below, you see that the wave is propagating at an angle to the shore (something around 45 degrees, maybe?). If you focus on the wave crest that is just offshore of that little obstacle in the water (curious enough, a piece of brick wall), you clearly observe that angle. But then looking at the next wave crest in-shore, it is almost parallel to the shore! Assuming that both crests come from the same wave field, so that the second one was in the same position as the other one only moments before (which I know it was because I observed it), something clearly happened between then and now.

Refraction of waves

Why do waves change direction as the water depth changes? As waves run from deep into shallow water, at some point they start to “feel” the bottom, which slows them down.

Or, more scientifically speaking, the dispersion relation for shallow water waves is a function of water depth: The shallower the water, the slower the waves. That means that if a wave crest is running on a slope with one side being in shallower water while the other one is still in deeper water, it will change direction towards the shallow water because the shallow side of the crest is slowed down while the deeper side keeps on moving faster, thus forcing the whole crest around a curve.

But in this picture series there is more to see: See how the wave crest gets deformed after it has passed that obstacle?

Diffraction of waves

This is a process called diffraction: The change of direction after a wave crest has passed either through a slit and then starts radiating from that slit as circle segments, or, in this case, an obstacle. The wave passing an obstacle is, in a way, the same as the wave passing through two wide slits which are very close to each other, only separated by the obstacle: The edges of the wave crest at the edges of the “slits” also start radiating out as circle segments!

One spot, so many things to observe!

And there are, of course, ships. What I wanted to show on this picture is a close-up of the turbulent wake of the ship, but it’s really difficult to see so I’ll let that pass for today.

And the picture below shows so much cool stuff: Waves radiating from that pylon. Ripples on the surface by a gust of wind. Wave crests getting a lot steeper as they run up on the slope. And, my main reason for posting: I really like how the wave is spilling as it breaks! :-)

Fluorescent night walk — following the stream through the lake into Kiel fjord!

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Luckily some of my friends are crazy enough to bring the UV lamps and go on a night walk with me, following the green fluorescent stream! (Don’t know what I am talking about? Check out the previous posts (post 1, post 2) on why there is fluorescent dye in a lake across my street and why that is exciting)

Following the water

It looks very spooky when all of a sudden in the middle of a park you come across something looking like the picture below. Well, you would probably not come across it if you didn’t know where to look, but you get my point. And once you found it, you can follow it downhill.

But don’t let yourself get distracted by signs on the trees, someone is trying to lead you in the wrong direction ;-)

Because what we were looking for was, of course, the same lake I have been posting about today and yesterday, except now it looks like the picture below. If you thought it was creepy by day you know nothing of creepy!

Creepy, but also fascinating! Of course I have to inspect it more closely.

Below my hand holding the UV torch while I was looking at all kinds of critters in the water (poor things!)

Science is, of course, team work. Especially when you want pictures, too ;-) Thanks Maria and Tom for such a spontaneous and exciting adventure!

Below, Tom is shining the UV lights down the little water fall so we can take pictures.

And here you see the view from the upper lake down the water fall into the lower reservoir. Next time I will definitely not do such a fluorescent night walk without a tripod and a better camera than my phone!

It might have been a bit of a hassle to find if you didn’t know where to look, but since I know exactly where that lake drains into Kiel fjord, we could follow the fluorescent water out the storm drain into the fjord!

Here we are at the top of the sea wall, looking down, and you see eddies of fluorescent water coming out of the storm drain and into the fjord. Super cool to see that the flow was coming out on the edges of the drain, and that it was eddying. And that, even though there was not a large flow coming out, it could be seen quite far into the fjord, at least as far as our torches could still light the surface. Very very cool tracer oceanography! That was one exciting evening!! :-)

Wave watching as official part of the program of a conference on chemistry and physics education!

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Yes, you read that correctly.

The German Society for Chemistry and Physics Education (GDCP)’s annual conference started out with a 2 hour cruise on Kiel fjord, during which the participants had the opportunity to choose between enjoying the sunshine and just doing whatever they liked, and several “guided tours” on either the sights of Kiel in general, or the biggest sight in Kiel, the water :-)

Preparation is everything: charts to learn to observe waves

I had prepared laminated charts to be used in case for some reason the weather wasn’t cooperating, and they definitely came in handy even during beautiful sunshine. On those charts, I used pictures that you’ve seen on this blog before, and contrasted them with typical physics text book illustrations, either sketches or ripple tank photos.

Since I was fairly busy talking, I only snapped two pictures:

The Oslo ferry right after turning inside the Kiel fjord, right when it starts backing up towards its berth, and, more interestingly, the turbulent wake. See how messed up the wave field is? It’ll take quite a while for all that turbulence to dissipate and for the sea surface to look as if nothing ever happened there!

And then our ferry’s wake. Here we see the turbulent propeller wake and one side of the feathery wavelets of the V formed by the wake.

Beautiful day to be on a ship!

Now, if you’d like to do a guided wave watching tour, you know where to find me… ;-)

Dye tracer “in the wild”, day 2

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This morning, the green lake looked different yet again.

If you remember yesterday’s pictures, we ended the evening with the lake being a fairly well mixed green color (picture on the right).

Now imagine my surprise when I came back in the morning and it looked like this:

The right side of the lake is still green, but the direct connection between inflow and outflow is an even brighter green! And the green inflow detaches once more at the tip of that little island (which it only did during the first observation yesterday, and two hours later the mixing had progressed around the tip).

There are only two ways I can think of how that could have happened:

a) During the night, there was a lot of un-dyed water added to the lake. Maybe through rainfall? But the effect would have been that the green color in the lake would have gotten diluted and, when the rain stopped, the inflowing water appears a lot greener than the surrounding lake water. Possible, even though I didn’t notice any rain during the night.

The other option is this:

b) Someone added more dye to the leaking pipes. This is the more probable explanation to me. The effect would be the same as above: A more intense inflow into a less intense lake.

In any case, the plume we are seeing now can only have been flowing with that intense a coloring into less green water for a couple of hours, otherwise the whole lake would have been mixed through and through.

I guess the easiest way to know which explanation is right would be (well, in addition to asking them directly) to have an objective measure of how green the water is, so that we would know if that changed over night or if the plume is really more intense now than yesterday. But with light that is always changing that is really not possible to say.

But this new green inflow is definitely beautiful: Look at the instabilities where it meets the stagnant lake water!

And more instabilities on the other side.

So those pictures were taken at around 7 in the morning. When I came back in the afternoon, the lake looked like this (sorry about the confusing lighting with the shadows and directly lit spots, can you ignore those and imagine what the color would look like under better light?):

Completely mixed and very very green! Interesting, isn’t it? So apparently the inflow stayed as intensely green as in the morning and, over the course of the day, mixed the whole thing.

P.S.: The company that puts the dye tracer in said on my Instagram @fascinocean_kiel that they are using uranine as dye, and that it’s completely safe for the environment. And, interestingly, that’s what we use in tank experiments under the name fluorescin, and that means that it is a fluorescent dye! I really need your UV light, Uta!! :-)

Dye tracers “in the wild”

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You know I love dye tracers (remember the beautiful fluorescent green we used in the 2017 experiments in Grenoble, when we got to play with the 13m diameter rotating tank?) but today I found some “in the wild” again — on the way back from my morning run & swim no less, in Kiel’s Forstbaumschule.

I’ve seen a dye tracer here several times before, and it’s basically just an indicator for a leak in the district heating (and everybody claims that it isn’t harmful to the environment despite its color).

Dye as a flow tracer

Spotting leaks would be very difficult if you just had normal water running into places where there is other normal water. Last winter you could clearly see that the dyed water was quite a lot warmer than the rest because it melted ice away where it went, but at temperatures like to day you might be able to see a thermal signature with thermal imaging equipment, but it is nowhere near as obvious as during winter.

But today my timing was lucky: The pipes can’t have been leaking for very long yet, because there were clear boundaries visible between the “old” lake water that wasn’t dyed yet, and the plume of dyed water entering into the lake and leaving it on the other side.

Dye as age tracer

So in a way the dye also acts as age tracer (since there are currently no other inflows into that lake. It would obviously be different if there were): the “old” water is still dye-free, whereas the “young” water is bright green. And then there are the regions where older and younger water mix and the color isn’t quite as intense.

Dye to visualize mixing

On the boundaries between the dyed water and the old lake water you see mixing in form of tiny eddies, and I’m pretty sure that when I go back this afternoon, the whole lake will be this awesome fluorescent color. And I am curious to see if there will still be flow structures visible or if it’ll all just be bright green :-)

Update: 2 hours and 11 hours later

And I went back. Twice.

Below you see how the coloring changes at the inflow mixes more and more with the lake water: left the picture taken at 7:15 am, then 9:15 am, then 7 pm. Fascinating! :)

Wave watching: A wake, another wake, and a mystery wave

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And we are wave watching again!

A ship’s wake and the different zones within

Here is the wake of the little ferry that goes across Kiel canal.

I love how you can see the different parts that a wake consists of: The V with the ship at its tip that consists of wavelets from the bow wave and that spreads outwards. And then the turbulent wake where the ship has physically displaced the water when sailing through, and that then has been thoroughly mixed by the ship’s propellers.

This second, turbulent wake actually changes the water’s surface for quite some time. Propellers put the water in rotation and it slowly entrains surrounding waters, and this turbulent motion looks substantially different from the “normal” sea surface. It can even be spotted from satellites long after the ships are gone. You’ve probably sometimes noticed streaks on the sea even though no ships were present — those might well have been the remains of wakes!

But speaking of ships that have sailed…

Wake of a ship that sailed past a while ago

Here is another example of a wake being visible quite some time after the ship has sailed past. However what we see here is the feather-y train of wavelets from the V reaching the shore.

While we were looking at it, my friend mentioned that the waves seemed to approach a lot more slowly than she had expected. That’s because the movement we first notice is the phase velocity of individual wave crests. But when you look closely, you can’t follow one individual wave crest for a very long time, it always appears and you have to start over. That’s because the signal, the V itself, only moves with the group velocity, which moves at half the velocity of the wave crests. That really looks confusing! Unfortunately I didn’t get a good movie of this. But there is always next wave watching session! :-)

And a mystery wave!

But then what would any wave watching session be without a riddle.

Any idea what caused the wave pattern below? Not the obvious, larger waves, but the concentric circle segments that radiate outwards from somewhere in the bottom right?

This is a case where it is really helpful when you recognize where the picture was taken, because there is some important information missing from the picture: The straight edge continues on for a little to the right, and then it opens up to a fairly long channel coming in.

However what you do see is the wind direction from the way the water is smooth right at the shoreline and then ripples start to form as you look further away: The wind is blowing out onto the water.

Now combine those two informations and you understand how that wave pattern was generated!

Diffraction at a “slit”

Wind-generated waves move as (more or less) straight crests out of the channel that opens into Kiel fjord just outside the right edge of the picture above. Then, suddenly, they aren’t bound at the sides any more, and what happens looks like diffraction at a slit (except the slit is fairly wide in this case): The straight crests turn and form 90 degree circle segments that radiate outwards. Voila!