<p>The dye diffuses vertically because it has a higher density. So when you put the dye in from the top, it wants to go to the bottom, albeit slowly. If it were more buoyant, it would float. It does diffuse horizontally, though much slower. This can be shown that, after a long time, the water will turn the color of the dye.</p>
<hr>
<p>As far as rotation affecting the dye, that is complicated. If you start the rotation before you put the dye in, you can get <a href="http://weathertank.mit.edu/links/projects/taylor-columns-introduction" rel="nofollow noreferrer">Taylor Columns</a>. Throw in different variables/change different things about the tank, and you can find different phenomenon (see the <a href="http://weathertank.mit.edu/links/projects" rel="nofollow noreferrer">Weather in a tank experiments</a>).</p>
<p>What you see in your experiment is called &quot;shear-induced diffusion.&quot; The movement of your pen makes shear, and that shear increases the diffusion rate. I would also think that the eddies generated help diffuse the dye.</p>

CC BY-SA 4.0
2020-09-24T19:30:48.030Z
Thank you so much for an great answer! I really appreciate your help :)
Mathomat55
CC BY-SA 4.0
2020-09-26T15:36:26.337Z
By the way, why do we introduce turbulence in rotating water? What does this have to do with the ocean or atmosphere? Just currious
Mathomat55
CC BY-SA 4.0
2020-09-26T17:56:32.547Z
Its usually for demonstrations. If you imagine an entire hemisphere projected onto a tub, you can think that the distance from the center is like latitude. So the rotation is like the rotation of the earth. So you can visualize things like Rossby waves, the general circulation, and similar features in the tub. The rotation can organize turbulence and generate. Check out this video that has a couple of video demonstrations: https://www.youtube.com/watch?v=uWdKVpQ94Ns
BarocliniCplusplus
CC BY-SA 4.0
2020-09-29T16:33:06.467Z
But adding turbulence, what does that mean for the real ocean? Is there a specific ocean-phenomenon we are trying to demonstrate when adding turbulence?
Mathomat55
CC BY-SA 4.0
2020-09-30T17:20:58.670Z
For the ocean? Sure. Turbulence in the ocean happens all the time. Taylor columns, Ekman pumping, or even the thermohaline/overturning circulation can be emulated to some extent.
BarocliniCplusplus
CC BY-SA 4.0
2020-10-01T08:22:25.650Z
Ohh, of course! Thank you @BarocliniCplusplus
Mathomat55
CC BY-SA 4.0
2020-09-24T17:13:08.790Z
2020-09-24T17:13:08.790Z
BarocliniCplusplus
<p>I have been doing some DIYnamics with rotating water and non-rotating water. So I had a bucket filled with water on a rotating table, and waited for it to be in solid body rotation. I then dropped some droplets of fooddye in the water. I observed that the dye sank vertical to the bottom of the bucket without mixing with the water horizontally. I remember that my teather called this 2D turbulence. However, I can't find a reason for why the dye does not mix horizontally. What is so special with rotating water, that makes 2D tubulence occur and not 3D?</p>
<p>After this experiment was done, we added more dye. But now we mixed it gently with a stroke of a pen, to make turbulence. I then see some spirals being formed, so the water must mix a bit horizontally now. But why? And how does the rotating water affect turbulence?</p>

<geophysics><earth-rotation><turbulence>
How does rotating water affect turbulence?
Mathomat55
# How does rotating water affect turbulence?

(Asked by: username_0 on 2020-09-24)

I have been doing some DIYnamics with rotating water and non-rotating water. So I had a bucket filled with water on a rotating table, and waited for it to be in solid body rotation. I then dropped some droplets of fooddye in the water. I observed that the dye sank vertical to the bottom of the bucket without mixing with the water horizontally. I remember that my teather called this 2D turbulence. However, I can't find a reason for why the dye does not mix horizontally. What is so special with rotating water, that makes 2D tubulence occur and not 3D?

After this experiment was done, we added more dye. But now we mixed it gently with a stroke of a pen, to make turbulence. I then see some spirals being formed, so the water must mix a bit horizontally now. But why? And how does the rotating water affect turbulence?

**Tags:** geophysics, earth-rotation, turbulence


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## 1 answers

### Answer by username_1 on 2020-09-24. Score: 1 | Accepted answer

The dye diffuses vertically because it has a higher density. So when you put the dye in from the top, it wants to go to the bottom, albeit slowly. If it were more buoyant, it would float. It does diffuse horizontally, though much slower. This can be shown that, after a long time, the water will turn the color of the dye.

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As far as rotation affecting the dye, that is complicated. If you start the rotation before you put the dye in, you can get [Taylor Columns](http://weathertank.mit.edu/links/projects/taylor-columns-introduction). Throw in different variables/change different things about the tank, and you can find different phenomenon (see the [Weather in a tank experiments](http://weathertank.mit.edu/links/projects)).

What you see in your experiment is called "shear-induced diffusion." The movement of your pen makes shear, and that shear increases the diffusion rate. I would also think that the eddies generated help diffuse the dye.

> **Comments:**
> **username_0**: Thank you so much for an great answer! I really appreciate your help :)  
> **username_0**: By the way, why do we introduce turbulence in rotating water? What does this have to do with the ocean or atmosphere? Just currious  
> **username_1**: Its usually for demonstrations. If you imagine an entire hemisphere projected onto a tub, you can think that the distance from the center is like latitude. So the rotation is like the rotation of the earth. So you can visualize things like Rossby waves, the general circulation, and similar features in the tub. The rotation can organize turbulence and generate. Check out this video that has a couple of video demonstrations: https://www.youtube.com/watch?v=uWdKVpQ94Ns  
> **username_0**: But adding turbulence, what does that mean for the real ocean? Is there a specific ocean-phenomenon we are trying to demonstrate when adding turbulence?  
> **username_1**: For the ocean? Sure. Turbulence in the ocean happens all the time. Taylor columns, Ekman pumping, or even the thermohaline/overturning circulation can be emulated to some extent.  
> **username_0**: Ohh, of course! Thank you @username_1  

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