Fake water spin experiments around the equator

[greypilgrim]

Hi.

I just visited the equator in Ecuador, where they perform those fake water spin experiments on and a few meters north and south of the line.
I know that the Coriolis effect is way too small on that scale, so how do they do it? They even let spectators pull the plug.

 

[kuruman]

I would guess that the drain is machined to be slightly inclined in order to start a clockwise or counterclockwise flow going when the plug is pulled. A test would be to swap sinks and see what happens to the flow.

 

[Herman Trivilino]

It's likely due to asymmetries in the shapes of the basins. It seems the easy way to construct this device is to just look around for old basins and test them.

 

[jbriggs444]

Watch him fill the basin in the second video. He pours the water in the direction of the eventual spin.

Edit to add...

There may be some tomfoolery in the first video. When he grabs the leaves out of the water, he sweeps the first handfull in a counter-clockwise manner and then pauses to watch the remaining leaves. This may be in order to cancel any slight rotation that he observes prior to unplugging the basin.

 

[greypilgrim]

It's likely due to asymmetries in the shapes of the basins. It seems the easy way to construct this device is to just look around for old basins and test them.

It's the same basin every time, they moved it.

Watch him fill the basin in the second video. He pours the water in the direction of the eventual spin.

That's probably it. Guess I'll have to do some experiments myself when I get home (which is far from the equator).

They had other experiments which were even more obvious BS, like trying to walk a straight line with your eyes closed or the guide pushing your extended arms down (I forgot if it was supposed to be easier or harder on the equator line compared to just next to it).
My favorite was balancing an egg on a nail head, which only a few of the participants succeeded to do. Apparently you can do it at home too, "but only on the 21st of March and September".

 

[Herman Trivilino]

It's the same basin every time, they moved it.

Oh, I didn't see that. Well, if that's the case, then it's like @jbriggs444 says, the direction of rotation is initiated by the way the water is poured into the basin.

Careful experiments were done in the 1960's by researchers at MIT. They used special hemispherical bowls that were carefully machined to be symmetrical. After filling the basins they waited a long time to eliminate any motions imparted by the filling process. In Boston they noted that drainage circulation was generally counterclockwise. They shipped the apparatus to Australia and the drainage there was clockwise.

 

[jrmichler]

Here's a photo of inlet vortices to an identical pair of culverts. It was peak flow from this springs snow melt, and the culvert inlets were submerged. The left vortex is rotating clockwise, the right vortex is rotating counterclockwise. Latitude is 45 deg North.

I'm too lazy to make the calculations, but it should be easy to show that the hydrodynamic forces due to water flow are several orders of magnitude larger than the Coriolis forces in this situation.

And, if somebody wants to make the calculation, the culvert outlets. Note that, while the inlets are submerged, the outlets are not running full. The water on the upstream side was about one foot below the road.