von Karman vortices and a side of SAL

[senorpepr]

While looking through MODIS imagery, I saw these awesome picture of von Karman vortices to the west of the Cape Verde Islands. Off the African coast is also some Saharan dust.

http://rapidfire.sci.gsfc.nasa.gov/real ... 00.1km.jpg

[LilNoles2005]

Thanks for that picture, very awesome.

[mobilebay]

This just goes to show that you can learn something everyday. Pepr, could you please tell me more about these vortices. This is the first I've seen or heard of them.

[senorpepr]

(Bare with me... hopefully I don't get too technical)

Well, the best way to describe these von Karman vortices is to imagine a stream. The water in our stream will act in place of the low-level flow which, in the picture above, is east to west. Okay, now in our "stream" imagine a rock just below the water's surface. The rock will act as an island, or in the picture, the Cape Verde Islands. As the water in the stream hits the rock, it "pools" up just upstream of the rock. As this pooling occurs, more water finds the path of least resistance and goes to one or the other side of the rock and it's associated "pool." As these water molecules hit the pool and slide off to the side to go around, they begin to slightly spin. As more and more of these molecules spin off the "pool," a mini vortice forms. They look like small whirlpools or eddy. Well, as for this picture, the air coming from Africa hits the Cape Verde Islands and creates a pool of air on the east side of them. (Slightly higher pressure) As this pool of air grows, more air from Africa will hit the pool and spin off of it. As these air particles spin off, they become like eddies or mini vortices. Of course, air isn't visible to the eye. So to make them visible, throw in some low-level clouds. In this picture, the clouds get caught up in the vortice and produces the swirl you see.

Hopefully that explains it better -- let me know if it doesn't.

[mobilebay]

(Bare with me... hopefully I don't get too technical)

Well, the best way to describe these von Karman vortices is to imagine a stream. The water in our stream will act in place of the low-level flow which, in the picture above, is east to west. Okay, now in our "stream" imagine a rock just below the water's surface. The rock will act as an island, or in the picture, the Cape Verde Islands. As the water in the stream hits the rock, it "pools" up just upstream of the rock. As this pooling occurs, more water finds the path of least resistance and goes to one or the other side of the rock and it's associated "pool." As these water molecules hit the pool and slide off to the side to go around, they begin to slightly spin. As more and more of these molecules spin off the "pool," a mini vortice forms. They look like small whirlpools or eddy. Well, as for this picture, the air coming from Africa hits the Cape Verde Islands and creates a pool of air on the east side of them. (Slightly higher pressure) As this pool of air grows, more air from Africa will hit the pool and spin off of it. As these air particles spin off, they become like eddies or mini vortices. Of course, air isn't visible to the eye. So to make them visible, throw in some low-level clouds. In this picture, the clouds get caught up in the vortice and produces the swirl you see.

Hopefully that explains it better -- let me know if it doesn't.

Thanks PEPR. That is one of the best jobs of describing something I've ever seen. I'm very impressed. Thought I knew pretty much everything in the Tropics, but that proved you can learn something new everyday.

[senorpepr]

These vortices can occur anywhere, really. Back in my European days I use to see them frequently in the winter time by the islands north of Scotland.

[HurryKane]

(Bare with me... hopefully I don't get too technical)

Well, the best way to describe these von Karman vortices is to imagine a stream. The water in our stream will act in place of the low-level flow which, in the picture above, is east to west. Okay, now in our "stream" imagine a rock just below the water's surface. The rock will act as an island, or in the picture, the Cape Verde Islands. As the water in the stream hits the rock, it "pools" up just upstream of the rock. As this pooling occurs, more water finds the path of least resistance and goes to one or the other side of the rock and it's associated "pool." As these water molecules hit the pool and slide off to the side to go around, they begin to slightly spin. As more and more of these molecules spin off the "pool," a mini vortice forms. They look like small whirlpools or eddy. Well, as for this picture, the air coming from Africa hits the Cape Verde Islands and creates a pool of air on the east side of them. (Slightly higher pressure) As this pool of air grows, more air from Africa will hit the pool and spin off of it. As these air particles spin off, they become like eddies or mini vortices. Of course, air isn't visible to the eye. So to make them visible, throw in some low-level clouds. In this picture, the clouds get caught up in the vortice and produces the swirl you see.

Hopefully that explains it better -- let me know if it doesn't.

Thanks PEPR. That is one of the best jobs of describing something I've ever seen. I'm very impressed. Thought I knew pretty much everything in the Tropics, but that proved you can learn something new everyday.

Also, what you see in the picture posted above is called a Karman vortex street. To stick with senorpepr's rock in the stream example, say you're on the bank looking down on the rock and the stream of water is flowing left to right. Vortices/eddies are shed off the rock into downstream flow in alternating fashion, one off the side of the rock away from you, then one off the side of the rock near you, and so forth. Here's some pictures and animations of a vortex street (the two-d animation best illustrates it): http://www2.icfd.co.jp/menu1/karmanvortex/karman.html

It's one of the more attractive fluid dynamic phenomena out there

[P.K.]

These vortices can occur anywhere, really. Back in my European days I use to see them frequently in the winter time by the islands north of Scotland.

I was just going to say that. Jan Mayen island is a good place to see a von Karman Vortex Street.

There is a good example of that here http://rapidfire.sci.gsfc.nasa.gov/realtime/2005097/crefl2_143.A2005097125000-2005097125500.250m.jpg - 7.5MB image or http://rapidfire.sci.gsfc.nasa.gov/realtime/2005097/crefl2_143.A2005097125000-2005097125500.1km.jpg = 570kB lower resolution version.

Edit - That was a good way of describing them without using any non-dimensonial parameters.

[cyclonaut]

Spectacular pic!

Thanks for the pic & the explanation.

[Wthrman13]

Edit - That was a good way of describing them without using any non-dimensonial parameters. Cool

But non-dimensional parameters are fun .

[senorpepr]

Another day of the vortices:

http://rapidfire.sci.gsfc.nasa.gov/real ... 00.1km.jpg

[P.K.]

Edit - That was a good way of describing them without using any non-dimensonial parameters. Cool

But non-dimensional parameters are fun .

But they may confuse some people on here though....