vortex-streets

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These images from Earth Observatory show islands creating ’von Karman vortices’ in the stratocumulus clouds. In the animation you can see that each vortex is formed from alternating sides of the object that is impeding flow of the fluid such as clouds or soap films.

In the first image, the water droplets in the clouds create an optical phenomena known as Glory, making the rainbow to the left. However, this is different from a rainbow, in that it is formed from backwards diffraction meaning they are always directly opposite to the Sun.

Another entry in the Princeton 2011 Art of Science competition.

“For this image, two artificial fish fins are placed side-by-side and flapped in-phase with each another as water flows past the fins (flow direction is up). Small hydrogen bubbles (the white part of the image) allow for the wake of the fins to be visualized. The interaction of the fins creates two repeating patterns of swirling vortices known as vortex streets.”

Most objects are not particularly aerodynamic or streamlined. When air flows over such bluff bodies, they can shed regular vortices from one side and then the other. This periodic shedding creates a von Karman vortex street, like this one stretching out from Isla Socorro off western Mexico. From the wind’s perspective, the volcanic island forms a blunt disruption to the otherwise smooth ocean. This vortex shedding is seen at smaller scales, as well, in the wind tunnel, in soap films, and in water tunnels. If you’ve ever been outside on a windy day and heard the electrical lines “singing” in the wind, that’s the same phenomena, too. With the right crosswind, radial bicycle spokes will buzz for the same reason as well!  (Photo credit: MODIS/NASA Earth Observatory)

We’ll ride the spiral to the end

and may just go where

no one’s been. 

Watch here in better quality.

Vortex (by David de la Mano)

Original photo (by David de la Mano)

Here more pics.

And here a video about his creations, shot by Martín Segredo.

More info about David de la Mano:

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A simple cylinder in a steady flow creates a beautiful wake pattern known as a von Karman vortex street. The image above shows several examples of this pattern. Flow is from bottom to top, and the Reynolds number is increasing from left to right. In the experiment, this increasing Reynolds number corresponds to increasing the flow velocity because the cylinder size, fluid, and temperature were all fixed. As the Reynolds number first increases, the cylinder begins to shed vortices. The vortices alternate the side of the cylinder from which they are shed as well as alternating in their sense of rotation (clockwise or counterclockwise). Further increasing the Reynolds number increases the complexity of the wake, with more and more vortices being shed. The vortex street is a beautiful example of how fluid behavior is similar across a range of scales from the laboratory to our planet’s atmosphere.  (Image credit: Z. Trávníček et. al)

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I took these photos between 3 and 4 AM the morning of Jan. 6th, 2014 in Chicago. This Monday is now known around town as the beginning of “CHIBERIA”, due to the quick and hard drop of temperatures into the double digits below 0°F - all thanks to the now famously referenced polar vortex. At the time these photos were taken the temperature was around -10°F with a windchill in the -30's°F. 

Von Karman Vortices Off Chile

Two small islands had a big impact on the skies over the Pacific Ocean in January 2013, creating paisley patterns that stretched 280 kilometers (175 miles). The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-color image on January 13, 2013.

The Juan Fernandez Islands are located roughly 800 kilometers (500 miles) off the Chilean coast. The biggest of these—Isla Alejandro (Alexander) Selkirk and Isla Robinson Crusoe—are volcanic islands situated along an east-west-trending submarine ridge. Each island boasts a tall summit. With an area of 52 square kilometers (20 square miles), Isla Alejandro Selkirk reaches an altitude of 1,650 meters (5,413 feet) above sea level. Slightly smaller, Isla Robinson Crusoe has a total area of 48 square kilometers (19 square miles), and reaches an altitude of 922 meters (3,025 feet).

The islands are tall enough to disturb air flow over the ocean. When an object such as an island interferes with the movement of air, von Karman vortices form in the air on the downwind (or leeward) side of the island. Also known as vortex streets, they are double rows of spiral eddies that are made visible by the clouds.

Flow over blunt bodies produces a series of alternating vortices that are shed behind an object. The image above shows the turbulent wake of a cylinder, with flow from right to left. Red and blue dyes are used to visualize the flow. This flow structure is known as a von Karman vortex street, named for aerodynamicist Theodore von Karman. The meander of the wake is caused by the shed vortices, each of which has a rotational sense opposite its predecessor. The rapid mixing of the two dyes is a result of the flow’s turbulence. In low Reynolds number laminar cases of this flow the structure of individual vortices is more visible. Similar flow structures are seen behind islands and in the wakes of flapping objects. (Photo credit: K. Manhart et al.)

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Seen:  Eye of the Vortex

Who: ERIC / website

Where: Place M, Shinjuku

It’s been Eric Week at Place M / Gallery M2 in Shinjuku- While the second floor gallery serves as the first public venue for his GOOD LUCK HONG KONG series, the third floor, Place M, is where to catch a second showing of his previously exhibited India work, Eye of the Vortex.

I mentioned here earlier that Eric and his Leica and his flash spent some time in India- the entire series can be seen on his website here, or handsomely printed as a photography book by Akaaka, which can be purchased here.

The pictures on the walls were every bit as exciting as I remembered them from September when they were pinned to the walls of Guardian Garden in Ginza. He is either unaware of the traditional Western approaches to photographing India, or simply doesn’t care. He keeps true to himself but not in a way that mocks those within range of his lens. Despite working on the streets with a flash Eric isn’t one of those brutally hardcore street photographers out to beat people down photographically- his subjects still retain their concerns and humanity. Even if a picture makes you grin it’s not at the expense of the subject but a joyous appreciation for the moment. I guess the illumination he employs works to enlighten everyone who is part of it. 

On site at Place M you’ll find three of his four books available for sale. They’re all fun- GOOD LUCK CHINA is great.  Look At this People is a snappy book and an entertaining video on Youtube with a song that makes me want to go out and take pictures as soon as I hear it.  Eye of the Vortex made my Favorite Photobooks of 2014 list.

youtube

Today’s post is largely brought to you by the fact that I have been sick the past four days and my fiance and I have been bingeing on Star Trek Voyager. At some point, we began wondering about the sequence from 0:30-0:49 in which Voyager flies through a nebula and leaves a wake of von Karman vortices. Would a starship really leave that kind of wake in a nebula?

My first question was whether the nebula could be treated as a continuous fluid instead of a collection of particles. This is part of the continuum assumption that allows physicists to treat fluid properties like density, temperature, and velocity as well-defined quantities at all points. The continuum assumption is acceptable in flows where the Knudsen number is small. The Knudsen number is the ratio of the mean free path length to a characteristic flow length, in this case, Voyager’s sizeThe mean free path length is the average distance a particle travels before colliding with another particle. Nebulae are much less dense than our atmosphere, so the mean free path length is larger  (~ 2 cm by my calculation) but still much smaller than Voyager’s length of 344 m. So it is reasonable to treat the nebula as a fluid.

As long as the nebula is acting like a fluid, it’s not unreasonable to see alternating vortices shed from Voyager. But are the vortices we see realistic relative to Voyager’s size and speed? Physicists use the dimensionless Strouhal number to describe oscillatory flows and vortex shedding. It’s a ratio of the vortex shedding frequency times the characteristic length to the flow’s velocity. We already know Voyager’s size, so we just need an estimate of its velocity and the number of vortices shed per second. I visually estimated these as 500 m/s and 2.5 vortices/second, respectively. That gives a Strouhal number of 0.28, very close to the value of 0.2 typically measured in the wake of a cylinder, the classical case for a von Karman vortex street.

So far Voyager’s wake is looking quite reasonable indeed. But what about its speed relative to the nebula’s speed of sound? If Voyager is moving faster than the local speed of sound, we might still see vortex shedding in the wake, but there would also be a bow shock off the ship’s leading edge. To answer this question, we need to know Voyager’s Mach number, its speed relative to the local speed of sound. After some digging through papers on nebulae, I found an equation to estimate speed of sound in a nebula (Eq 9 of Jin and Sui 2010) using the specific gas constant and temperature. Because nebulae are primarily composed of hydrogen, I approximated the nebula’s gas constant with hydrogen’s value and chose a representative temperature of 500 K (also based on Jin and Sui 2010). This gave a local speed of sound of 940 m/s, and set Voyager’s Mach number at 0.53, inside the subsonic range and well away from any shock wave formation.

Of course, these are all rough estimates and back-of-the-envelope fluid dynamics calculations, but my end conclusion is that Voyager’s vortex shedding wake through the nebula is realistic after all! (Video credit: Paramount; topic also requested by heuste11)