# vortex-streets

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.”

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)

The above image is the Kármán vortex street caused by wind flowing around the Juan Fernández Islands off the Chilean coast. It is a delight for the eyes, portraying the turbulent side of nature.

## Vortex shedding

In fluid dynamics, vortex shedding is an oscillating flow that takes place when a fluid such as air or water flows past a bluff (as opposed to streamlined) body at certain velocities, depending on the size and shape of the body.

In this flow, vortices are created at the back of the body and detach periodically from either side of the body. These linear chain of spiral eddies are called von Karman vortice

The fluid flow past the object creates alternating low-pressure vortices on the downstream side of the object. Ergo, the object will tend to move toward the low-pressure zone.

## Airplanes and  Lift

The main thing to know is that a difference in pressure across the wing–low pressure over the top and higher pressure below–creates the net upward force we call lift.

Upon reaching a certain velocity, the aircraft’s lift is more than its weight and as a result, the aircraft takes off .

## Vortex Induced Vibrations

When the vortices are not formed symmetrically around the body (with respect to its midplane), different lift forces develop on each side of the body, thus leading to motion transverse to the flow.

These are know in fluid dynamics as vortex-induced vibrations (VIV). They are important factor to be taken into consideration because constant vibration of any object leads to fatigue and eventually failure.

## Chimneys and their spiraled heads

Tall chimneys constructed of thin-walled steel tube can be sufficiently flexible that, in air flow with a speed in the critical range, vortex shedding can drive the chimney into violent oscillations that can damage or destroy the chimney.

They are  protected from this phenomenon by installing a series of fences (sometimes called strakes or spoilers) at the top and running down the exterior of the chimney for approximately 20% of its length. The fences are usually located in a helical pattern.

The fences prevent strong vortex shedding with low separation frequencies.

Final Thoughts

In this post i wanted to introduce you this amazing phenomenon that occurs in nature known as vortex shedding. We will look at the technical side of this phenomenon in another post.

Have a great day!

EDIT: If this fascinated you,do check out this NASA article

`Nike Air Vortex Retro`

Spring and Lafayette, taken as I was running towards the train to go home at 3 o'clock yesterday afternoon…

A Von Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid around a rounded obstacle.

`Nike Vortex Vintage Sneaker`
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## “Mystique Jungle” by Ingress Vortices

In Between Waking State And That Of The Dream

Hieroglyphs Invoked By The Supernatural Team

Mandala Of Glyphs, Solstice, Equinox And Eclipse

Art Vortex, Exploration Of The Landscapes Extreme

Mystique Theme, Evergreen Jungle, Ocean, Emotion

Aquamarine, Buddha Gong, Sonic Shield, Inspiration

Sensation, Scent Of Lavender Field, Deep Dream

Soul Returns To The Body, Wake Up Feeling Healed

Mixed media on paper
Size: 17.5 H x 25 W x 0.1 cm

Ingress Vortices - Facebook - Twitter - Flickr - Tumblr - Saachi Art

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I love this man…  [x] [x]

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)

Two (Artificial) Fish Swimming Side-by-Side (Birgit Boschtisch, Peter Dewey, Alexander Smits)

die schönste miss baywatch

How I feel when I talk to Cullen on my designated “you are NOT going to romance Cullen again” playthrough.

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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.

Soap films can create remarkable flow visualizations when illuminated with monochromatic (single color) light. Each of the photos above shows a flow moving from left to right with a small object near the left creating an obstruction. In the top two images, the objects are cylinders; in the lower one it’s a flat plate tilted at 45 degrees. All of the objects shed vortices as the flow moves past. These vortices alternate in direction – the first spins clockwise, the next counter-clockwise, then clockwise again and so on. This pattern is known as a von Karman vortex street and can even show up in the atmosphere! (Image credit: D. Araya et al.)