|Above: A Von Karman vortex street downwind of the Canary Island chain in the eastern Atlantic Ocean off the coast of Africa on Feb. 27, 2015. Image credit: NASA Worldview.|
Editor's note: Meteorologist Tom Niziol, recently retired as winter weather expert on the Weather Channel, will be filing occasional guest posts here at Category 6.
Von Karman vortices are spectacular in satellite imagery, are more common than they may sound, and, at least in one case, they have proven to be damaging.
As a meteorologist, one of my favorite quotes comes from Lewis Fry Richardson, an English mathematician, physicist and meteorologist who pioneered modern mathematical techniques of weather forecasting. It goes like this:
“Big whirls have little whirls, That feed on their velocity; And little whirls have lesser whirls, And so on to viscosity.”
What does this have to do with Von Karman vortices? Read on and it will be revealed.
The term “vortex” is simply defined as a mass of whirling fluid or air, especially a whirlpool or whirlwind. Von Karman vortex streets are a linear chain of spiraling whirlwinds. They were named after Theodore Von Karman, a co-founder of NASA's Jet Propulsion Laboratory, regarded by many as the outstanding aerodynamic theoretician of the twentieth century. Although this phenomenon was known and studied by other scientists back through the 20th century, Von Karman’s research became synonymous with this phenomenon.
Vortex streets occur in liquids as well as gases. As fluids move toward a cylinder that is sticking up into the prevailing flow, that flow will split around the cylinder. The edge of the fluid that is in contact with the cylinder wall is slowed by friction. This will cause a spin in the flow on either side of the cylinder, creating eddies. Those eddies form in a regular pattern, first one, then the other on the downwind side of the object, and may continue to spin as they move downstream with the prevailing flow. The animation below shows the development of the vortices downwind of either side of the object.
Under specific conditions, the atmosphere can produce Von Karman vortex streets as prevailing winds blow around an object such as a mountain peak or island that sticks up into the flow. If there is a thin deck of clouds within the air stream, those clouds act like dusting fingerprints, revealing the beautiful details of the disturbed flow.
Using satellite imagery, you can find many places on the globe where Von Karman vortex streets develop. One of my favorites is Jeju Island, located south of the Korea Peninsula. It is a roughly 45-mile long and 20-mile wide island (72 x 32 kilometers) with a lone peak that extends to 6400 feet (1950 m) above the water. That mountain peak serves as the perfect object to generate the vortex streets on prevailing northerly winds.
|Figure 1. Diagram of Jeju Island (inset), which sits off the south end of the Korean Peninsula. Image credit: Tom Nizol.|
Recently, Jeju produced an impressive vortex street, captured by the Japanese Himawari satellite. Below is an animation of that event, the island is located at the top of the frame and the vortices are produced at regular intervals as the wind splits around the mountain peak under northwest winds.
|Figure 2. Himawari satellite loop showing the Von Karman vortex street on December 12, 2019 downwind of the island of Jeju in the Korean Strait. Image credit: NASA Worldview.|
As long as you can satisfy the requirements to produce Von Karman vortices—which include something like an island that “sticks up” into the air flow, a favorable wind speed, and cloud cover that can help mark or visualize the wind footprint—
, they can occur elsewhere on the globe.
Below are two other examples from some of my favorite vortex hot spots. Another example is at the top of this post. See if you can determine the wind direction for each of these based on what you have read so far.
|Figure 3. A Von Karman vortex street downwind of the island of Guadalupe off Mexico’s Baja Peninsula on July 20, 2019. Image credit: NASA Worldview.|
|Figure 4. This Von Karman vortex street is located in the Southern Hemisphere, in the Fernandez Island Chain off the coast of Chile on Feb. 2, 2019. Image credit: NASA Worldview.|
Beyond the atmosphere
Von Karman vortex streets are found elsewhere in nature, where they can lead to both positive and negative results.
In regard to negative impacts, let’s think about other structures which might stick up into the airflow, like cylindrical buildings or power plant cooling towers. Architects and engineers are well aware of Von Karman vortices, and they design buildings to mitigate their impact.
In November 1965 a Von Karman vortex street occurred at the Ferrybridge 3 coal-fired power station in West Yorkshire, England. There was a group of eight cooling towers, each over 300 feet tall.
Four of those towers held strong against the buffeting effects of the vortex street. However, the other four, a bit farther downwind of the first towers, experienced the full buffeting effect of those vortices. Three of those four towers collapsed.
|Figure 5. Two of the three 350-foot cooling towers at Ferrybridge, Yorkshire, that were brought down by a Von Karman vortex street on Nov, 1, 1965. The three towers were rebuilt and the other five bolstered. The entire Ferrybridge C complex was decommissioned in 2016 and is being demolished. Image credit: Keystone/Getty Images.|
Let’s end our lesson on Von Karman vortices on a good note, literally.
The same physics that produces the alternating vortices around a cylindrical-shaped object that is put into the airflow can cause a taut string on an instrument to vibrate and produce a sound.
This is the theory behind the Aeolian harp, a musical instrument that is played by the wind. Named for Aeolus, the ancient Greek god of the wind, this instrument can be traced back to biblical times. The motion of the wind across a string causes the Von Karman vortex street, and those alternating vortices will make the string vibrate, resulting in a musical tone.
I can’t close this out without giving all the weather geeks out there a chance to search for your own Von Karman vortex streets using satellite imagery. You can do this using NASA Worldview, starting with the island locations I showed above.