The von Karman vortex street of shed vortices that form the wake of a stationary cylinder are a classic image of fluid dynamics. Here we see a very different wake structure, also made up of vortices shed from a cylindrical body. This wake is formed by two identical cylinders, each rotating at the same rotational rate. Their directions of rotation are such that the cylinder surfaces in between the two cylinders move opposite the flow direction (i.e. top cylinder clockwise, bottom anti-clockwise). This results in a symmetric wake, but the symmetry can easily be broken by shifting the rotation rates out of phase. (Photo credit: S. Kumar and B. Gonzalez)
Tag: vortex shedding
Flapping Wakes
As a flapping object moves through a fluid, many patterns of vortices can form in its wake. The familiar von Karman vortex street, so often seen in clouds or behind cylinders, is only the beginning. In the photo above, a symmetric foil flaps in a vertical soap film; as the amplitude and frequency of the oscillation varies, the wake patterns it produces change dramatically. From left to right, a) a von Karman wake; b) an inverted von Karman wake; c) a 2P wake, in which two vortex pairs are shed with each cycle; d) a 2P+2S wake, in which two vortex pairs and two single vortices are shed per cycle; e) a 4P wake; and f) a 4P+2S wake. See some of these flows in action in these videos. (Photo credit: T. Schnipper et al.)
Volcanic Vortices
The volcanoes of the South Sandwich Islands, located in the South Atlantic, have a notable effect on cloud formation in this satellite photo. Visokoi Island, on the right, sheds a wake of large vortices that distort the cloud layer above it. On the left, Zavodovski Island’s volcano does the same, with the added effect of low-level volcanic emissions, which include aerosols. These tiny particles provide a nucleus around which water droplets form, causing an marked increase in cloud formation visible in the bright tail streaming off the island. (Photo credit: NASA, via Earth Observatory)
Brine Shrimp Swimming
For small creatures, swimming is dominated by viscosity. Here researchers use particle image velocimetry (PIV) to explore the flow field around brine shrimp. Its motion is divided into two vorticity-generating phases–the wide power stroke where the shrimp generates most of its forward motion and the recovery stroke where the shrimp returns its starting position while generating as little motion and drag as it can. (Video credit: B. Johnson, D. Garrity, L. Dasi)
Supersonic Flow Around a Cylinder
This numerical simulation shows unsteady supersonic flow (Mach 2) around a circular cylinder. On the right are contours of density, and on the left is entropy viscosity, used for stability in the computations. After the flow starts, the bow shock in front of the cylinder and its reflections off the walls and the shock waves in the cylinder’s wake relax into a steady-state condition. About halfway through the video, you will notice the von Karman vortex street of alternating vortices shed from the cylinder, much like one sees at low speeds. The simulation is inviscid to simplify the equations, which are solved using tools from the FEniCS project. (Video credit: M. Nazarov)
Artificial Fins in Tandem
For this image, two artificial fish fins are placed side-by-side and flapped in phase. Flow in the image is upward. The wakes of the fins interact in a complicated vortex street. Researchers hope that studying such flows can help in designing the next generation of autonomous underwater vehicles. (Photo credit: B. Boschitsch, P. Dewey, and A. Smits)
Cloud Streets from Space
Cloud streets flowing south across Bristol Bay hit the Shishaldin and Pavlof volcanoes, which part the air flow into distinctive swirls called von Karman vortex streets. As air flows around the volcano, a vortex is shed first on one side, then the other. Although the usual example for this type of flow is the wake of a cylinder, vortex streets can extend behind any non-aerodynamic body immersed in a flow. The same phenomenon is responsible for the singing of power lines in the wind. As astronaut Dan Burbank observes, “It’s classic aerodynamics, but on a thousands of miles scale.” (Photo credit: Dan Burbank, NASA)
Pitching Plate Flow Viz
This photograph uses fluorescent dye to visualize the wake behind a rigid flat plate pitching about its leading edge. A vortex is shed from the plate twice in each cycle of oscillation. These vortices entangle, producing the structured wake above. The top photo shows a side view of the wake, the bottom photo is a top view. (Photo credit: J. Buchholz and A. Smits)
Vortex Street Sim
This numerical simulation shows a von Karman vortex street in the wake of a bluff body. As flow moves over the object, vortices are periodically shed off the object’s upper and lower surfaces at a steady frequency related to the velocity of the flow. The simulation takes place in a channel; note how the thickness of the boundary layers on the walls increases with downstream distance, forcing a slight constriction on the vortex street in the freestream.
