Tag: wake

Walking in the Wake of a Cylinder
A cylinder in a flow produces a series of alternating vortices known as a von Karman vortex street. Changing the flow speed and rotating the cylinder both allow researchers to tune the frequency of these shed vortices. What happens to an object in the wake?
For a simple hydrofoil tethered to the cylinder, the object wends back and forth along the vortices. But when that hydrofoil sits at the end of a double-pendulum, something very interesting happens. The whole apparatus follows a consistent trajectory similar to a human walking gait. Researchers are using this motion to build a robot that will help physical therapy patients regain a natural walking style. (Image and video credit: A. Carleton et al.)
May 8, 2023
Swimming in Line
When swimming in open waters, it pays to keep your ducks (or your goslings!) in a row. A recent study examined the waves generated behind adult water fowl and found that babies following directly behind them benefit from their wake. In the right spot behind its mother, a duckling sees 158% less wave-drag than it would when swimming solo. That’s such a large reduction that the duckling actually gets pulled along! And the advantage doesn’t just help one duckling; a properly-placed duckling passes the benefit on to its siblings as well. So any duckling that stays in line has a much easier time keeping up, but those who slip out of the ideal spot will have a much tougher time. (Image credit: D. Spohr; research credit: Z. Yuan et al.; via Science News; submitted by Kam-Yung Soh)
November 30, 2021
Flexible Filament Reduces Drag
Most shapes aren’t streamlined for fluid flow. We call these bulky, often boxy shapes, bluff bodies. Above, we see two examples of a bluff body, a flat plate, in a soap film. On the left, the plate sits perpendicular to the soap film’s top-to-bottom flow. Two large, counter-rotating vortices form behind the plate and a wide wake stretches behind it.
On the right, we see the same flat plate but now a long, flexible filament is attached to either end. As the flow moves past, it deforms the filament, creating a rounded shape. Researchers found that, under the right conditions, this flexible afterbody could reduce drag on the object by up to 10%. (Image and research credit: S. Gao et al.)
September 2, 2020
Trails from a Delta Wing
Rhodamine (red) and fluorescein (green) dyes highlight the complex flows around a delta wing. To visualize the flow, researchers painted the apex of the delta wing with rhodamine, which gets drawn into the core of the wing’s leading edge vortex. The green fluorescein dye was added to the wing’s trailing edge, where it gets pulled into the secondary structure of the vortices. A laser illuminates the flow, making even the most delicate wisps of dye shine. As the wake behind the wing develops, the dyes reveal growing instabilities along the vortices. Given time and space, these instabilities will grow large enough to destroy any order in the wake, leaving behind turbulence. (Image and research credit: S. Morris and C. Williamson; see also poster)
November 5, 2019
Asymmetric Wakes
When a ship moves through water, it leaves a distinctive V-shaped wake behind it. In the nineteenth century, Lord Kelvin made some of the earliest theoretical studies of this phenomenon, calculating that the arms of the V should have an angle of about 39 degrees, known as the Kelvin angle. But that theoretical result doesn’t always hold in practice.
More recently, researchers calculated and experimentally verified an extension to Kelvin’s theory, one which accounts for what’s going on below the water. They found that any shear in the currents below the surface can strongly affect the shape of a boat’s wake, altering angles and creating asymmetry between the two sides. The results have practical consequences, too: they help predict the wave resistance ships will encounter when traversing areas with substantial subsurface shear, like near the mouths of river deltas. (Image credit: M. Adams; research credit: B. Smeltzer et al.; submitted by clogwog)
September 4, 2019
Entraining Bubbles
If you stand on a bridge and watch the current flow past pylons below, you’ll see disturbances marking the wakes. Dragging a rod – or an oar – at a high enough speed through the water creates something similar: a wavy cavity in the fluid surface that surfs along behind the rod. The faster you pull the rod, the harder you’ll have to work, until that wake becomes so turbulent that it begins entraining air bubbles, like the tiny ones seen above. Once entrainment starts, the drag coefficient drops somewhat, presumably due to changes in the pressure distribution around the rod. The characteristics of air entrainment change with object size as well. Larger rods can entrain air through the cavity and not just in the wake. (Image and research credit: V. Ageorges et al.)
July 31, 2019
Reshaping the Wake to Decrease Drag
When it comes to the aerodynamics of cars, there’s only so much streamlining one can do. In the end, most cars have a certain boxy-ness as a matter of practicality; they do, after all, have to carry people and things. But that doesn’t mean we’re stuck with the level of drag those shapes entail.
For cars and other non-streamlined objects, much of their drag comes from their wake, which usually contains a large, asymmetric, and unsteady recirculation region. In a new wind tunnel study, scientists used air blasts to reshape this wake, making it more symmetrical, even when the wind direction did not align with the car model. That reduced the drag by 6%. They’re now experimenting with adding additional nozzles along the non-windward edges of the model to see if they can reduce drag even further.
Although this appears to be the first time this technique has been tested for road vehicles, the idea of blowing air to improve aerodynamics is well-established, particularly in aviation. (Image credit: V. Malagoli; research credit: R. Li et al., submitted by Marc A.)
May 1, 2019
Powdery Trails
Because air and water are colorless and transparent, we cannot see most of the flows around us – but they’re always there. In a recent series, photographer Jess Bell has been capturing images of jumping dogs trailing a colorful powder wake. There’s no compositing in the photos. Bell puts powder on the dogs, then photographs them as they jump. The results show the billowing, turbulent wakes left by the dogs. I particularly like how you can see the stream of powder coming from some of the dogs’ ears. For more of Bell’s work, check out her website and Instagram. (Image credit: J. Bell; via PetaPixel and Rakesh R.)
February 8, 2019
Dinosaurs, Propellers, and Hiding Objects
The latest FYFD/JFM video is out, and it’s all about the interactions between structures and flows! We learn about plesiosaur-inspired underwater robots, how turbulence affects air-water interfaces, and how adding a tail can help hide an object in a flow. If you missed one of the previous episodes in this series, you can find them all here. (Image and video credit: T. Crawford and N. Sharp)
November 14, 2018
Swirls of Color
These beautiful swirls show the wake downstream of a thin plate. Here water is flowing from left to right and dye introduced on the plate (upstream and unseen in the photo) curls up into vortices. The vortices in the top row rotate clockwise, while the vortices along the bottom rotate anti-clockwise. This pattern of alternating vortices is extremely common in the wakes of objects and is known as a von Karman vortex street. Similar patterns are seen in soap films, behind cylinders, in the wakes of islands, and behind spaceships. (Image credit: ONERA, archived here)
October 24, 2018