Tag: cavitation

Supercavitating Penguins
[original media no longer available]
Penguins, already fluid dynamicists by nature, have developed clever methods of increasing their speed to escape from the leopard seals that prey on them. In the clip above, notice from 1:55 onward as the penguins swim for the surface and leap onto the ice – they leave a trail of bubbles in their wake. The penguins are using supercavitation to decrease their drag. When the penguins first dive in to the water, they splay their feathers out in the air and then lock them closed in the water, trapping pockets of air beneath them. When the need for a burst of speed arises, the penguin shifts its feathers to release the air, coating most of its body in a layer of bubbles. Because the drag in air is much less than the drag in water, this enables the bird to achieve much higher speeds than they normally do when swimming.
April 24, 2012
Granular Eruptions
Granular flows, which are made up of loose particles like sand, often display remarkably fluid-like behavior. Here researchers explore the behavior of granular flows when a solid impacts them at high speed. The sand, unlike a fluid, does not have surface tension, yet we still observe many of the same behaviors. Like a fluid, the sand splashes and creates cavities and jets as it deforms around the fallen object. The sand even “erupts” as submerged pockets of air make their way back to the surface.
April 4, 2012
Cavity Collapse
When a solid object is driven into a quiescent liquid, a cavity is formed. As the cavity collapses jets–a type of singularity–form. In this video, researchers explore the effect of the geometry of a disk being driven into water on the shape of the cavity formed and how it collapses. As in this video of droplet impacts on posts of different geometries, there’s a lovely symmetry in the results. (Video credit: O. Enriquez et al)
December 6, 2011
Stone-Skipping Physics
Many people have learned to throw skipping stones across a pond or lake, but how many have considered the physics of how it happens? In this video, researchers use high-speed video to explore the skipping of various balls across water. The deformation of the ball as well as the shape of the cavity its impact creates determines whether it rebounds off the water’s surface.
November 29, 2011
Bullet Shock Wave and Cavitation
A 9mm bullet impacts a falling jet of water. High-speed video reveals the formation of a shock wave inside the jet. Because this shock wave is confined inside the jet, it causes strong secondary cavitation–the bubble that seems to explode in front of the bullet.
October 25, 2011
To Splash or Not to Splash?
Hydrophobic surfaces tend to repel water while hydrophilic ones attract it. This video explores the effects that hydrophobic and hydrophilic surface coatings can have on spheres when dropped in water. There are noticeable differences in splash formation and wake shape. For more, see this research paper.
August 8, 2011
High Hopes
This gorgeous high-speed video captures bubbles, droplets, wakes, cavitation, coalescence, jets, and lots of surface tension at 7000 fps. The authors unfortunately haven’t indicated whether this is air in water or something more viscous, but regardless there are some great phenomena on display here. # (via Gizmodo)
July 7, 2011
Propeller Cavitation
Cavitation occurs in moving liquids when the local pressure–in this case, at the tip of the propeller–drops below the vapor pressure. The fast-moving fluid transitions to a gas phase, creating a tip vortex of water vapor even though the propeller is completely submerged.
June 28, 2011
Underwater Explosions
As powerful as explosions can be above ground, they are even more dangerous underwater. Since water, unlike air, is incompressible, the pressure wave at the front of an underwater explosion is not damped to the extent it would be in air. A high-pressure, high-temperature bubble of gas also forms in the explosion, and, as with cavitation, if the bubble collapses near metal, the damage can be extensive. (via Gizmodo)
March 23, 2011
Propeller Cavitation
Gas bubbles can form in a flowing liquid in areas where the pressure drops below its vapor pressure. This process, called cavitation, is a major problem for engineers because the collapse of the bubbles upon entering a high pressure area can damage metal surfaces. Shown here is cavitation on a fully submerged boat propeller.
March 15, 2011