December 12, 2012
Grains Cause Meteorite Impact Waves To Spread Unevenly
April Flowers for redOrbit.com - Your Universe Online
Scientists at Duke University have gained a new microscopic picture of the way the energy of a meteorite or a missile transfers to sand and dirt grains by observing high-speed video of projectiles slamming into a bed of disks.The study, published in Physical Review Letters, showed that the transfer is jerky, not smooth as the researchers believed it would be.
"It was surprising just how un-smooth the slow-down of the intruding object was," Duke physicist Robert Behringer said.
The results of this study may change the way scientists model impacts of meteorites and missiles and their effects in the future.
Scientists have previously made the assumption that the slowing down would be smooth and that any sound wave traveling through granular material would move in a regular, uniform pattern, similar to the way noise from hands clapping diffuses through the air evenly in all directions. Behringer and his team, including Lou Kondic of the New Jersey Institute of Technology (NJIT), used high speed video to show a very different behavior for the sound wave than had been previously believed.
The researchers used an ultra fast camera to track the collision energy generated as they shot bronze disks into a narrow bed of photoelastic grains. The films shows that as energy shifted from the disks to the beads, the disks lose most of their energy in intense, sporadic acoustic pulses along networks of grains, called force chains, in the bed of beads.
"This phenomenon was so hard to observe before because of how fast the force chains travel," Behringer said. The standard movie rate is about 30 frames per second. To capture the path of energy down the force chains, the scientists had to use a camera that could capture 40,000 frames per second, 1300 times faster than a normal video, because the sound pulses move at such high speeds.
The disks were shot at speeds up to 6.5 meters per second, or about 15 miles per hour. The force chains started moving the energy away from the intruding object on impact, dumping it deep into the bed of disks like the drainpipes of a septic system carrying waste from a house.
The team noted that because the speed of the disks was under sonic or super-sonic speeds, the patterns of energy transfer could be substantially different.
"For supersonic speeds, it's kind of like the car chases that happen in markets in movies. People can't get out of the way fast enough. Similarly the pulses wouldn't clear the chain networks and the forces would back up rather than get carried away from the intruder," Behringer said.
The team may try experimenting with sonic or supersonic speeds soon, though it requires different grain particles. Behringer added that once a missile or meteor dropped below sonic speeds, the energy transfer would be sporadic and jerky, just as their data showed.