Blacksburg, Va. -- Wind, water, and waves erode billions of tons of soil from the earth's surface. As a result, many rivers are plagued with excessive amounts of suspended sediment. According to the U.S. Environmental Protection Agency, such eroded sediment is the largest nonpoint source pollution in the environment.
While the mechanism responsible for soil erosion may seem obvious wind, water and wave forces breaking apart particles in fact, the precise conditions or criterion that sets a particle free from its mates has not been identified. For 72 years, scientists and engineers have been working with a time-averaged force criterion, originally proposed by A. Shields, an American engineer, to describe threshold conditions for sediment to become mobile.
Now, a team of Virginia Tech College of Engineering faculty members and graduate students have demonstrated that sustained spikes in turbulence are responsible for dislodging particles, whether on land or in the water. They report their research results in the October 31 issue of Science in the article, "The Role of Impulse on the Initiation of Particle Movement Under Turbulent Flow Conditions."*
Scientists and engineers have long suspected that turbulence, an ubiquitous feature of natural fluid flow phenomena, was part of the equation. Anyone who has flown has experienced turbulence. So a guess that turbulence is the culprit was still not sufficiently informative.
"There has been a need to develop a method that accounts for the role of turbulence on soil erosion in a quantitative way," said civil and environmental engineering Professor Panos Diplas, lead author on the research."If you measured the velocity of the air flowing across a fixed place in the middle of Virginia Tech's drill field, you would see that velocity fluctuates wildly," Diplas said.
"Wildly and randomly," said mechanical engineering Associate Professor Clint Dancey, co-author.
|Contact: Susan Trulove|