Synopsis: Grainy picture

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Credit: Y. Ding et al., Phys. Rev. Lett. (2011)

Drag Induced Lift in Granular Media

Yang Ding, Nick Gravish, and Daniel I. Goldman

Published January 13, 2011

Lift and drag—the forces that allow an airplane to take off and land—are normally associated with an object moving in a liquid or gas. Now, a team of scientists reports in Physical Review Letters how an object’s shape affects these forces when it moves through a granular medium, like sand or beads.

Yang Ding and his colleagues at the Georgia Institute of Technology, US, designed a simple experiment to measure the upward (lift) and shear (drag) forces on differently shaped rods moving through a bed of millimeter-sized glass beads. The rods, which had either a circular, square, or half-circle cross section, were suspended from a moving platform and dragged—with their long axis perpendicular to the direction of motion—through the beads. A force sensor on the platform measured the resultant lift and drag forces on the moving rods, which Ding et al. compared with numerical simulations and a theoretical model.

The group finds that lift is downwards on the half-cylinder rod, but upwards on the square and circular rods. Variations on this type of study could help scientists understand how body shape aids a sand-burrowing animal or how to optimize the design of desert-roving robots. – Jessica Thomas

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