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The Use of Spheres to Measure Lift and Drag on Wind-Eroded Soil Grains¹

ABSTRACT

Experiments were conducted to determine the relative magnitude of lift and drag on small spheres, similar to soil grains, at different heights in a fluid boundary surface and to determine if these forces are related to movement of soil grains in saltation.

Lift on the average was about 75% that of drag when a sphere was at zero height. Lift decreased with height and virtually ceased a short distance above ground. The greater the ground roughness and drag velocity, and therefore the steeper the velocity gradient, the higher lift extended. It is concluded that lift is caused by a steep wind velocity gradient near the ground.

Drag on the spheres was generally much greater than lift. Drag increased directly with an increase in wind velocity and was apparently due to direct pressure of the wind against the sphere.

It is evident from these experiments that lift alone is too small to cause the saltating grains to rise, as they do, essentially vertically. The predominantly vertical rise apparently is the way the saltating grains rebound from the ground.

¹ Contribution from Soil and Water Conservation Research Division, ARS, USDA, with the Kansas, Agr. Exp. Sta. cooperating. Department of Agronomy Contribution No. 715. Presented before Div. I, Soil Science Society of America, Dec. 5, 1960, at Chicago, Ill.

² Soil Scientist, Western Soil and Water Management Research Branch, SWCRD, ARS, USDA, Manhattan, Kans.

Received for publication December 19, 1960. Accepted for publication February 13, 1961.