Threshold Speed and Grain Size
WHEN the flow of air or of water over a flat bed of loose grains is gradually increased, there comes an instant when a few grains here and there begin to be dislodged and moved downstream by the force of the fluid upon them. At the instant of dislodgement any individual grain so moved is being turned about its leeward points of contact with the underlying grains, as indicated in Fig. 27. The force of the grain’s immersed weight, acting through the centre of gravity, is pulling the grain backwards and downwards; and this is just overbalanced by a drag force due to the fluid acting parallel to the general surface but at some rather indefinite height above the points of support.
KeywordsWind Tunnel Critical Diameter Threshold Velocity Sand Surface Wind Strength
Unable to display preview. Download preview PDF.