A study of the number of sand grains lifting off per unit time and per unit sand bed area

摘要

Aeolian sand flow is an especial two-phase flow of gas and solid resulted from sands carried by wind to cause the spatial displacement. To simplify this complex process, researchers have divided it into four interacting subprocesses: jumping of sand grains, movement of sand grains in the airflow, changes of the airflow due to the moving sand grains, and impacts between falling sand grains and sand grains in the underlying sand bed. Although it simplifies Aeolian sand flow enough to permit the development of models of these subprocesses, no theory of Aeolian sand flow will be complete without a consideration of coupling among the subprocesses. In addition, two key physical variables must be defined: the probability density distribution for the initial liftoff velocities of saltating sand grains and the number of sand grains lifting off per unit time and per unit sand bed area (also called liftoff rates). Little data exist on liftoff rates because of the difficulty of direct measurements and theoretical analysis. This paper combines wind tunnel experimental data with a theoretical analysis to provide a preliminary solution based on the probability density distribution for the initial liftoff velocities of sand grains. The result shows that liftoff rates increase rapidly with increasing of friction wind velocities. The relationship between friction wind velocities and liftoff rates can be expressed as: N = A 1 2 + 2A 1 B 1 u * + B 1 2 u * 2, A 1 = ?9858.87, B 1 = 14,830.07, R 2 = 0. 97. Although the results of this study require additional experimental validation, they provide a strong basis for future research.