Research On Laws Of Incipient Motion For Blown Sand

In the study on two-phase flow of blown sand, its mechanism, that the starting mechanism of sand under the action of the wind field is the basic and important research topics. The sand on the ground under the wind action leaving the sand surface and beginning to move is called incipience of the blown sand, it signifies the beginning of wind erosion. The wind erosion is one of the main powers causing land degradation in arid and semiarid area, and the threshold wind velocities is the key factors affecting wind erosion, so the threshold wind velocities of sand particles has been attracted extensive attention of researchers.The main achievements made in this paper are as follows:1. Based on the analysis of the forces acting on the particles on the sand surface in wind-field, a formula for computing the threshold wind velocity over the slope is derived. The threshold wind velocities obtained from this formula are compared with those got by experimental measurement, the result suggests that the formula is reasonable. Then, the influences of the slope angle, wind direction angle, charge mass ratio, electrostatic force and aerodynamic lift on the sand particle's threshold wind velocity are discussed in detail. The numerical results show that the threshold wind velocity becomes bigger with the increment of the wind direction angle at the same slope angle. However, in the same wind direction,, the threshold wind velocity becomes smaller with the increment of the slope angle in leeward slope. When in week electric field, the effect of electrostatic force on the threshold velocity can be neglected; while in strong electric field, this effect is much more significant, and the threshold wind velocity decreases obviously with the increment of the charge mass ratio. The threshold wind velocity decreases obviously with consideration of the aerodynamic lift, which demonstrates that the effect of aerodynamic lift on the threshold velocity cannot be neglected, either.2. Considering the hiding exposure action among non-uniform sands, the common tangential angle of bed material, whose common tangential line passes through the tangential point of the particles, is introduced in this paper as a parameter to indicate the position of the particles, then, the relation between the common tangential angle and exposure degree is established. Based on the critical condition that the particles on the sand surface is to move, variation of common tangential angle is then used to determine a criterion for non-uniform sand incipience. Relating the hiding exposure action with the position of the particles on the bed and its grain size during incipient motion, based on the static conditions of sand critical incipience, a formula for computing the threshold wind velocity of non-uniform sand is derived. The threshold wind velocities obtained from this formula are compared with those got by experimental measurement, and the correlative coefficient between them is 0.938, which suggests that the formula is reasonable.3. Based on the existing sand flow experiment, we develop the method of trapping creep sand particles to measure creep-sediment-transport rate at different wind speed, then the sand particle's threshold wind velocity can be obtained while the creep-sediment-transport rate is equal to zero. And that the wind speed is defined as the sand particle's integrated wind velocity. Finally, the threshold wind velocities obtained from our experimental measurement are compared with Bagnold's, and the result suggests that our experiment is reasonable.