Vertical distribution of wind–sand interaction forces in aeolian sand transport

Abstract

The interaction forces between wind and sand particles are important for particle movement in a blowing sand cloud. These interaction forces include drag force, Magnus force and Saffman force. By using a numerical simulation technique, the vertical distributions of these forces are acquired. The discrete particle model is used to simulate windblown sand movement, in which the gas phase is described by the volume-averaged Navier–Stokes equations, and each particle is directly tracked by solving the particle motion equations considering the inter-particle collisions. In two-dimensional simulations, the wind flow is along the positive direction of the x-coordinate axis, and the height direction is along the positive direction of the y-coordinate axis. The results show that, the mean drag force on a particle in the horizontal direction increases with height, while the total drag force density in each height layer decreases with height. In the positive y and negative y directions, the magnitude of the mean Magnus force on a particle tends to increase with height, while the magnitude of the total Magnus force density decreases with height. The Saffman force mainly occurs in the positive y direction. In this direction, the mean Saffman force on a particle approximates to a constant, except for the region near the sand bed surface, while the total Saffman force density decreases sharply with height below the height of about 3 cm and is close to zero above the height of about 3 cm. In the positive and negative y directions, the magnitude of the mean Magnus force on a particle is more than that of the mean Saffman force. The number density of anticlockwise rotational particles approximates to that of the clockwise rotational particles. Above the height of about 3 cm, the number density of anticlockwise and clockwise particles decays exponentially with height. The probability density function of particle angular velocity at different heights can be described as a normal function. Two kinds of particle shear stress (one from the dispersed phase Reynolds stress, the other from the drag force) are different near the sand bed surface but almost the same at other heights.