Measurement of the movement parameters of saltating sand over a flat sand bed using a high-speed digital camera

Abstract

The movement parameters of saltating sand are basic physical characteristics in aeolian physics, but remain poorly understood because of limitations in the available measurement technology. High-speed photography provides an efficient method to track the trajectories of saltating particles. From a digital recording of the trajectory, movement parameters can be derived. However, accurate and quick reconstruction of particle trajectories from video images is not yet possible, as this requires the use of prohibitively laborious manual methods. In addition, light reflection from the bed decreases contrast between near-surface particles and the bed. This makes saltation information near the surface difficult to obtain, even though this data is vitally important for studying particle entrainment processes and the mechanisms by which particles leave the surface. In this paper, we describe a novel method for tracking the trajectories of saltating particles. Using motion-detection algorithms based on the subtraction of consecutive images and dynamic thresholds, we developed a new series of high-contrast images from the original images. We then tracked particles by combining a manual operation with a computerized operation, after which a polynomial fitting method could be used to obtain various movement parameters of the saltating particles, such as the lift-off velocity and angle, impact velocity and angle, characteristic length and height, and instantaneous velocity vector at each point in the trajectory. The new method can quickly and accurately calculate the movement parameters of saltating sand particles in air and near the bed surface.