High-speed photography in measuring the velocity of sand particles in an air/particle two-phase flow

The velocity of saltating sand is a key parameter in aeolian movement,but remains poorly understanding because of limitations in the available measurement technology.High-speed photography illuminated by intensive and continuous laser provides an efficient method to track the trajectories of saltating particles.From a digital recording of the trajectory,particle velocity and other 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 the 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 used motion-detection algorithms based on the subtraction of consecutive images and developed a new series of high-contrast images from the original images.We then describe two novel methods for tracking the trajectories of saltating particles and determining the velocity field in a cloud of blowing sand,respectively.We tracked particles by combining a manual operation with a computerized operation,and then the movement parameters of saltating sand particles in air and near the bed surface,such as the lift-off velocity and angle,impact velocity and angle,characteristic length and height,and instantaneous velocity and accelerated velocity vector at each point in the trajectory were quickly and accurately calculated by polynomial fitting method.Compared with traditional manual track method,this novel technique can easier to obtain a large number of continuous points within one or more trajectories.We evaluated the velocity field by a novel two-frame PTV algorithm based on the concept of maximizing the match efficiency.This approach overcomes some disadvantages of the conventional PTV methods,such as similar movement patterns within a small region,a uniform particle distribution,and high particle density.For the flow of saltating particles,there is high heterogeneity in the particle velocities due to the existence of two distinct grain populations:ascending and descending grains.Therefore,this improved PTV method not only can satisfy the requirement for a large sample size but also is more suitable for measuring the velocity of sand particles in an air/particle two-phase flow.The results shed new light on the complicated mechanisms involved in sand saltation and should prove useful in formulating and evaluating more accurate theoretical models of saltation.