Analysis of creeping sand trajectories using high-speed photography

High-speed digital photography was adopted to record sand creep motion in atmospheric boundary layer wind tunnel. Algorithms as non-negative subtraction, dynamical threshold and bidirectional particle tracking were applied to abstract digital creeping trajectories out of image sequence. Based on lift-off velocity distribution, statistical analysis of trajectory characteristic parameter including velocity growth rate and duration of trajectory indicated that, majority of flow-initiated particles tended to velocity maintenance or acceleration, and probabilities of both particles’ sudden stagnation and acceleration before stagnation improved as friction velocity U* increased; nevertheless, distribution of velocity growth rate of collision-initiated creeping trajectories behaved stable and much more independent of U* variation, whose concentrating on [0,1] demonstrated the overall velocity-decaying feature of collision-initiated creep motion. Durations of both kinds of trajectory followed the Gauss Distribution, from which a duration-decaying tendency with increase of U* were obtained.