A study of particle splash on developing ripple forms for two bed materials

A custom-designed Particle Tracking Velocimetry (PTV) system was developed to measure particle splash dynamics in two materials: mixed quartz sand and ground acrylic particles approximating the density of snow. The strengths and limitations of the PTV system are described in the context of five experiments carried out at friction velocities slightly greater than the threshold friction velocity for particle entrainment and saltation. Analysis of particle collisions on the bed surface (splash) demonstrates that the simulated snow particles impact the surface at a smaller angle than quartz sand. They also eject a greater number of particles, though at lower velocities and angles than observed for the sand particles. These relations appear to be highly sensitive to the porosity of the bed surface, while particle shape and elasticity play a secondary role. With regard to PTV measurements carried out along the wind-aligned axis of migrating aeolian ripples, particles ejected from the lee slope tend to be slightly larger than those on the stoss slope, and in general are ejected with a lower speed and greater angle to the surface. Irrespective of the type of granular material, when the average impact angle is close to or greater than the negative slope of the evolving bedform, the particle ejection rate from the stoss slope was measured to be approximately twice that for the lee slope. This empirical observation confirms the assumption made in models of ripple development that the ejection flux governs the bedform migration rate. However, systematic under-prediction of this rate by an early model suggests that additional processes, inclusive of surface creep, must also be accounted for.

Research Highlights
► PTV was used to measure splash for sand and acrylic (simulated snow) beds.
► Acrylic particles impact the surface at a smaller angle and eject more particles.
► Particles ejected from the lee slope of ripples are larger and move slower.
► Results suggest bedform migration is governed by ejection flux.