Two-Dimensional Particle Motions during Rarefied Transport in a Static
Bath: Implications for Bedload Diffusion
- Sarah Williams,
- David Furbish
David Furbish
Vanderbilt University, Vanderbilt University
Author ProfileAbstract
Most particle motions on Earth's surface are fundamentally stochastic
and often occur under rarefied transport conditions. Every particle
makes a unique path along the bed, similar but distinct from the paths
of all other particles in motion, until it loses enough kinetic energy
to become disentrained. The details of a particle's motion are
determined by the amount of energy added and extracted during each
moment of travel. Thus, particle motions physically reflect the complex
energy dynamics at play and are a building block of morphodynamic
theory. A full appreciation of this energy balance is needed to properly
describe the motion of particles and associated disentrainment under
different transport conditions. Often multidimensional behaviors occur
during transport as both a result of and influence on these particle
scale energy dynamics. One such phenomenon is that of particle-scale
random walking during transport which results in diffusion over short
timescales in both the downstream and transverse directions. We have
adopted the Galton board as the fundamental conceptual model on which to
create a mechanistic yet probabilistic formulation of particle
diffusion. Here we provide a data set of two-dimensional particle travel
distances supplemented with high-speed videos of particle-surface
collisions collected during laboratory experiments to characterize the
influence of shedding fluid vortices and angularity on collisional
distances and two-dimensional travel for particles at low Reynolds
numbers. Such a description is consistent with diffusion from the
top-down and may be distinct from the bottom-up, or surface roughness,
controlled random walking that other studies have explored. Preliminary
analysis shows that spherical particles experience jiggling motions
resulting in transverse displacement in the absence of surface roughness
and this behavior is further exaggerated for particles of natural
angularity. We hope to clarify the influence of the particle Reynolds
number in top-down and bottom-up spreading.