Why Do Fluxes Near the Granular Bed Scale Differently Than Within the
Transport Layer?
Abstract
Some aspects of the dynamics of aeolian transport over a flat sediment
bed have been thoroughly investigated and are relatively well
understood. The interactions between grains in transport and the wind
give rise to well-known dynamical scaling laws for the fluxes and
concentrations of grains in most of the transport layer. However, recent
work has revealed a sudden shift in these scaling laws near the granular
surface and below. While the vertical flux of grains in the transport
layer scale linearly with excess wind shear stress, the vertical flux
near the granular surface—the ‘erosion rate’—scales linearly with
wind speed. Analysis of numerical modeling results reveal that
near-surface horizontal and vertical fluxes are important for the
instability that leads to wind ripple growth and stabilization as well
as ripple propagation. A few main open questions are: What are the
physical mechanisms behind the scaling of the erosion rate with wind
speed? Could they arise from the small subpopulation of high-energy
grains, who’s characteristics scale differently than the average grain
in transport? As these grains move downward from the free-wind layer, do
they tend to retain their properties as they pass through the feedback
layer, delivering their energy, momentum and scaling directly to the
bed? Do collisions between grains near and within the bed, which
redistribute energy and momentum from high-energy impacts, play a key
role in determining the scaling of near-bed fluxes? How important are
potential collective effects that can occur when impacts with sufficient
energy to excite the bed occur close together in time and space? An
answer to these questions would help complete our understanding of the
physics of aeolian transport, with repercussions that shed light onto
the emergence and propagation of wind ripples. Using a detailed grain
scale numerical model, we are investigating the dynamics of grains near
the granular bed, and what saltation properties drive these dynamics.
Preliminary results, including velocity distributions near the bed,
indicate that the signal from high-energy grains that traverse the
feedback layer from above reaches the bed surface, consistent with the
hypothesis that the surface erosion rate is related to this small
population of grains who’s characteristics scale with the free-wind
speed.