  The transport of heavy particles in a medium that consists of fluid and solid
phases such as stream gravel beds, cracked soils and wetlands is affected by
processes such as attachment-detachment, gravity and drag, and by mixing
processes that are induced by Taylor dispersion and mechanical dispersion. This
paper addresses an additional dispersion mechanism which is induced by
gravitational settling and is a result of the coupling between the modified
particle concentration (the change of particle number density due to settling)
and the lateral velocity profiles at the subscale. Heavy particles that move in
areas of low horizontal velocity (e.g., near solid surfaces and wake regions)
settle closer to the release source as compared to particles in high velocity
regions. The macroscopic concentration field of such suspensions is influenced
by the ratio between the settling velocity and the subscale distribution of the
horizontal velocities. The objective of the study is to isolate and quantify
this type of dispersion using controlled flow scenarios. We used a Taylor brush
geometry (an array of vertical grooves) to numerically solve the flow.
Particles were released from a vertical plane, generating a constant flux. The
subscale Eulerian concentration was compiled from simulated trajectories and
then spatially averaged to generate macroscopic concentration fields. The
results show that this settling-induced dispersion is significant in regions
near the source and that it cannot be modeled using Fick's law type of
formulations. A parametric investigation shows that the location of the highest
dispersion flux is linearly proportional to both the groove spacing and depth.
A proposed model that estimates this location is used to evaluate where
settling-induced dispersion should not be ignored.
