  Low Stokes number particles at dilute concentrations in turbulent flows can
reasonably be approximated as passive scalars. The added presence of a drift
velocity due to buoyancy or gravity when considering the transport of such
passive scalars can reduce the turbulent dispersion of the scalar via a
diminution of the eddy diffusivity. In this work, we propose a model to
describe this decay and use a recently developed technique to accurately and
efficiently measure the eddy diffusivity using Eulerian fields and quantities.
We then show a correspondence between this method and standard Lagrangian
definitions of diffusivity and collect data across a range of drift velocities
and Reynolds numbers. The proposed model agrees with data from these direct
numerical simulations, offers some improvement to previous models in describing
other computational and experimental data, and satisfies theoretical
constraints that are independent of Reynolds number.
