  One of the most enigmatic science question concerning inertial particle
transport by a turbulent boundary layer flow is the value of the turbulent
Schmidt number as the ratio of particle diffusivity and turbulent eddy
viscosity. Using direct acoustic measurement of turbulent particle flux
profile, and two-phase flow turbulence-resolving numerical simulation, it is
demonstrated that turbulent dispersion of particles is reduced rather than
enhanced when predicted with existing literature model. The explanation lies in
the misleading assumption of settling velocity in quiescent water to estimate
the turbulent particle diffusivity while direct measurements and simulations of
turbulent particle flux support the occurrence of settling retardation.
