  The way particles interact with turbulent structures, particularly in regions
of high vorticity and strain rate, has been investigated in simulations of
homogeneous turbulence and in simple flows which have a periodic or persistent
structure e.g. separating flows and mixing layers. The influence on both
settling under gravity and diffusion has been reported and the divergence
(compressibility) of the underlying particle velocity field along a particle
trajectory has been recognized as an important quantity in quantifying these
features. This paper shows how these features can be incorporated in a formal
way into a two-fluid model of the dispersed particle phase. In particular the
PDF equation for the particle velocity and position is formerly derived on the
basis of a stochastic process that involves the statistics of both the particle
velocity and local compressibility along particle trajectories. The PDF
equation gives rise to contributions to both the drift and particle diffusion
coefficient that depend upon the correlation of these quantities with the local
carrier flow velocity. Key Words: turbulent structures, particle dispersion,
drift, PDF approach
