  We introduce a simple model of active transport for an ensemble of particles
driven by an external shear flow. Active refers to the fact that the flow of
the particles is modified by the distribution of particles itself. The model
consists in that the effective velocity of every particle is given by the
average of the external flow velocities felt by the particles located at a
distance less than a typical radius, $R$. Numerical analysis reveals the
existence of a transition to clustering depending on the parameters of the
external flow and on $R$. A continuum description in terms of the number
density of particles is derived, and a linear stability analysis of the density
equation is performed in order to characterize the transitions observed in the
model of interacting particles.
