  Lagrangian tracking of particle pairs is of fundamental interest in a large
number of environmental applications dealing with contaminant dispersion and
passive scalar mixing. The aim of the present study is to extend the
observations available in the literature on relative dispersion of fluid
particle pairs to wall-bounded turbulent flows, by means of particle pair
tracking in direct numerical simulations (DNS) of a turbulent channel flow. The
mean-square change of separation between particle pairs follows a clear
ballistic regime at short times for all wall distances. The Eulerian structure
functions governing this short-time separation are characterised in the
channel, and allow to define a characteristic time scale for the ballistic
regime, as well as a suitable normalisation of the mean-square separation
leading to an overall collapse for different wall distances. Through fluid
particle pair tracking backwards and forwards in time, the temporal asymmetry
of relative dispersion is illustrated. At short times, this asymmetry is linked
to the irreversibility of turbulence, as in previous studies on homogeneous
isotropic flows. The influence of the initial separation (distance and
orientation) as well as the influence of mean shear are addressed. By
decomposing the mean-square separation into the dispersion by the fluctuating
velocity field and by the average velocity, it is shown that the influence of
mean shear becomes important at early stages of dispersion close to the wall
but also near the channel centre. The relative dispersion tensor $\Delta_{ij}$
is also presented and particularly the sign and time evolution of the
cross-term $\Delta_{xy}$ are discussed. Finally, a ballistic cascade model
previously proposed for homogeneous isotropic turbulence is adapted here to
turbulent channel flows. Preliminary results are given and compared to the DNS.
