  Dispersion of a passive scalar from concentrated sources in fully developed
turbulent channel flow is studied with the probability density function (PDF)
method. The joint PDF of velocity, turbulent frequency and scalar concentration
is represented by a large number of Lagrangian particles. A stochastic
near-wall PDF model combines the generalized Langevin model of Haworth & Pope
with Durbin's method of elliptic relaxation to provide a mathematically exact
treatment of convective and viscous transport with a non-local representation
of the near-wall Reynolds stress anisotropy. The presence of walls is
incorporated through the imposition of no-slip and impermeability conditions on
particles without the use of damping or wall-functions. Information on the
turbulent timescale is supplied by the gamma-distribution model of van Slooten
et al. Two different micromixing models are compared that incorporate the
effect of small scale mixing on the transported scalar: the widely used
interaction by exchange with the mean (IEM) and the interaction by exchange
with the conditional mean (IECM) model. Single-point velocity and concentration
statistics are compared to direct numerical simulation and experimental data at
Re_\tau=1080 based on the friction velocity and the channel half width. The
joint model accurately reproduces a wide variety of conditional and
unconditional statistics in both physical and composition space.
