  Passive scalar dynamics in wall-bounded turbulence is studied via Direct
Numerical Simulations of plane channel flow, for a friction Reynolds number
$Re_* = 160$ and a Schmidt number $Sc=1$. Peculiar to the present research is
that the spatial resolution reaches far beyond what has been employed in
similar past studies. Our aim is to examine the statistics of the most
dissipative events across the various layers of the channel flow, and to
compare them to the homogeneous isotropic case, where the recent studies by
Schumacher et al (2005) and Watanabe and Gotoh (2007) have described a range of
scalar micro-scales that require extremely high spatial resolution to be
properly resolved.
  Resolution effects are observed on integral-scale quantities such as the mean
profiles of the scalar dissipation and its variance. By examining probability
distributions, it is found that the finest resolution is essential for
correctly computing small-scale statistics in the near-wall region of the
channel. As expected, this high-resolution requirement extends outwards to the
channel centerline, where the behavior of isotropic turbulence is recovered.
However, high-intensity scalar dissipation events are overemphasized by
marginal resolution near the wall, while they are underemphasized in the
central region.
