Very Fine Near-Wall Structures in Turbulent Scalar Mixing

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 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 (non-local) 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 behaviour of isotropic turbulence is recovered. However, marginal resolution, that underemphasizes high-intensity scalar dissipation events in the nearly-isotropic central region, is found to overemphasize them significantly near the wall.