Very fine near-wall structures in turbulent scalar mixing

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.