Very fine near-wall structures in scalar mixing

TL;DR

This study uses high-resolution DNS to analyze near-wall scalar mixing in turbulent channel flow, revealing the importance of resolution for accurately capturing small-scale dissipation events and comparing wall-bounded and isotropic turbulence.

Contribution

It demonstrates that extremely high spatial resolution is crucial for accurately resolving small-scale scalar dissipation events in wall turbulence, extending understanding beyond previous studies.

Findings

High resolution is essential for accurate small-scale statistics near the wall.

Resolution affects the representation of intense scalar dissipation events.

Isotropic turbulence behavior is recovered at the channel centerline.

Abstract

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.