Statistics of vorticity alignment with local strain rates in turbulent premixed flames

Nilanjan Chakraborty

doi:10.1016/j.euromechflu.2014.01.002

Abstract

The instantaneous alignment of the vorticity vector with local principal strain rates is analysed for statistically planar turbulent premixed flames with different values of heat release parameter and global Lewis number spanning different regimes of combustion. It has been shown that the vorticity vector predominantly aligns with the intermediate principal strain rate in turbulent premixed flames, irrespective of the regime of combustion, heat release parameter and Lewis number. However, the relative alignment of vorticity with the most extensive and compressive principal strain rates changes based on the underlying combustion conditions. Detailed physical explanations are provided for the observed behaviours of vorticity alignment with local principal strain rates. It has been shown that heat release due to combustion significantly affects the alignment of vorticity with local principal strain rates. However, the mean contribution of the vortex-stretching term in the transport equation of enstrophy remains positive for all cases considered here, irrespective of the nature of the vorticity alignment.

Highlights

The alignment of the vorticity vector with local principal strain rates is of fundamental importance for the understanding and modelling of turbulent fluid motion, as the alignment statistics directly affect the nature of the vortex-stretching mechanism [1]
It has been found that vorticity aligns with the intermediate principal strain rate in all cases considered here, which is consistent with previous analyses for both non-reacting and reacting flows [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
Vorticity vector shows non-negligible alignment with the most extensive principal strain rate and this alignment increases in the heat releasing zone for high values of Karlovitz number (i.e. Ka > 1) for flames with Le ≈ 1.0, which is consistent with previous analyses on turbulent reacting flows [13,14,15,16]

Summary

Introduction

The alignment of the vorticity vector with local principal strain rates is of fundamental importance for the understanding and modelling of turbulent fluid motion, as the alignment statistics directly affect the nature of the vortex-stretching mechanism [1]. The vorticity vector shows a significant extent of alignment with the most compressive principal strain rate eγ for the major part of the flame, except the unburned gas side in the corrugated flamelets regime flame (i.e. case A) considered here, and this tendency increases towards the heat releasing zone of the flame (i.e. 0.7 ≤ c ≤ 0.9).

Results

Conclusion