Abstract
In the framework of turbulence-flame interaction, the flame is characterized by the gradient of a reactive scalar such as the progress variable, whereas the turbulence is represented by the vorticity and the strain rate. Quantitative assessment of this interaction is performed trough the study of the coupled transport between these quantities that are subject to the effects of heat release and chemical reactions. The present analysis aims at improving the understanding of the small scale turbulence – flame interaction properties, through the introduction of an additive decomposition of the strain rate and vorticity fields into their local and non-local components. The respective role of the local and non-local effects is studied for a broad range of Karlovitz numbers, by virtue of direct numerical simulations (DNS) of turbulent, premixed, lean, and statistically planar flames of methane-air. In the conditions of the present study, the alignment between flame front normals and the strain rate is found to be dominated by the local contribution from the strain rate tensor.
Highlights
Turbulence-flame interactions are often quantitatively investigated by means of characterizing the two-way coupling between the turbulent flow field and the gradient of the progress variable c that evolves according to
The direct numerical simulations (DNS) database consists of two simulations of an inlet–outlet configuration of statisticallyplanar turbulent premixed lean methane flames evolving in a box with periodic lateral boundary conditions
The conditional average of the TSI term, as well as its local/non–local decomposition, as a function of the progress variable is scrutinized in fig
Summary
One of the key findings of this family of analysis, conducted either on the passive and reactive scalars with passive chemical reactions or flame characterized with a high turbulence intensities, is the preferential alignment of the scalar gradient with the most compressive direction e3A [1, 2, 3] This behaviour is altered in weak to moderate turbulence, where the flame–induced thermal expansion affects the turbulence–scalar interaction and the normals are aligned with the most extensive direction e1A [2, 4]. By performing a complex Schur decomposition of the velocity gradient tensor Keylock [5], dissociated the normal/local dynamics of the velocity gradient from the other dynamics of the velocity gradient in the context of non–reacting homogeneous isotropic turbulence (HIT) Thanks to this additional decomposition, supplementary elements of explanation of the behaviour of strain and vorticity alignments are provided.
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