Abstract
In turbulent flames, chemical species mixing rates are controlled to a large extent by the geometric alignment of the species composition gradients with the local velocity gradients. This alignment indeed characterizes the turbulence--scalar interaction (TSI), which is one of the leading-order source terms in the scalar dissipation rate (SDR) evolution. In situations featuring density variations, which may be relevant either to (i) non-reactive but multiphase or compressible flows, or to (ii) reactive flows such as those considered herein, it should be acknowledged that there is still some controversy about the role of dilatational effects and its connection with the rotation of the strain-rate tensor principal axes. These two issues are analyzed by considering direct numerical simulation databases of flame kernel growth in homogeneous isotropic turbulence (HIT). Two distinct conditions of turbulence--combustion interaction (TCI) are considered: the first is a quasi-laminar reference case while the second displays some local thickening of the preheating zone. Special emphasis is placed on the kinematics of the scalar gradient orientation, which is studied through the direct inspection of the reactive scalar gradient orientation budget. To the best of the authors' knowledge, this is first time that such a budget is scrutinized in premixed combustion conditions. The analysis shows that, for the flow conditions that are presently investigated, the role associated to the rotation of the strain-rate eigenframe is of paramount importance: it is the leading-order term in the scalar gradient orientation budget. This study also confirms that, in the vicinity of the flame, the resulting evolution opposes the rise of the reactive scalar gradient norm (i.e., the rise of the SDR) but the corresponding effects are lessened as the normalized root mean square (RMS) of the velocity fluctuations is increased.