Enstrophy transport conditional on local flow topologies in different regimes of premixed turbulent combustion

Abstract

Enstrophy is an intrinsic feature of turbulent flows, and its transport properties are essential for the understanding of premixed flame-turbulence interaction. The interrelation between the enstrophy transport and flow topologies, which can be assigned to eight categories based on the three invariants of the velocity-gradient tensor, has been analysed here. The enstrophy transport conditional on flow topologies in turbulent premixed flames has been analysed using a Direct Numerical Simulation database representing the corrugated flamelets (CF), thin reaction zones (TRZ) and broken reaction zones (BRZ) combustion regimes. The flame in the CF regime exhibits considerable flame-generated enstrophy, and the dilatation rate and baroclinic torque contributions to the enstrophy transport act as leading order sink and source terms, respectively. Consequently, flow topologies associated with positive dilatation rate values, contribute significantly to the enstrophy transport in the CF regime. By contrast, enstrophy decreases from the unburned to the burned gas side for the cases representing the TRZ and BRZ regimes, with diminishing influences of dilatation rate and baroclinic torque. The enstrophy transport in the TRZ and BRZ regimes is governed by the vortex-stretching and viscous dissipation contributions, similar to non-reacting flows, and topologies existing for all values of dilatation rate remain significant contributors.

Statistical behaviors of conditioned two-point second-order structure functions in turbulent premixed flames in different combustion regimes

“…cases D-F) is governed by vortexstretching, dilatation and molecular dissipation terms. [14][15][16][17]74,75 In these cases, enstrophy drops monotonically from the leading edge and the components 1 , 2 and 3 behave in a similar manner within the flame. [14][15][16] This further suggests that the effects of anisotropy can be significant for flames in the wrinkled flamelets and corrugated flamelets regime but these effects progressively weaken in the thin reaction zones regime with increasing Karlovitz number.…”

“…Moreover, the vorticity and sub-grid flux statistics obtained from simple chemistry 14,60 DNS are found to be qualitatively consistent with those obtained from detailed chemistry DNS. 17,61 Furthermore, it has previously been shown that several models developed based on simple This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.…”

“…(∇ × ∇ )/ 2 ) plays a key role in the vorticity ⃗ ⃗⃗ = ∇ × ⃗⃗ transport and enhances 2 and 3 . [13][14][15][16][17]74,75 By contrast, the vorticity transport in cases with ′ ⁄ ≫ 1 (e.g. cases D-F) is governed by vortexstretching, dilatation and molecular dissipation terms.…”

“…[1][2][3][4] To date, limited investigations have been directed to the analysis of correlation functions and spectra [5][6][7][8] or structure functions [9][10][11] in turbulent reacting flows. It is well-known that the effects of heat release on turbulent fluid motion significantly modify both the vorticity or/and enstrophy [12][13][14][15][16][17][18][19] and turbulent kinetic energy [20][21][22][23][24] transports in premixed turbulent flames, e.g. through the baroclinic torque and dilatation rate arising from thermal expansion.…”

“…through the baroclinic torque and dilatation rate arising from thermal expansion. Furthermore, it has been argued [25][26][27][28] and shown both experimentally [29][30][31][32] and computationally [12][13][14][15][16][17][18][19][20][21][22][23][24] that significant amplification of velocity fluctuations known as flame-generated turbulence 25,28 can occur in flames under certain conditions. There are other manifestations of the heatrelease effects [32][33][34] and interested readers are referred to Refs.…”