Scaling and collapse of conditional velocity structure functions in turbulent premixed flames

Whitman, Samuel; Towery, Colin A.Z.; Poludnenko, Alexei; Hamlington, Peter E.

doi:10.1016/j.proci.2018.07.010

Cited by 23 publications

(26 citation statements)

References 31 publications

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“…The analyses by Sabelnikov et al 9,11 focussed on the two-point velocity correlation statistics in the context of second-order structure functions for weakly turbulent premixed flames in the corrugated flamelets regime 40 where the flame thickness remains smaller than the Kolmogorov length scale. By contrast, Whitman et al 10 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.…”

Section: Introductionmentioning

confidence: 95%

“…The structure functions ( ⃗, ⃗, ), correlation functions ( ⃗, ⃗, ) and spectra obtained using ( ⃗, ⃗, ) are of paramount importance in the fundamental understanding of the spatial structure of turbulent flows and have been analysed in detail for non-reacting turbulent flows. [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.…”

Section: Introductionmentioning

confidence: 99%

“…The signature of heat release should be reflected in the structure functions, which are widely used for experimental data processing [1][2][3][4] , fundamental physical understanding 38 and developing models. 39 To date, only a limited number of investigations [9][10][11] have focussed on the statistical behaviours of the structure functions in turbulent premixed flames. The analyses by Sabelnikov et al 9,11 focussed on the two-point velocity correlation statistics in the context of second-order structure functions for weakly turbulent premixed flames in the corrugated flamelets regime 40 where the flame thickness remains smaller than the Kolmogorov length scale.…”

Section: Introductionmentioning

confidence: 99%

“…The effects of heat release are expected to be strong under the conditions analysed by Sabelnikov et al 9,11 , whereas the effects of turbulence are commonly assumed to dominate under high Karlovitz number (i.e. ≫ 1 where Karlovitz number is the ratio of chemical timescale to small-scale turbulent timescale) conditions investigated by Whitman et al 10 Thus, the behaviours of the structure function in the flames analysed by Whitman et al 10 are expected to resemble the corresponding behaviours in the non-reacting turbulence. These analyses address two limiting behaviours in terms of Damköhler number (i.e.…”

mentioning

confidence: 96%

“…ratio of large-scale turbulent timescale to chemical timescale) and Karlovitz number . For example, the conditions analysed by Sabelnikov et al 9,11 deal with the limiting behaviours for ≫ 1 and ≪ 1, whereas the analysis by Whitman et al 10 is relevant to < 1 and ≫ 1, with the conditioned structure functions being differently defined in these two studies.…”

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confidence: 99%

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

et al. 2019

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The second-order structure functions and their components conditioned upon various events have been analysed for un-weighted and density-weighted velocities using a Direct Numerical Simulation (DNS) database. The heat release due to combustion has been shown to have significant influences on the structure functions and their components conditioned on different mixture states. The use of densityweighted velocities changes the relative magnitudes of differently conditioned structure functions but does not reduce the scatter of these magnitudes. The structure functions conditioned to constant-density unburned reactants at both points and normalized using the root-mean-square velocity conditioned to the reactants are larger at higher values of mean reaction progress variables ̅ (deeper within the flame brush), with this trend being not weakened with increasing turbulence intensity ′⁄. These results indicate that, contrary to a common belief, combustion-induced thermal expansion can significantly affect the incoming constant-density turbulent flow of unburned reactants even at ′⁄ and Karlovitz number as large as 10 and 18, respectively. The statistical behaviours of the structure functions reveal that the magnitude of the flame normal gradient of the velocity component tangential to the local flames can be significant and it increases with increasing turbulence intensity. Moreover, the structure functions conditioned on both points in the heat release zone bear the signature of the anisotropic effects induced by baroclinic torque for the flames belonging to the wrinkled flamelets and corrugated flamelets regimes. These anisotropic effects weaken with increasing turbulence intensity in the thin reaction zones regime.

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Section: Introductionmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 96%

mentioning

confidence: 99%

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

et al. 2019

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Influence of Thermal Expansion on Potential and Rotational Components of Turbulent Velocity Field Within and Upstream of Premixed Flame Brush

Lipatnikov

Sabelnikov

Nikitin

et al. 2020

Flow Turbulence Combust

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Direct Numerical Simulation (DNS) data obtained earlier from two statistically stationary, 1D, planar, weakly turbulent premixed flames are analyzed in order to examine the influence of combustion-induced thermal expansion on the flow structure within the mean flame brushes and upstream of them. The two flames are associated with the flamelet combustion regime and are characterized by significantly different density ratios, i.e. = 7.53 and 2.5. The Helmholtz-Hodge decomposition is applied to the DNS data in order to extract rotational and potential velocity fields. Comparison of the two fields shows that combustion-induced thermal expansion can significantly change the local structure of the incoming constantdensity turbulent flow of unburned reactants by significantly increasing the relative magnitude of the potential velocity fluctuations when compared to the rotational velocity fluctuations in the flow. Such effects are documented not only within the mean flame brush, but also well upstream of it. The effect magnitude is increased by the density ratio , with the effects being well (weakly) pronounced at = 7.53 (2.50, respectively). Moreover, the potential and rotational velocity fields can cause opposite variations of the local area of an iso-scalar surface , =const within flamelets by generating the local strain rates of opposite signs.

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Detonation initiation by compressible turbulence thermodynamic fluctuations

Towery

Poludnenko

Hamlington

2020

Combustion and Flame

Self Cite

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Theory and computations have established that thermodynamic gradients created by hot spots in reactive gas mixtures can lead to spontaneous detonation initiation. However, the current laminar theory of the temperature-gradient mechanism for detonation initiation is restricted to idealized physical configurations. Thus, it only predicts conditions for the onset of detonations in quiescent gases, where an isolated hot spot is formed on a timescale shorter than the chemical and acoustic timescales of the gas. In this work, we extend the laminar temperature-gradient mechanism into a statistical model for predicting the detonability of an autoignitive gas experiencing compressible isotropic turbulence fluctuations. Compressible turbulence forms non-monotonic temperature fields with tightly-spaced local minima and maxima that evolve over a range of timescales, including those much larger than chemical and acoustic timescales. We examine the utility of the adapted statistical model through direct numerical simulations of compressible isotropic turbulence in premixed hydrogen-air reactants for a range of conditions. We find strong, but not conclusive, evidence that the model can predict the degree of detonability in an autoignitive gas due to turbulence-induced thermodynamic gradients.

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Scaling and collapse of conditional velocity structure functions in turbulent premixed flames

Cited by 23 publications

References 31 publications

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

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

Influence of Thermal Expansion on Potential and Rotational Components of Turbulent Velocity Field Within and Upstream of Premixed Flame Brush

Detonation initiation by compressible turbulence thermodynamic fluctuations

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