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.
sigma=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
sigma, with the effects being well (weakly) pronounced at sigma=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  c( x ,t) =const within flamelets by generating the local strain rates of opposite signs.

中文翻译：

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热膨胀对预混火焰刷内部和上游湍流速度场势分量和旋转分量的影响

对早先从两个统计平稳、一维、平面、弱湍流预混火焰获得的直接数值模拟 (DNS) 数据进行分析，以检查燃烧引起的热膨胀对平均火焰刷内及其上游的流动结构的影响。两种火焰都与小火焰燃烧状态有关，并以显着不同的密度比为特征，即 ? 西格玛 = 7.53 和 2.5。Helmholtz-Hodge 分解应用于 DNS 数据以提取旋转和潜在速度场。两个场的比较表明，与流动中的旋转速度波动相比，燃烧引起的热膨胀可以显着增加潜在速度波动的相对幅度，从而显着改变未燃烧反应物的传入恒定密度湍流的局部结构。这种影响不仅在平均火焰刷中被记录下来，而且在它的上游也有记录。效应量级通过密度比 σsigma 增加，在 sigma=7.53（分别为 2.50）时效果很好（弱）显着。此外，势场和旋转速度场会导致等标量表面的局部区域发生相反的变化 ?? c( x ,t) =const 在小火焰内通过产生相反符号的局部应变率。