Turbulence and fluid-front area production in binary-species, supercritical, transitional mixing layers
Turbulence and fluid-front area production in binary-species, supercritical, transitional mixing layers
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二元物质、超临界、过渡混合层中的湍流和流体前沿区域产生
DOI：
10.1063/1.1688326
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发表时间：
2004
影响因子：
4.6
通讯作者：
J. Bellan
中科院分区：
文献类型：
--
作者：
N. Okong'o;J. Bellan
通过对O_2-H_2和C_7H_(16)-N_2两个物系的时间、超临界混合层的直接数值模拟得到的过渡态数据库的分析，以阐明特定物种的湍流方面和流体解体的特征。尽管所有层的演化都以高密度梯度量级(HDGM)区的形成为特征，但由于特定的、较小的初始密度层结，C_7H_(16)-N_2层表现出比可比的O_2-H_2层更高的生长和更多的全球分子混合以及更大的湍流水平。而O_2-H_2体系过渡态较小的密度梯度和较低的质量分数梯度表明，在局部基础上，混合层表现出增强的混合，这归因于混合溶解度的增加和混合的近理想化。这些热力学特性导致O_2-H_2层比C_7H_(16)-N_2层有更大的不可逆熵产生(耗散)。最大的O_2-H_2耗散主要集中在初始密度层结边界扭曲的HDGM区，而最大的C_7H_(16)-N_2耗散主要集中在HDGM区，这是两种流体混合的结果。为了了解流体的崩解，计算了与质量分数梯度垂直的流体锋面的面积产量，它是在随锋面和流动之间的相对速度移动的坐标系中进行的。在横流局部基础上，C_7H_(16)-N_2层产生更多的面积，并且随着扰动波长的减小和初始雷诺数的增大，面积产量增加。最活跃的区域产生层也显示出最大的垂直涡度和质量分数梯度矢量的概率。对面积生产方程中各项的分析表明，C_7H_(16)-N_2层的压力梯度项均方根贡献很大，这是由于压力梯度大的区域与HDGM区域重合所致。这种重合归因于真实气体行为，这是物种系统特有的，因为两个物种系统的压力梯度和密度梯度的排列相似。质量分数梯度与应变率的对准也取决于物种系统。涡量与物系和初始条件无关，涡度优先与中间应变率特征方向一致，表明涡粘性模型不适用于湍流超临界混合。
Databases of transitional states obtained from direct numerical simulations of temporal, supercritical mixing layers for two species systems, O_2–H_2 and C_7H_(16)–N_2, are analyzed to elucidate species-specific turbulence aspects and features of fluid disintegration. Although the evolution of all layers is characterized by the formation of high-density-gradient magnitude (HDGM) regions, due to the specified, smaller initial density stratification, the C_7H_(16)–N_2 layers display higher growth and increased global molecular mixing as well as larger turbulence levels than comparable O_2–H_2 layers. However, smaller density gradients and lower mass-fraction gradients at the transitional state for the O_2–H_2 system indicate that on a local basis, the layer exhibits an enhanced mixing, this being attributed to the increased mixture solubility and to mixture near-ideality. These thermodynamic features are found responsible for a larger irreversible entropy production (dissipation) in the O_2–H_2 compared to the C_7H_(16)–N_2 layers. The largest O_2–H_2 dissipation is primarily concentrated in HDGM regions that are distortions of the initial density stratification boundary, whereas the largest C_7H_(16)–N_2 dissipation is located in HDGM regions resulting from the mixing of the two fluids. To understand fluid disintegration, the area production of a fluid front perpendicular to the mass fraction gradient is calculated in a coordinate system moving with the relative velocity between the front and the flow. On a cross-stream local basis, the C_7H_(16)–N_2 layers produce more area, and area production increases with smaller perturbation wavelengths combined with larger initial Reynolds numbers. The most active area-producing layer also exhibits the largest probability of having perpendicular vorticity and mass-fraction-gradient vectors. Analysis of the terms in the area production equation shows a large pressure-gradient-term root mean square contribution for the C_7H_(16)–N_2 layers, due to the coincidence of regions with large magnitudes of pressure gradient with HDGM regions. Such coincidence is attributed to real-gas behavior, which is species-system specific, as the alignment of the pressure gradient and density gradient is similar for both species systems. The alignment of the mass fraction gradient with the strain rate is also species-system dependent. Independent of species system and of the initial conditions, the vorticity is preferentially aligned with the intermediate strain-rate eigendirection, indicating that eddy-viscosity-type models are not adequate for turbulent supercritical mixing.