Scalar mixing at the outer edge of high density turbulent jets

变密度圆射流是工程应用中最重要的混合装置之一。它们最突出的特点是，射流流体和周围流体的密度比决定了射流的穿透，从而决定了两种流体的混合。传统上接受的缩放的中心线衰减的速度和标量的密度比的平方根导出的两种流体的小密度差。对于较大的密度差，密度比的换算基本上是经验性的，并且取决于射流外缘卷吸的换算。周围的无旋流体被卷入射流的旋涡部分的流体力学过程还没有很好地理解。因此，它不能很好地描述现有的模拟方法，如大涡模拟（LES）。即使对于恒定密度射流，实验和LES模拟的平均被动标量的条件配置文件，以及在标量的概率密度函数（PDF）之间也出现了很大的差异。对于高密度射流，完全缺少分辨良好的实验数据。因此，本建议希望调查瑞利散射实验，直接数值模拟（DNS）和LES模拟的组合效果的大密度比和大的变化的涡量在湍流圆形射流的外缘标量混合和所产生的卷吸。实验将进行两个雷诺数的六氟化硫（SF6）射流到三个不同的coflow，即到纯N2，到He-N2混合物和到纯氦气，从而达到密度比高达34.5。实验将使用高重复率的平面瑞利散射来解决，基于泰勒斯假设，三维密度场。将对这六种实验情况进行DNS和LES模拟，目的是再现实验。各种LES配方的未解决的数量将通过比较计算的条件配置文件和标量PDF与实验和DNS结果进行测试。测试将使用最佳估计的概念，以获得最佳的涡流粘度子过滤器模型与不稳定性校正。对于标量PDF，将选择先前导出的复合PDF模型中的参数的不同公式，并确定不同选择的误差。

Variable density round jets are among the most important mixing devices in many engineering applications. Their most prominent feature is the fact that the density ratio between that of the jet fluid and that of the surrounding fluid determines the penetration of the jet and thereby the mixing of the two fluids. The traditionally accepted scalings of the centerline decay of velocity and scalars with the square root of the density ratio were derived for small density differences of the two fluids. For large density differences the scaling with the density ratio is essentially empirical and depends on the scaling of the entrainment at the outer edge of the jet. The fluid mechanical process by which the surrounding irrotational fluid is entrained into the vortical part of the jet is not well understood. It therefore cannot not be well enough described by existing simulation methods like Large Eddy Simulations (LES). Even for constant density jets substantial differences show up between experiments and LES simulations in conditional profiles of the mean passive scalar, as well as in the Probability Density Function (PDF) of the scalar. For high density jets well resolved experimental data are missing entirely. The present proposal therefore wants to investigate by Rayleigh scatter experiments, by Direct Numerical Simulations ( DNS ) and by LES simulations the combined effect of a large density ratio and a large change of vorticity at the outer edge of turbulent round jets on scalar mixing and the resulting entrainment. The experiments will be performed for two Reynolds numbers of sulfur-hexafluoride (SF6) jets into three different coflows, namely into pure N2, into a He-N2 mixture and into pure helium, thereby attaining density ratios up to 34.5. The experiments will use high repetition planar Rayleigh scattering to resolve, based on Taylors hypothesis, the three-dimensional density field. DNS and LES simulations will be performed for these six experimental cases with the aim to reproduce the experiments. Various LES formulations for the unresolved quantities will be tested by comparing the calculated conditional profiles and the scalar PDF with the experiments and the DNS results. The tests will use the concept of optimal estimators to obtain an optimal eddy viscosity sub-filter model with intermittency corrections. For the scalar PDF different formulations of the parameters in a previously derived composite PDF model will be chosen and the errors for the different choices will be determined.