Scalar mixing at the outer edge of high density turbulent jets

在许多工程应用中，变密度圆形射流是最重要的混合装置之一。它们最显著的特点是射流流体的密度与周围流体的密度之比决定了射流的穿透，从而决定了两种流体的混合。传统上接受的速度中心线衰减的标度和密度比的平方根标度是在两种流体密度差很小的情况下推导出来的。对于较大的密度差，密度比的标度基本上是经验的，取决于射流外缘夹带的标度。周围的无旋转流体被带入射流的垂直部分的流体力学过程尚不清楚。因此，现有的模拟方法如大涡模拟（LES）不能很好地描述它。即使对于恒定密度的射流，在平均被动标量的条件剖面以及标量的概率密度函数（PDF）中，实验和LES模拟之间也显示出实质性的差异。对于高密度喷流，完全没有很好的实验数据。因此，本文希望通过瑞利散射实验、直接数值模拟（DNS）和LES模拟来研究大密度比和湍流圆形射流外缘涡度的大变化对标量混合和由此产生的卷带的综合影响。实验将以两种雷诺数的六氟化硫（SF6）射流进入三种不同的共流，即进入纯N2，进入He-N2混合物和进入纯氦，从而获得高达34.5的密度比。实验将利用高重复平面瑞利散射在泰勒假设的基础上解析三维密度场。我们将对这六个实验案例进行DNS和LES模拟，以重现实验结果。通过将计算的条件剖面和标量PDF与实验和DNS结果进行比较，将对未解析量的各种LES公式进行测试。试验将使用最优估计器的概念来获得具有间歇修正的最优涡流粘度子过滤器模型。对于标量PDF，将选择先前导出的复合PDF模型中参数的不同公式，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.