Large-eddy simulation (LES) experiments have been performed using the Parallelized LES Model (PALM). A methodology for validating and understanding LES results for plume dispersion and concentration fluctuations in an atmospheric-like flow is presented. A wide range of grid resolutions is shown to be necessary for investigating the convergence of statistical characteristics of velocity and scalar fields. For the scalar, the statistical moments up to the fourth order and the shape of the concentration probability density function (p.d.f.) are examined. The mean concentration is influenced by grid resolution, with the highest resolution simulation showing a lower mean concentration, linked to larger turbulent structures. However, a clear tendency to convergence of the concentration variance is observed at the two higher resolutions. This behaviour is explained by showing that the mechanisms driving the mean and the variance are differently influenced by the grid resolution. The analysis of skewness and kurtosis allows also the obtaining of general results on plume concentration fluctuations. Irrespective of grid resolution, a family of Gamma p.d.f.s well represents the shape of the concentration p.d.f. but only beyond the peak of the concentration fluctuation intensity. In the early plume dispersion phases, the moments of the p.d.f. are in good agreement with those generated by a fluctuating plume model. To the best of our knowledge, our study demonstrates for the first time that, if resolution and averaging time are adequate, atmospheric LES provides a trustworthy representation of the high order moments of the concentration field, up to the fourth order, for a dispersing plume.

中文翻译：

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无限雷诺数下中性边界层被动羽流浓度波动大涡模拟的收敛性和能力

使用并行 LES 模型 (PALM) 进行了大涡模拟 (LES) 实验。介绍了一种验证和理解 LES 结果的方法，用于在类似大气的流动中羽流扩散和浓度波动。广泛的网格分辨率被证明对于研究速度和标量场的统计特征的收敛性是必要的。对于标量，检查高达四阶的统计矩和浓度概率密度函数 (pdf) 的形状。平均浓度受网格分辨率的影响，最高分辨率的模拟显示较低的平均浓度，与较大的湍流结构有关。然而，在两个更高的分辨率下观察到浓度方差收敛的明显趋势。通过显示驱动均值和方差的机制受网格分辨率的不同影响来解释这种行为。偏度和峰度的分析还允许获得羽流浓度波动的一般结果。不考虑网格分辨率，一系列 Gamma pdf 很好地代表了浓度 pdf 的形状，但仅超出了浓度波动强度的峰值。在早期的羽流扩散阶段，pdf 的矩与波动羽流模型生成的矩非常吻合。据我们所知，我们的研究首次表明，如果分辨率和平均时间足够，大气 LES 可以可靠地表示浓度场的高阶矩，直到四阶，