Abstract
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.
Highlights
The dispersion of substances from a small punctiform source in an atmospheric turbulent flow is a physical phenomenon of capital importance in many ecological, environmental, and industrial applications
√ where Δ = 3 ΔxΔyΔz with Δx, Δy, Δz being the grid spacing in the x, y and z directions, respectively and l is the SGS mixing length; (ii) the computation of the scale-interaction term describing the transfer of energy between resolved and subgrid scales (Brown 1994; Heinze et al 2015; Mironov and Sullivan 2016); and (iii) the residual of the turbulent kinetic energy (TKE) budget
The most salient points of the comparison among resolutions were that, despite the good consistency of one-point second-order velocity statistics, the two-point correlation analysis showed that the highest resolution simulation (2048) developed larger turbulent structures, characterized by longer length scales and time scales
Summary
The dispersion of substances from a small punctiform source in an atmospheric turbulent flow is a physical phenomenon of capital importance in many ecological, environmental, and industrial applications. Atmospheric dispersion experiments (e.g., Mylne and Mason 1991; Mylne 1992, 1993; Jørgensen and Mikkelsen 1993; Yee et al 1993a, b; Mole and Jones 1994) providing concentration-fluctuation statistics have some characteristics that make them unsuitable for the purpose of validating LES results These are, (1) the extremely small scalar sources used which are unreachable even for nowadays LES, (2) sensors located quite far downwind from the source so that the effects related to the source size have been mostly forgotten, and (3) emissions and measurements are located very close to the ground, where the effect of the wall-model parametrization in the LES are critical because most of the energy may not be explicitly resolved.
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