On the Convergence and Capability of the Large-Eddy Simulation of Concentration Fluctuations in Passive Plumes for a Neutral Boundary Layer at Infinite Reynolds Number

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.

Concentration Fluctuations from Localized Atmospheric Releases

“…Fackrell and Robins 1982a, b;Nironi et al 2015) the ratio between the size of the source and that of the larger scale eddies has a great impact on the concentration statistics. This feature makes the LES results on concentration statistics particularly sensitive to the grid resolution (Ardeshiri et al 2020). To avoid this, the size of the grid cell should be much smaller compared to that of the source, a condition that has not been adopted in most of the studies published so far.…”

“…An accurate investigation on the effect of the grid resolution on the concentration fluctuation statistics (up to the fourth moment) was very recently presented by Ardeshiri et al (2020) using the open source code parallelized LES model (PALM, Maronga et al 2015). Notably, by spanning a wide range of grid refinement (the source was resolved from a minimum of one to a maximum of 8 3 grid cells), Ardeshiri et al (2020) showed that the dependence of concentration statistics on the grid size is not monotonic and explained the mechanism by which grid resolution affects concentration fluctuations. They also showed that the gamma PDF is an excellent model for concentration fluctuations from point sources, but only for downwind positions beyond the peak of concentration fluctuation intensity.…”

“…More recent results have been converging on the choice of the gamma distribution as best PDF model for the concentration from a point sources at least over a certain range of downwind distances from the source (e.g., Yee and Skvortsov 2011;Marro et al 2015;Ardeshiri et al 2020), Table 2 Table extended from Efthimiou et al (2016a) Publication Distributions Hanna (1984c) Exponential F Lewellen and Sykes (1986b) Clipped normal F Sawford (1987) Clipped normal a , Exponential, Lognormal F Dinar et al (1988) Clipped normal a , Exponential L Yee (1990) Clipped normal a , Exponential, Lognormal F Mylne and Mason (1991) Clipped normal a , Exponential…”

“…This last term is unclosed. According to the phenomenological description of the dispersion process outlined above, meandering is mainly linked to the production of scalar variance close to the source (e.g., Fackrell and Robins 1982b;Ardeshiri et al 2020), while relative dispersion is mostly linked to its dissipation (e.g., Sykes et al 1984;Cassiani et al 2005a). Equation 4, its applications, and closures are further discussed in Sect.…”

Technical note: Interpretation of field observations of point-source methane plume using observation-driven large-eddy simulations

“…Despite the vast amount of idealized studies, the performance of LES has not been validated against many experimental field studies. (Steinfeld et al, 2008;Ardeshiri et al, 2020;Rybchuk et al, 2020). For instance, the Prairie Grass experiment (Barad, 1958) still serves as a common reference for LES studies.…”