Effects of grid resolution on large eddy simulation of plume dispersion in a neutral boundary layer

A set of high resolution large eddy simulation (LES) has been performed to investigate the effect of grid resolution on statistical characteristics of the velocity and scalar fields to find an evidence of convergence in the turbulent fields of the numerical experiments. The scalar field is generated by the dispersion from a localized small source in a neutrally stratified wind tunnel boundary layer. The LES results are analyzed with the aid of published experimental measurements of Nironi et al. (2015) and Fackrell and Robins (1982), phenomenological model (Gifford, 1959), and theoretical arguments on scalar mixing (Duplat & Villermaux, 2008). The characteristics of the fluctuating concentration field including moments up to the fourth order and the concentration PDF are compared in details. The results confirm that LES convergence is difficult to observe and this is exacerbated for the scalar field. Our analysis shows that the LES turbulent flow statistics are in good agreement with the experiments and the results consistently improve by increasing the grid resolution with small but visible changes. Contrary to expectation that mean concentration is minimally sensitive to the grid resolution, the highest resolution simulation shows lower mean concentration which is linked to have larger flow structures. However, the convergence in scalar fluctuations is observed if the grid resolution is high enough which shows that the mechanisms driving the evolution of the mean and the variance of the plume are different and are differently influenced by the numerical resolutions. Skewness and Kurtosis show that, even for dispersion in an unbounded domain, the Gamma PDF model correctly represents the concentration PDF over wide range of distance in the dissipative phase of plume evolution, while the meandering is a better representation in the early phase of dispersion.