Large-eddy simulation of dispersion: comparison between elevated and ground-level sources

Abstract

Large-eddy simulation (LES) is used to calculate the concentration fluctuations of passive plumes from an elevated source (ES) and a ground-level source (GLS) in a turbulent boundary layer over a rough wall. The mean concentration, relative fluctuations and spectra are found to be in good agreement with the wind-tunnel measurements for both ES and GLS. In particular, the calculated relative fluctuation level for GLS is quite satisfactory, suggesting that the LES is reliable and the calculated instantaneous data can be used for further post-processing. Animations are shown of the meandering of the plumes, which is one of the main features to the numerical simulations. Extreme value theory (EVT), in the form of the Generalised Pareto distribution (GPD), is applied to model the upper tail of the probability density function of the concentration time series collected at many typical locations for GLS and ES from both LES and experiments. The relative maxima (defined as maximum concentration normalized by the local mean concentration) and return levels estimated from the numerical data are in good agreement with those from the experimental data. The relative maxima can be larger than 50. The success of the comparisons suggests that we can achieve significant insight into the physics of dispersion in turbulent flows by combining LES and EVT.

Large-Eddy Simulation of Dispersion from Line Sources in a Turbulent Channel Flow

“…There have been some earlier studies that used LES methods -for a point source in a street canyon (Walton and Cheng, 2002), a line source in a street canyon (Liu and Barth, 2002) and ground-level and elevated point sources in turbulent boundary layers (Sykes and Henn, 1992), for example. Also, Xie et al (2004) have predicted possible extreme concentrations from groundlevel and elevated point sources in boundary layers. However, scalar dispersion even in classical flows like plane channels or turbulent boundary layers at high Re is a challenging task due not least to the requirement of fine resolution, the existence of high intermittencies and the presence of steep scalar gradients.…”

“…So as an initial test of the LES approach, a conceptually fundamental problem was addressed: that of dispersion from a line source in a fully developed channel flow at relatively high Re number; this should be even simpler than predicting dispersion from a point source in turbulent boundary layers, e.g. Xie et al (2004). The former is essentially a two-dimensional problem with well-defined upper and bottom boundary conditions in an axially homogeneous flow, whereas the latter is a three-dimensional problem usually with an artificially designed upper boundary condition.…”

Large-Eddy Simulation of plume dispersion within various actual urban areas

“…There are typically three approaches, e.g. extreme value theory (Xie et al, 2004), probabilistic approaches (Csanady, 1973;Hanna, 1984), and deterministic approaches (Bartzis et al, 2007). The theoretical models can reasonably provide peak concentrations when statistic values of plume concentrations (mean concentrations, concentration variances, etc.)…”

Large-Eddy Simulation of Dispersion from Surface Sources in Arrays of Obstacles

“…k s is the subgrid turbulent diffusivity and is given by ν s /Sc s where ν s is the subgrid viscosity and Sc s is the subgrid Schmidt number whose value was set to 0.9 (typical of the value used for the large scale field in RANS methods and in earlier LES studies -e.g. Xie et al (2004);Cai et al (2008)). k m is the molecular diffusivity and is defined as ν/Sc m .…”