Investigation of Concentration Fluctuations for continuous point sources by high resolution Large Eddy Simulation and Stochastic Modeling

Concentration fluctuations from continuous small sources in a neutral boundary layer were investigated by mean of high-resolution Large Eddy Simulation (LES). The data set includes concentration moments up to the fourth order and shows the range of validity of the Gamma probability density function (PDF) model for the concentration fluctuations and the transition to a  Gaussian PDF for ground sources. We also investigated systematically the off-center line peaks in the concentration variance showing that they are persistent for ground level sources while they disappear according to theoretical arguments for elevated sources. The analysis includes a thorough investigation of the distribution of the most energetic components in the frequency domain by using spectral analysis of the LES results and a stochastic model based on simple theoretical arguments. This analysis supports the picture that the peak in concentration variance frequency distribution is related to both the plume dispersion geometry and the turbulent flow. The results also confirm the recent literature findings that for an elevated source the peak in the concentration variance frequency distribution is almost independent from the crosswind location for a given downwind distance from the source. To our knowledge no previous LES or wind tunnel study had the completeness of the current study.