Modelling extreme concentrations from a source in a turbulent flow over a rough wall

The concentration fluctuations in passive plumes from an elevated and a ground-level source in a turbulent boundary layer over a rough wall were studied using large eddy simulation and wind tunnel experiment. The predictions of statistics up to second order moments were thereby validated. In addition, the trend of relative fluctuations far downstream for a ground-level source was estimated using dimensional analysis. The techniques of extreme value theory were then applied to predict extreme concentrations by modelling the upper tail of the probability density function of the concentration time series by the generalised Pareto distribution. Data obtained from both the simulations and experiments were analysed in this manner. The predicted maximum concentration (Γ0) normalized by the local mean concentration (Cm) or by the local r.m.s. of concentration fluctuation (crms), was extensively investigated. Values for Γ0/Cm and Γ0/crms as large as 50 and 20, respectively, were found for the elevated source and 10 and 15, respectively, for the ground-level source.