The effects of parametric uncertainties in simulations of a reactive plume using a Lagrangian stochastic model

A combined Lagrangian stochastic model with micro-mixing and chemical sub-models is used to investigate a reactive plume of nitrogen oxides (NOx) released into a turbulent grid flow doped with ozone (O3). Sensitivities to the model input parameters are explored for different source NOx scenarios. The wind tunnel experiments of Brown and Bilger (1996) provide the simulation conditions for the first case study where photolysis reactions are not included and the main uncertainties occur in parameters defining the turbulence scales, source size and reaction rate of NO with O3. Using nominal values of the parameters from previous studies, the model gives a good representation of the radial profile of the conserved mean scalar Γ¯NOx although slightly over predicts peak mean NO2 concentrations Γ¯NO2 compared to the experiments. The high dimensional model representation (HDMR) method is used to investigate the effects of uncertainties in model inputs on the simulation of chemical species concentrations. For this scenario, the Lagrangian velocity structure function coefficient has the largest impact on simulated Γ¯NOx profiles. Photolysis reactions are then included in a chemical scheme consisting of eight reactions between species NO, O, O3 and NO2. Independent and interactive effects of 22 input parameters are studied for two source NOx scenarios using HDMR, including turbulence parameters, temperature dependant rate parameters, photolysis rates, temperature, fraction of NO in total NOx at the source and background ozone concentration [O3]. For this reactive case, the variance in the predicted mean plume centre Γ¯O3 is caused by parameters describing both physical (mixing time-scale coefficient) and chemical processes (activation energy for the reaction O3+NO). The variance in predicted plume centre Γ¯NO2 and root mean square NO2 concentration γNO2′, is strongly influenced by the fraction of NO in the source NOx, and to a lesser extent the mixing time-scale coefficient. Adjusting the latter gives improved agreement with the Brown and Bilger experiment. Some weak parameter interactions are observed.