Large-eddy simulation of the turbulent dispersion of a reactive plume from a point source into a neutral atmospheric boundary layer

We consider the dispersion from a point source in the neutral atmospheric boundary layer. The emission from the point source consists of a species A which reacts with an ambient species B present in the boundary layer. The study is carried out by means of large-eddy simulation (LES). The main emphasis lies on the effect of turbulent mixing on chemistry in determining the concentration levels in the plume. First, we consider a relatively slow reaction which is characterized by a reactive time scale that is much larger than the turbulence time scale. Wind-tunnel experiments exist for this case (Builtjes, 1983. In: Wispelaere, C. (Ed.), Air Pollution Modeling and its Application II. Plenum Press, New York, pp. 59–84) and with these experimental data we validate our LES model. A comparison between computational and experimental data shows excellent agreement for the mean concentrations on the plume centreline. Second, we consider the case of a relatively fast reaction which is characterized by a reaction time scale smaller than the turbulent time scale. In this case the reaction is limited by the turbulent mixing. First we consider the effect of mixing by the resolved scales obtained from the LES. The simulation results demonstrate that the total reaction rate is reduced considerably compared to the ideal case in which species are completely mixed. Next we consider the influence of subgrid scale mixing. To account for this influence a subgrid model for the chemistry is adopted. The simulation results show that the subgrid scale mixing plays a prominent role close to the source.