Lagrangian micromixing modelling of reactive scalar statistics. Scalar mixing layer in decaying grid turbulence

A Lagrangian micro-mixing numerical model estimates the concentration statistics of reactive pollutants (NO, O3 and NO2) of a scalar mixing layer in decaying grid turbulence. A stochastic macro-mixing scheme computes the fluid particle trajectories, which describe the turbulent flow (Lagrangian turbulence), whereas the micro-mixing scheme interaction by the exchange with the conditional mean (IECM), implementing a new formulation for the mixing time, represents the dissipation of concentration turbulent fluctuations due to molecular diffusion. The non-homogeneous reaction-dominated limit (NHRDL) of the conserved scalar theory simulates chemical reactions. The numerical model integrates these three schemes and is validated by comparison with experimental and direct numerical simulation (DNS) data, whereas inter-comparisons with other numerical models are also reported. The study focuses on the reliability of Lagrangian stochastic modelsin representing micro-scale pollutant dispersion (air quality) and the importance of representing chemical reactions depending on instantaneous concentrations rather than their means.