Fast Model to Compute the Concentration Covariance of Two Passive Scalars from Their Mean Concentration Field

The modeling of chemical reactions between two species at small scales where the chemical equilibrium is not yet attained and the segregation effect may not be negligible depends on the knowledge of the covariance of the concentration field. Unfortunately, while the mean concentration field determination is almost a settled matter, the evaluation of the second moments of the concentration field is still an open problem. In this framework, the fluctuating plume model has proved to be a versatile and powerful tool to evaluate the whole concentration field, nevertheless the Lagrangian simulation of the plume barycenter meandering in real cases is still a challenging task. Following Cassiani and Giostra (Atmos Environ 36:4717–4724, 2002) we use the hypotheses of the fluctuating plume model to develop a simple and fast method to derive the concentration field in real situations without using a Stochastic Lagrangian model, but using a mean concentration field, either measured or numerically simulated. The model is tested against a wind tunnel experiment data-set and the results are presented and discussed.