A large-eddy simulation (LES) using the atmospheric code ARPS is performed to study the passive scalar dispersion downstream of an obstacle. An immersed boundary method has been introduced to take into account the obstacle. To simulate the scalar dispersion, instead of resolving the passive scalar transport equation, fluid particles containing scalar are tracked in a Lagrangian way. The results of the LES are compared with the experiments of Vinc¸ont et al. [1]. In those experiments, simultaneous measurements of the velocity and scalar concentration fields have been made in the plume emitting from a two-dimensional line source flushed with the wall. The source is one obstacle height downstream of a two-dimensional square obstacle located on the wall of a turbulent boundary layer. Our simulations predict the qualitative and quantitative features of the experimental results.

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