Atmospheric Dispersion Downstream a Two-Dimensional Obstacle: Experimental Evaluation of Turbulence Closure Models

This study investigates the turbulent dispersion of pollutants in the wake of a two-dimensional square obstacle. Utilizing Laser Doppler Anemometry and Particle Image Velocimetry, we characterized the flow dynamics, identifying a recirculation zone downstream of the obstacle, marked by high shear and increased turbulent viscosity, and playing a crucial role in turbulent momentum exchange. We evaluated the turbulence kinetic energy budget, estimating its dissipation rate, and found traditional isotropy and Taylor hypothesis methods inadequate within the wake region. Furthermore, we explored pollutant dispersion from a linear source located downstream the obstacle. Analysis of mean concentration and variance revealed that the log-normal distribution is most effective for modelling concentrations within the recirculating region, while the Gamma distribution suits areas outside it. Testing various closure models for turbulent mass fluxes highlighted the limitations of the Simplified Gradient Diffusion Hypothesis model, favouring more complex closure models for longitudinal trends, though these still faced challenges with intensity estimation. The Simplified Gradient Diffusion Hypothesis model proved robust for vertical mass fluxes, with satisfactory results in turbulent diffusivity and turbulent Schmidt number calculations. The experimental results serve as a benchmark for validating numerical simulations and assessing the accuracy of closure models typically employed in pollutant dispersion modelling.