A formulation for the street canyon recirculation zone based on parametric analysis of large eddy simulations
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2018 Hungarian Meteorological Service. All Rights Reserved. Poor air quality continues to be a common problem for the bigger cities. Urban air quality models operate in the street scale and are capable of calculating the concentrations of pollutants for each street. These models are relatively easy and cheap to use, so they can be an excellent tool for many tasks, such as city planning and moderation of the traffic. To be efficient, they rely heavily on simple mathematical formulas that model the various flow phenomena between buildings and the atmosphere, to calculate the dispersion as a function of geometry and meteorological conditions. The sum of recirculating vortices that control the pollutant dispersion, has been collectively named as the recirculation zone. Current models use semi-empirical parameterisations of the recirculation zone, which are backed up by numerous field and wind tunnel experiments. In this paper, a strategy is discussed and applied, to possibly find an objective description of the recirculation zone, as a function of the street canyon geometry and the flow and dispersion characteristics. Specifically, the quadrant analysis of turbulence is applied on the results of wind flow and pollution dispersion, obtained from a Large Eddy Simulation (LES) in a wide street canyon with building height to street width ratio equal to 1/3. The quadrant analysis implemented here uses the fluctuations of the vertical velocity and the concentration. The two of the four events that clean the air in a street canyon, are added to create a parameter named as Cleaning Effect. The possibility of using this value to define different regions in the street canyon and thus the recirculation zone is discussed, in a preliminary use of this method.