Extending the capability of Large Eddy Simulations to model dispersion in urban areas

Abstract

A synthetic turbulence and temperature fluctuation generation method was developed in Large–Eddy Simulations (LES) to investigate the effects of inflow turbulent conditions, thermal stratification and wind direction on flow and dispersion over a rural–to–urban transition region. The modelling approach was validated by comparing predictions of mean velocity, turbulent stresses and point–source dispersion against data from a wind tunnel experiment. The depth of the internal boundary layer (IBL) that developed from the leading edge of the block array was determined using the wall–normal turbulent stress method that I developed in this study. The method allowed the location of the interface to be more clearly defined by using a power law formula of similar form to those derived in previous studies. The development of the IBL was analysed in relation to the dispersion from a ground–level point source within the urban array. It was found that the vertical transport of pollutant was constrained by the presence of the IBL so leading to trapping of material in the canopy layer. The effects of weakly stable stratification (i.e. with the Richardson number Ri ≤ 1) on turbulence and line source dispersion were investigated over the same rural–to–urban transition region. Vertical profiles of wall–normal turbulent stress showed that the height and the growth rate of the IBL were sensitive to the thermal stability and the turbulent kinetic energy (TKE) prescribed at the inlet. Furthermore, it was found that increasing the stable stratification level reduced the vertical transport of pollutant which increased the volume–averaged concentration within the canopy. The transport of pollutant below and above the canopy was analysed for a mean wind direction inclined at angles to the main streets (i.e. 15◦ and 45◦ ) of the urban array. The natural ventilation below the canopy due to wind variation was found to be greatly improved at the 45◦ compared to the normal wind direction (i.e. 0◦ ). For Ri = 0.2 the effects of wind direction on turbulence and dispersion were found to dominate the effects of weak thermal stratification. Lastly, an LES study of a field experiment confirmed that dispersion scenario is greatly sensitive to variations of wind direction which may produce large concentration differences even in the far–field.

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ORCID for Zhengtong Xie: orcid.org/0000-0002-8119-7532

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