Large-eddy simulation study on the effect of vehicle-induced turbulence and exhaust fumes on wind flow and pollutant dispersion in urban street canyons
Final Report Abstract
The transport and dilution of pollutants at street level are the result of the interaction between the atmospheric (turbulent) wind flow, buildings and other obstacles, and vehicular traffic. A good understanding of these processes is crucial in terms of local air quality management as it improves the ability for predicting the spatial distribution of pollutants. Extensive research studies have been conducted on the wind flow and transportation of pollutants within urban street canyons. However, the impact of vehicle-induced effects (VIE) on the flow and pollutant dispersion have been considered only in few studies. The research in this project ties precisely with the point of representing, understanding and quantifying the effects of vehicleinduced turbulence (VIT) on the wind flow and pollutant dispersion in street canyons. For this purpose we used high-resolution turbulence-resolving numerical modeling (a so-called LES model). This was achieved by extending and applying the PALM model system, which has been mainly developed at Leibniz University Hannover. We were able to adequately represent explicitly moving vehicles by implementing a simplistic method for modeling the vehicle induced-effects. The concept of the so-called inducedvelocity method (IVM) is based on the assumption that the frictional drag is much smaller than the form drag and can thus be neglected. The form drag, however, is represented by prescribing a fixed velocity to the obstacles grid volume on the model’s Cartesian grid, which equals the driving speed of the vehicle. These imposed velocities are then advected at driving speed and direction within the LES grid. In our validation study we were able to prove that the IVM is well suited to represent typical vehicle wakes in LES models. This holds also for the case of moving vehicles in idealized street canyons and consequences for pollutant dispersion. The general pattern of the wind field as well as the dispersion characteristics is qualitatively well reproduced by the IVM and for the investigated traffic cases. Our investigation of the effect of VIE on the wind flow and dispersion of pollutants in idealized street canyons showed that VIE leads to a redistribution of concentrations in the street canyon, but does not favor exchange with the flow above the canyon. This means that reductions in time-averaged concentrations take place locally, but lead to a corresponding increase elsewhere. In line with previous studies, a reduction in concentrations at street-level was existent but only at the leeward wall. It has also been shown that the increase rises with a higher influence of vehicle-induced movement (i.e. with higher vehicle speed / wind speed ratio). The percentage increase in the mean concentration was between 1.5 % and 12.2 %, with the highest increase occurring at a wind speed of 5 m s−1 and vehicle speed of 15 m s−1 . The last part of our study focused on the joint influence of VIE and deferentially heated surfaces. Here we were able to that the movement induced by traffic opposes the thermal buoyancy flow, inhibiting the exchange with the free flow above the canyon and increasing the mean concentration in street canyons by up to +22 %. Only in case of heating of the windward wall does traffic lead to a reduction in mean concentrations by -15 %. The combined effect of VIE and thermodynamics, however, nevertheless leads to a reduction in mean concentrations in street canyon in all investigated cases by up to 28 %.
Publications
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Introduction and validation of a simplistic method to represent vehicle-induced turbulence in high-resolution large-eddy simulations. Copernicus GmbH.
Motisi, Giovanna & Maronga, Björn
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Introduction and validation of a simplistic method to represent vehicle-induced turbulence in high-resolution large-eddy simulations, PALM Model Conference 2023, Offenbach am Main, Germany, September
Motisi, G. & Maronga, B.
