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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - URBANRAD (Radiative transfer effects on air pollution dispersion in urban areas: from the street scale to the neighbourhood scale)

Teaser

Summary

Air pollution from traffic and industrial emissions is one of the key issues for modern metropolises, because of urban population growth. Modelling and understanding pollution dispersion at a large scale will help to improve the design of our cities, buildings and traffic systems and to attain a sustainable environment for areas of dense population. Among the different physical mechanisms to predict, thermal radiation is of primary interest because convection and dispersion of pollutants are driven by thermal effects. The radiation absorption and emission by the atmosphere (mainly due to water vapour and carbon dioxide) or by the building surfaces and the radiation scattering by clouds and fog modify the local energy balance. These phenomena are coupled with the air flow and should be taken into account for an accurate modelling. However, in most pollution dispersion simulations, radiation is not taken into account or is crudely modelled. Radiative transfer calculation is still a computational issue because the radiative intensity field is an intricate function of the wavenumber and the direction of the photons, space and time. Coupled simulations of radiative transfer and urban flows require new efficient numerical methods and physical models for an accurate prediction and a better understanding of pollution dispersion at the neighbourhood scale.

This project aims at understanding the effects of radiative transfer on air pollution dispersion in urban areas at both the street scale and the neighbourhood scale. Numerical models have been developed for that purpose, that will help design and manage our cities, buildings and traffic systems in order to produce sustainable, safer, healthier, and more comfortable urban environments. Radiation modelling have been coupled with fluid dynamics, pollutant transport and environmental conditions to produce the first model able to take into account radiation transport effects at the street scale. Numerical simulations at the street scale have been carried out and have shown substantial effects of radiative transfer in low-wind conditions, for a mid-latitude summer atmosphere. In addition, a novel strategy has been developed to improve the computational efficiency of thermal radiation transport calculations. It consists in optimising the angular resolution in space and for each absorption coefficient class to get the minimum error on the radiative source term. The use of such adaptive method will be key to perform numerical simulations at the neighbourhood scale.

Work performed

Computational Fluid Dynamics is widely used as a predictive tool to evaluate people\'s exposure to pollutants in urban street canyons. However, in low-wind conditions, flow and pollutant dispersion in the canyons are driven by thermal effects and are potentially affected by longwave (infrared) radiation due to the absorption and emission of water vapour contained in the air. These effects are mostly ignored in the literature dedicated to air quality modelling at this scale and the purpose of this work is to study them. The Large-Eddy-Simulation of an air flow in a single 2D canyon with a heat source on the ground has been considered and the dispersion of a tracer was monitored once the statistically steady regime was reached. Incoming radiation was computed for a mid-latitude summer atmosphere and canyon surfaces were assumed to be black. Water vapour was the only radiating molecule considered and a global model was used to treat the spectral dependency of its absorption coefficient. Flow and radiation fields were solved in a coupled way using the finite element solvers Fluidity and Fetch. Results have shown significant effects of thermal radiation on flow patterns and tracer dispersion. When radiation is taken into account, the air is heated far from the heat source leading to a stronger natural convection flow. The tracer is then dispersed faster out of the canyon potentially decreasing people\'s exposure to pollution within the street canyon. These results have been presented at the AGU Fall Meeting in San Francisco in December 2016. A journal publication is also in preparation.

In order to enhance the computational efficiency of radiative transfer calculations, a goal-based angular adaptivity method has been developed, which is suitable for non grey media and when the radiation field is coupled with an unsteady flow. The method optimises the angular resolution according to the radiative source term (the goal) which is the key physical quantity for the coupling between flow and thermal radiation. The angular resolution is allowed to vary anisotropically in space and for each absorption coefficient class built from a global model of the radiative properties of the medium. The angular discretisation is based on a Haar wavelet expansion, which is a hierarchical version of the discrete ordinates expansion and does not require any angular interpolation in space when adapting. The method has been tested on a coupled flow-radiation on the 2D street canyon problem from a coupled instantaneous temperature field. Compared to a uniform Haar wavelet resolution, a goal-based adapted resolution requires 5 times less angular basis functions and 6.5 less CPU time to reach the same level of accuracy in the radiative source term. In addition, the adapted resolution proved to be accurate when used in unsteady coupled calculations. The gain in computational time will help to investigate larger computational domains and more realistic flow regimes that involves a larger range of temporal and spatial scales. This work has been accepted for publication in JQSRT and has also been presented in a thermal radiation symposium in Marseille.

Final results

To the best of our knowledge, this study is the first attempt to quantify gas radiation effects on pollutant dispersion in street canyons. This work will help improving the accuracy of air quality models and designing new urban environments that decrease people\'s exposure to air pollution. Additional impacts are expected on building energy efficiency as thermal radiation affects indoor air flows and heat transfer. First results obtained in a limited range of configurations have shown significant effects of gas radiation that need to be confirmed in more realistic flow regimes. Future work will combine gas radiation effects with building\'s radiation, cloud scattering and environmental conditions. In addition, coupled simulations at the neighbourhood scale will also be performed with the help of adaptive numerical methods.

The angular adaptivity method developed in this project is expected to have a strong impact in the field of radiative transfer modelling for both research and industry. Compared to the state of the art finite volume/discrete ordinates method, adaptivity is potentially much more efficient in terms of computational time and memory requirements for a given accuracy. Many application areas are concerned, like atmospheric sciences but also nuclear engineering or combustion. In future work, we plan to further enhance the computational efficiency of the adapted calculations by performing load balancing in parallel after adapting, to ensure an even distribution of work. Another perspective would be to extend the application of the method to heterogeneous media that are encountered for instance in combustion processes.