Numerical investigation on the urban heat island in an entire city with an urban porous media model

This paper considers a city to be the porous media with internal heat sources and derives a porous media model for the turbulent flow and heat transfer in the city. The presented model may be an appropriate method for the numerical study of the urban heat island (UHI) in an entire city with appropriate spatial resolution and computing capacity. We proposed a method to convert the building heat source intensity based on ground surface area into that based on building volume. With the method, the heat flux of the building heat source can be consistent with the building density. We adopted the single domain approach to account for the interface conditions and used the spatial distribution of the porosity to define the parameters in the porous urban region and the clear fluid region. The urban porous media model was verified by comparing its numerical results with the volume-averaged results of the micro-scale model. Moreover, the urban porous media model was applied to examine the effects of urban anthropogenic heat and wind speed on the spatial UHI of the urban area and the leeward rural area. The results showed that an increase in anthropogenic heat intensity by 100 W m−2 led to an increase in UHI intensity by 1.683 K. The increase in wind speed had a prominent mitigation effect on the UHI at a lower wind speed, but the decrease rate of UHI intensity decreased as the wind speed increased. The UHI intensity decreased as the height increased and the UHI almost vanished as the height reached five times the height of the urban canopy. The UHI intensity decreased rapidly as the airflow entered the leeward rural area, but the UHI still existed at a distance of 5 km from the urban area.