CFD analysis of transpirational cooling by vegetation: Case study for specific meteorological conditions during a heat wave in Arnhem, Netherlands

The transpirational cooling of vegetation as a measure to mitigate outdoor air temperatures was investigated for a street canyon in the city center of Arnhem, the Netherlands for the meteorological conditions of an afternoon hour on a hot summer day during a heat wave with wind of speed 5.1ms^-1 at 10m above ground and direction along the canyon. Computational Fluid Dynamics (CFD) simulations with locally applied vegetation in the street, i.e. avenue-trees, facade greening, roof greening and all three combined, were performed. The 3D steady-state Reynolds-averaged Navier-Stokes (RANS) equations were closed by the realizable k-ε turbulence model extended with source and sink terms to represent the effects of vegetation on air flow. By specifying a cooling power term in the energy equation, the transpirational cooling by vegetation was accounted for. The strongest cooling by a single vegetative measure was obtained with the avenue-trees with mean and maximum temperature reductions at pedestrian level of 0.43°C and 1.6°C, respectively. Facade greening resulted in rather small changes with mean and maximum reductions of 0.04°C and 0.3°C, respectively. For roof greening no noticeable reductions inside the canyon were found. In the case of a combination of all vegetative measures, cooling in terms of spatial distribution and intensity overall resembled a linear superposition of those of the vegetative measures solely applied with 0.52°C mean and 2.0°C maximum temperature reduction. Overall, the cooling was restricted to the vicinity of the vegetative measures, i.e. up to a distance of a few meters.