Simulated impacts of trees on heat and wind flows in an urban open space

Interactions between trees and urban environment have been studied by many authors and lots of positive points have been underlined through their researches. With pertinent trees planting, several improvements are expectable for aesthetic, visual, historical or symbolic purpose, for runoff, noise and pollutant control and from a microclimatic point of view. Some of these actions seem be obvious but it should be noticed that few information are available from literature for precise quantitative results.

Most of existing results are based on measurements and can be considered only as specific data in a certain context. What happens if vegetation is introduced in a mineral space or if a building is constructed instead of a park ? Extrapolations of measurement are often not possible, so only numerical simulations can afford advice to urban planners on the way they may improve urban environment.

In this paper, numerical modeling of heat and mass transfers in urban environment are discussed with special attention to physical behavior of trees. A coupling strategy between a thermal and a CFD code has been developed in order to take into account all the thermal exchanges and phenomena like evapotranspiration, mitigation of solar heat gain, reduction of the short wave and long-wave radiation from soil to environment and modification of wind flow patterns.

Results of parametric studies are presented in the case of an urban street in Montpellier (Southern France) under hot summer conditions. Size, shape and drag coefficient have been tested for different kind of vegetation. The effects of trees presence on surface and air temperature, wind patterns and velocities, and turbulent kinetic energy are discussed. Despite relatively low impact of air temperature, applications to outdoor thermal comfort improvement, pollutant dispersion or energy savings are possible with the numerical tool presented in this study.