Numerical investigation of effects of trees on cross-ventilation of an isolated building

Trees are often planted next to buildings for shade, evapotranspiration cooling, and as windbreaks. As windbreaks, trees slow down wind, consequently reducing the natural ventilation of the buildings that they shield. This reduction in natural ventilation has such parameters as the trees' locations, geometry (height (h), width (w), thickness (t)), foliage (e.g., shape and thickness), leaf area density (LAD), and leaf drag coefficient (Cd). This study investigated how these parameters impact the cross ventilation of a generic building (5 m x 5 m x 5 m) exposed to an atmospheric boundary layer wind flow using Reynolds Average Navier-Stokes (RANS)-based Computational Fluid Dynamics (CFD) simulations. Using source terms, trees were modeled as a porous medium, which decelerates wind speed and increases turbulence generation and dissipation. The effects of trees on cross ventilation were quantified by calculating ventilation rate reduction and conducting a local sensitivity analysis (LSA). As the LSA revealed, LAD, h, and Cd were the three parameters with the most influence on reducing cross-ventilation rates, which are strongly and linearly dependent on LAD and Cd. Cross ventilation rates vary with w and d nonlinearly despite their minor impact. Moreover, conical or oval foliage with a narrow upper part affects cross ventilation less than cuboidal or V-shaped foliage with a wider upper section. This study recommends trees shorter than 0.25H, and thinner than 0.168H with sparse and conical foliage to be planted upstream of a building, but not within 1H-3.5H, to minimize adverse effects on cross ventilation.