Assessing the effect of citrus plant on the improvement of the outdoor thermal environment using wind tunnel and ground-based Leaf Area Index measurements

Abstract

This study presents the preliminary results of the ongoing research project “Re.Nature Cities”, in which  the ability of street trees to act as an effective measure against increased urban air temperatures is evaluated via experimental and simulation means. In the existing literature, numerous studies highlight that the addition of street trees inside the canyons of urban areas may result in a significant reduction of the peak ambient summer Tair, having also a prominent effect on outdoor thermal comfort regulation. Yet, street trees also impact urban ventilation as they act as barriers, disturbing the wind flow and affecting buildings’ energy needs and thermal comfort; the positive effect of wind sheltering during the cold winter period, can be thus significantly counterbalanced during the warmer periods of the year. The existing evidence reveals that the green elements’ implementation in the built environment without holistically accounting for all the vegetation-air-buildings interactions, can even exacerbate human discomfort and deteriorate indoor natural ventilation. Based on the above, this study evaluates the mitigation potential of a tree type that is commonly encountered in Greek cities – the citrus- since it has low irrigation needs and high drought tolerance. An integrated experimental campaign, employing wind tunnel measurements, albedo and Leaf Area Index/Leaf Area Density (LAI/LAD) measurements is conducted so as to define of the aerodynamic, thermal and foliage characteristics of real trees.  Wind tunnel measurements of total drag are carried out in a wind tunnel section of 3.5m width and 2.5m height, while LAI measurements are conducted using a plant canopy analyzer, with the LAD of each layer (1 m/layer) then calculated from LAI by empirical equations. The obtained values are then used as input parameters in the vegetation model of the ENVI-met microclimate model, which is employed for the evaluation of the thermal environment of typical building blocks in Greece, considering different planting patterns and vegetation coverage scenarios. The experimental database of foliage, thermal and aerodynamic characteristics of common urban tree species, along with the detailed microclimatic simulations of typical urban districts provide a valuable tool for decision-making regarding the optimal vegetation coverage and the planting pattern for urban areas.