Modeling the influence of vegetation and water pond on urban microclimate

Abstract

The beneficial influence of trees and water ponds on summer comfort in urban spaces was studied experimentally in situ and in wind tunnels but the modeling needs further development to become effective in practical applications. This paper introduces a numerical approach based on coupling the CFD model of airflow, in which the influence of trees is considered as source terms, and the radiation exchange, completed with thermal conduction. The CFD, radiation and thermal conduction models use the same discretization grid at their common boundaries. The model was used to estimate the influence of trees and water ponds in a real town square. Comparison of results between two situations, with and without vegetation and water pond, indicate that surface temperatures are reduced in presence of trees and the comfort is improved.

Introduction

Urban environment modifies microclimate in certain ways. The geographical location and the structure of the city, i.e. regional meteorology, morphology, geometrical configuration, building materials, vegetation, water and human activities, have important influences on the urban climate (Mestayer and Anquetin, 1994, Santamouris, 2001). In general, surface is rougher, less windy and often drier in cities than in the surrounding rural zones due to the replacement of the vegetation by less permeable surfaces such as buildings and paved streets. Consequently, the air in urban area is warmer leading to the development of urban heat island (Akbari et al., 2001). The heat island intensity can result in up to 10 K temperature difference between the dense urban area and the surrounding rural zones (Santamouris et al., 2001).

Numerous experimental and numerical studies were carried on in order to analyze the urban climate. Hence, to improve urban climate, several strategies have been proposed: more vegetation, higher albedo (Akbari et al., 2001), or water ponds favoring the evaporative cooling (Nishimura et al., 1998, Givoni and La Roche, 2000). Evaporative cooling is arguably one of the most efficient ways of passive cooling for buildings and urban spaces in hot regions (Givoni, 1991, Ken-Ichi, 1991).

The green areas in cities improve the urban landscape, can regulate the urban climate by increasing the moisture content of the air and reducing the air temperature (Honjo and Takakura, 1990–1991), and provide better comfort. Experimental data show that the cooling effect of parks, due to combined effect of shade and evapotranspiration, can result in an ambient temperature reduction by 1–4.7 K (Barradas, 1998, Ca et al., 1998, Upmanis, 1998, Shashua-Bar and Hoffman, 2000). The temperature difference depends on the park size and the distance to the park. In addition, green-shade modifies building cooling and heating loads by reducing incident solar radiation and surface temperature (Simpson, 2002). Several authors report that shade surfaces with green canopy are 5–20 K cooler than sunlit surfaces (Fig. 1). Strategic planting of trees around buildings can reduce summer air-conditioning energy use by 10–35% (Rosenfeld et al., 1995, Raeissi and Taheri, 1999), and even 80% in specific situations (DeWalle et al., 1983). On the other hand, vegetation reduces the wind speed which may reduce infiltration of outside air, effectiveness of natural ventilation, and convective cooling of building surfaces (Akbari, 2002). In some cases, vegetation increases the latent load for air conditioning by adding moisture to the air through evapotranspiration (Huang et al., 1987).

Nishimura et al. (1998) proposed novel water facilities (waterfall, spray fountain) to create comfortable urban microclimate. The air temperature measurements on the leeward side highlighted a reduction by approximately 3 K of the temperature in the water vaporisation period and that the effect of the water system from 14 p.m. to 15 p.m. can be felt until approximately 35 m of the water system.

Previous experimental studies were conducted in wind tunnel or in situ measurements. To date, few approaches are based on numerical modeling (Honjo and Takakura, 1990–1991, Bruse and Fleer, 1998, Vinet et al., 2000, Sánchez and Alvarez, 2004) although this promises to be an effective method to investigate the impact of vegetation on urban microclimate. In addition, few studies are reported on the effect of water ponds.

The purpose of this study, pursuing the work already undertaken in previous research (Vinet, 2000, Robitu et al., 2004), is to model the phenomena in outdoor environment. In particular, this paper presents the development of a numerical model in quasi-steady state, which allows the evaluation of the impact of trees and water ponds on the urban thermal environment and the comfort of pedestrians.