Prediction of wind environment and thermal comfort at pedestrian level in urban area

Abstract

This paper reviews the recent developments in CWE research for predicting the pedestrian level wind and thermal environments in urban areas, primarily achieved by the researchers in the field of environmental engineering in Japan. First the progress in turbulence models in the last decade and in their applications to the problems related to wind climate is briefly reviewed, and the results of Architectural Institute of Japan (AIJ) collaborative project in cross comparisons of CFD results of wind environments are presented. Next, recent achievements in the field of modeling canopy flows for reproducing the aerodynamic and thermal effects of trees, buildings and automobiles are outlined. Examples of numerical results obtained using tree and vehicle canopy models are shown to demonstrate the significant effects of stationary and non-stationary subgrid scale flow obstacles on turbulent flowfield within street canyons.

Introduction

The world's urban population was 2.9 billion (47.2%) in 2000 and is expected to rise to 5 billion (60.2%) by 2030. During 2000–2030, the world's urban population is projected to grow at an average annual rate of 1.9% (World Urbanization Prospects, 2001). In line with the rapid urbanization and growth of urban population, there are increasing concerns regarding the quality of urban environment. In this respect, urban thermal environment is one of the major concerns, which had lead to numerous researches on this topic. The urban thermal environment has been worsened by the Urban Heat Island (UHI) effects. The UHI is now regarded as one of the most serious urban environmental problems in the world. The contributing factors of UHI include less vegetation in city area, absorption of solar energy input by concrete and paved surfaces, multiple heat reflections from canyon structures of high-rise buildings, anthropogenic heat releases from air-conditioning systems, automobiles, etc. In order to reduce the UHI effects, various mitigating measures have been proposed. The most commonly applied is tree planting. The monetary benefits of urban trees are difficult to quantify because these trees can provide numerous private and public benefits. The latter includes improving thermal environment, reducing air pollution and community noise problems, enhancing biodiversity and meliorating aesthetics (Akabari et al., 1992; McPherson and Rowntree, 1993; Rowntree, 1989). Thus, the existence of tree covered ground surface is one of the most essential factors to be considered in urban design. Accurate reproduction of aerodynamic effects of trees is also very significant for predicting wind environment in urban area.

Wind environment is one of the most important factors to be considered in UHI study as it has significant influence on UHI effect and outdoor thermal comfort. The conventional urban wind environment assessment methods only took into consideration the influence of topographic features and geometry of buildings (cf. Fig. 1(1)). These methods may be inadequate to reflect the real conditions of city where there are objects of various scales within the street canyons. Stationary objects (cf. Fig. 1(2)) such as trees and telephone boxes, and non-stationary objects (cf. Fig. 1(3)) such as moving vehicles and swinging hanging-signboards (commonly seen in Asian countries), may alter the surface roughness to certain extent. In order to obtain accurate quantitative data for urban wind environment assessment, these objects must not be overlooked. In recent years, canopy models for reproducing the aerodynamic and thermal effects of trees, buildings and automobiles have been developed and applied to various problems related to urban climate. The canopy models were incorporated into the meteorological mesoscale models. The simulation methods for mesoscale and microscale climates were then integrated into the total simulation system using the nested grid technique.

The growth of CWE applications in the past decade had greatly expanded the scope of wind engineering. Now the application of CWE ranges from the microclimate around a human body to the mesoscale climate in urban area. The aims of this paper are to present the progress of CWE researches for predicting pedestrian level wind environment around buildings primarily achieved by the researchers in the field of environmental engineering in Japan, together with a brief review on turbulence modeling for CWE applications to problems related to wind environment and cross comparison of predicted results with various turbulence models for several test cases, and to demonstrate the significant effects of stationary and non-stationary objects (tree canopy and vehicle canopy, respectively) on turbulent diffusion process within street canyons.