Wind tunnel tests on the relationship between building density and pedestrian-level wind velocity: Development of guidelines for realizing acceptable wind environment in residential neighborhoods

Abstract

The main purpose of this study is to reveal the relationship between the building density and the average wind velocity at pedestrian level in residential neighborhoods. This paper firstly presents the results of wind tunnel tests on 22 residential neighborhoods selected from actual Japanese cities. The results show that there is a strong relationship between the gross building coverage ratio and the mean wind velocity ratio. Secondly, the wind environment evaluation for case study areas is performed by using the wind tunnel results and the climatic conditions of several major Japanese cities. The development method of guidelines for realizing acceptable wind environment in residential neighborhoods using the gross building coverage ratio is proposed.

Introduction

Wind conditions at pedestrian level in general urban area affect the ventilation of buildings as well as the comfort and safety of pedestrians. Moreover, the pedestrian-level winds play an important role in the dispersion of traffic-related pollutants and diffusion of heat especially in urban areas. Therefore, it is necessary to understand the pedestrian wind environment around buildings for taking them into account in the urban design.

The characteristics of the wind over an actual ground-surface are directly affected by the roughness elements, and the flow patterns vary locally [1]. Givoni [2], for instance, pointed out that the wind profile described by a simple mathematical equation such as the power law does not represent the real wind conditions near the urban ground level. Building configuration, e.g. its height, width, building arrangements and density, are some of the significant factors affecting wind velocity near the ground level. There are several wind tunnel and numerical studies [3], [4], [5], [6], [7], [8] examining the effects of building configurations on the urban wind fields. Most of those studies applied the simplified and regularly aligned building blocks as urban roughness elements, but there are quite few studies clarifying the above effects by using the actual urban situations. Using these simplified blocks allows researchers to govern significant or insignificant parameters on urban configurations efficiently, but they do not necessarily represent the complex configurations of the real urban space.

Street canyon refers to a street with buildings lined up continuously along both sides [9]. Many field experiments have been carried out in street canyons of the actual urban areas to examine the effects of street canyon configurations, e.g. the ratio of building height to the width of street (aspect ratio), on the wind fields [10], [11], [12], [13]. In most of these studies, the field experiments are carried out in one or two streets in an actual city. There are relatively few studies involved in larger areas such as the whole residential neighborhood.

It is usually assumed that an increase in building density reduces the wind velocity in the urban area [2], [14]. Givoni [2] also argued that the reduction in wind velocity with the building density is due to the increased friction near the ground. However, it is widely recognized that such friction influence increases with the building density to a single peak and then declines [15], [16]. Oke [16] reported that when the above roof flow is perpendicular to the canyons, wind flow pattern is described in terms of three types of flow regimes, depending on the aspect ratios. The above paper shows that when the aspect ratio exceeds the threshold value a stable circulatory vortex is established in the canyon and the bulk of the above roof flow does not enter the canyon [16]. Nevertheless, it should be noted that actual residential areas comprise canyons of various dimensions. In these circumstances, it may be difficult to explain the decrease in the average pedestrian-level wind velocity of the whole area by the above three flow regimes alone.

The main purpose of this study is to reveal the relationship between the building density and the average wind velocity at pedestrian level in residential neighborhoods through a series of wind tunnel tests, using the actual urban models. Building density of a residential neighborhood is described using some different measures, e.g. dwelling density, building coverage and floor area ratio. The present study, however, covers the gross building coverage ratio and gross floor area ratio of the neighborhood in particular. The height of buildings may also greatly affect the pedestrian-level wind velocity, but it extends beyond the scope of the present paper. This paper firstly presents the results of wind tunnel tests on 22 residential neighborhoods selected from actual Japanese cities and analyzes the above building density and average wind velocity relationship. Secondly, the wind environment evaluation for case study areas is performed by using the wind tunnel results and the climatic conditions of several major Japanese cities. The development method of guidelines for realizing acceptable wind environment in residential neighborhoods using the gross building coverage ratio is proposed.