ANALYSIS OF WIND CONDITIONS AROUND A BUILDING DEVELOPMENT AS A PART OF ITS FORM DESIGNING PROCESS, A CASE STUDY

The need to ensure healthy and comfortable environmental conditions for city residents is one of the assumptions that define the concept of sustainable urban development. Factors, such as feeling the temperature, air movement, air smell or humidity, contribute to the quality in use of urban space and, indirectly, also the buildings themselves. Modern cities, especially the largest cities, face problems of air pollution, insufficient air exchange rate and overheating, all of which appear in built-up areas. Among the factors causing such a situation, a high concentration of buildings and hardened surfaces in cities may be mentioned. Solar energy is accumulated through building walls and surfaces, whereas the release of solar energy is impeded by the wind velocity slowdown, a characteristic phenomenon in case of cities, which is estimated at 20–30% in relation to air movement in city outskirt areas [1]. In spite of the slowdown mentioned, varied wind phenomena in a relatively small area, such as turbulence and rapid airflow acceleration and air stagnation may occur around buildings simultaneously. The dependence of these phenomena on the shape of buildings and the distance between them is clearly visible and proven [2, 3, 4]. Within the scope of environmental wind engineering, knowledge is developed which deals with the issues of wind comfort for pedestrians and of the possibilities for ventilating urban spaces.

The following paper concerns the possibility of including the above mentioned issues in the process of urban and architectural design in case of designing medium-high urban building developments, on the example of a particular project. In accordance with the current legal situation in Poland, aerodynamic analyses are required only for high-rise building projects, as well as for building developments located in aeration corridors. The paper is aimed to test the validity of the application of aerodynamic tests also while designing dense urban development units. Moreover, it concerns the possibility of introduction of such tests in the concept phase of a project, when basic decisions regarding the shape of buildings and their arrangement are made. Design methods based on the factual data concerning common practices and legal conditions in Poland have been juxtaposed to research methods in the field of aerodynamics. The subject of the research concerns a conceptual design of a building development that is intended to fill the urban quarter at the junction of Street Woronicza and Aleja Niepodległości (Independence Avenue) in Warsaw. In the process of forming buildings, experimental studies in wind tunnel, using oil visualization method have been applied. The method allows for a relatively prompt qualitative identification of the airflow around the building on the pedestrian level (the studies were conducted at the Department of Aerodynamics, Faculty of Power and Aeronautical Engineering, Warsaw University of Technology).

Research in the field of environmental wind engineering has been intensively developed since the second half of the 20^th century. Such studies belong with the field of physics known as fluid dynamics (FD). It is believed that a research which led to a breakthrough in connecting environmental wind engineering directly to architecture was the study by Gandemer, who, in 1975, identified and described the twelve so-called aerodynamic effects created around detached buildings and buildings arranged in the form of simple systems [5]. This study provided the basis for creating design guidelines for architects and urban planners. Gandemer applied tunnel experiments, which were then developed at the turn of the 80s and 90s by subsequent researchers (Hussain and Lee, Brown and DeKay, and, especially, Oke).

The experiments helped to understand the nature of wind phenomena in a three-dimensional system and were acknowledged as an important element of the pro-ecological approach to design in the 1990’s [6]. The importance of phenomena occurring in arrangements known as street canyons, namely the spaces between two parallel lengthened buildings, the typical street layouts in cities, has been emphasised in the experiments. Simultaneously, studies on numerical methods of wind simulation (computational fluid dynamics CFD) were being developed [7].The first achievements concerning space ventilation between buildings in the area of CFD include there search in 1993 by Walker, Shao and Willscoroft. This research method continues to be explored till the present day [8], especially in the scope of the connection between building development geometry and the airflow around it. Especially complex systems corresponding to the actual geometric characteristics of cities pose a great challenge [2, 9]. Despite the significant development in research methods, also numerical ones, with certain exceptions, aerodynamic research still fails to be included in spatial planning processes, whereas the level of knowledge displayed by architects and urban planners on this subject is insignificant.

The issues raised in the present paper concern wind conditions that occur between buildings and create wind comfort for pedestrians (it was assumed to appear at 1.5 m above the ground). The presence of the comfort requires protection against both, violent gusts of wind and against wind turbulence (the phenomena occur at building edges or in building narrowing). Moreover, it is crucial to prevent air stagnation from occurring. The latter leads to an insufficient ventilation in the spaces between buildings, overheating in the summer, the retention of unpleasant odours and air pollution.

From the research quoted by, among other researchers, Daniels [6], it stems that in the wind direction perpendicular to the street axis, uninterrupted ventilation in street space occurs when the parameter h/w (“h” is the height of buildings and “w” is the distance between them) is less than 0.37. As the value of the parameter is increased, the air ventilation gradually reduces, whereas above the h/w value equal to 0.74, the air exchange stops completely. Similar data is also provided in publications by further authors (namely, Oke, Harmana, Okamoto, Castro and Robins). Observing the implementations of contemporary development housing estates, as well as the buildings that fill of the existing downtown development of large Polish cities, it is easy to identify buildings with geometry posing such a risk [4].

In the present paper, attempt is made to verify, using a particular example, whether by means of designing buildings to fill of an urban quarter in accordance with the principles commonly applied in development investments in Poland, a risk of uncomfortable aerodynamic phenomena arises and whether such risks can be counteracted by introducing proper improvements in the shape of buildings.

First, a building structure concept was created for the building development that was intended to fill the selected plot. The function assumed for the building was of residential and service nature. During the research, the plot was not subject to the local development scheme, which would provide design guidelines. Thus, the form of the building was shaped according to the following criteria:

Once the urban conditions had been analysed, a comb-shaped building was formed (Figure 1). In such a way, courtyards were formed that open towards the south, thus provided with good access of light. The presence of courtyards offers a possibility to develop a continuity with the existing green areas and introduce such areas into the interior of the projected building development quarter. Moreover, such a shape of the designed building makes it possible to provide a good lighting of rooms in it. Bearing in mind the existing residential building situated to the west (38 m high), the designed building development has been moved away from the plot border by a distance of over 7 m. A distance of about 30 m between the buildings was obtained in this way. This allowed to maintain the current conditions of insolation for the existing building.

Figure 1.

A varied height of the building was assumed (36, 28 and 20 m in order to harmonize it better with the heights of neighbouring buildings) and two wide gates were introduced (for the sake of a better communication between the street and the courtyards)

As a result, an initial concept for the building development shape was created. In the form a layout that covers also neighbouring buildings, the structure was subjected to tests in the wind tunnel. The research resulted in a general diagnosis of wind conditions and in identification of problem areas. On the basis of the test, the shape of the designed building development was adjusted and subjected again to experimental research in the tunnel. The research resulted in the decision to introduce another modification to the shape of the building. The resulting version of the project was considered final and thus it was evaluated. The scheme of the described action plan is shown in Fig. 2.

Figure 2.

Experimental studies in the aerodynamic tunnel of medium-speed were used. The oil visualization method was selected [4, 10, 11, 12], as it provides qualitative results. The picture presenting the results, although it fails to provide reference to numerical values, makes it possible to identify the nature of wind conditions prevailing at the pedestrian level, which is an important zone, due to the presence of both, too violent gusts of wind and air stagnation. It is therefore an adequate method for the initial phases of the project. At the same time, the method is relatively simple and not that time-consuming, as compared to other experimental methods.

The research was conducted in a tunnel of 1 mx 1m measuring space cross-section. The model of the designed building with the surrounding building developments, made on 1:500 scale, was attached to the base, made of black glass, in the shape of a circle whose diameter equalled 1 m. The arrangement elements designed and introduced intentionally at the tunnel inlet made it possible for the researchers to depict the wind velocity profile commonly found in urban areas. The base was painted with a mixture of oil and white marker. During the wind simulation, the higher the wind velocity that exerted impact on the glass surface in a given area, the more oil mixture was blown off the surface. Having been stabilised, the obtained image enabled the researchers to read the wind flow directions and identify zones of wind acceleration, swirling and suppression.

The results yielded by the research, though only of qualitative character, allow conclusions to be drawn referring to a high probability of occurrence of problems concerning wind comfort in vicinity of the exemplary building development layout, originally assumed for the sake of research. Moreover, the results indicate the possibilities for improving the situation by means of such shape adjustments that lead to the “loosening” of a too dense urban layout. The research technique adopted for the study proved to be a correct one, although it would be reasonable to compare its effectiveness with numerical methods. Such methods tend to be more modern and cheaper, as well as they provide researchers with greater opportunities in terms of the promptness of obtaining results and in terms of the amount of results they yield. Still, such methods are less reliable than tunnel tests, in addition to being burdened with a larger margin of error. Currently, it is recommended to combine both methods, especially at the initial stages of research, to use the experimental research to properly select the input parameters for numerical simulations and to check the comparability of results in control situations [3, 11, 13]. Thus, the wind tunnel method described in the present paper could serve two purposes – initial verification of the building structure concept and the appropriate setting of numerical research, useful for further detailed research, should such research be required.

The research conducted has proven that the regulations applicable in Poland fail to take sufficient account of issues related to wind conditions in the areas of building developments. These regulations oblige architects to meet numerous criteria regarding the formation and location of buildings, including, aspects regarding the quality of microclimatic conditions, so as to ensure the minimum conditions of insolation and lighting of rooms with the use of daylight. However, the regulations fail to specify issues related to the field of aerodynamics, whereas studies on the impacts the newly designed building exerts, both on its immediate surroundings and larger complexes or new districts are not required. As a result, with the exception of precisely defined situations, any modifications to aerodynamic conditions remain uncontrolled, eventually which fact resulting in deterioration of comfort felt by users of urban spaces and buildings. The issues, in fact, provide untapped potential of possibilities for the care of the quality of the microclimate, both on a small areas scale and the city as a whole.

Insufficient air exchange intensifies the occurrence of the urban heat island phenomenon, that is characteristic for urban areas, together with its consequences (e.g. increased mortality, increase in energy consumption for cooling buildings, increased amounts of ozone). Other problems include increased pollution and concentration of exhaust gases, fog retention unpleasant odours retention. Rapid gusts and wind swirls, in turn, result not only in discomfort or even threats to passers-by, but may also lead to uncontrolled blowing of snow and dirt, damage to building elements.

It would be advisable to consider cases of urban and architectural design in which wind engineering research should be obligatory. Examples of such cases should include complementing the city dense building development, a group of buildings of varying heights, or entirely new quarters of building developments. Particular attention should be paid to areas of diversified spatial structure and function, as well as to very dense, intense building developments. It is in the surroundings of such complex building development layouts that the most diverse aerodynamic phenomena arise. It is, however, difficult to predict such phenomena solely on the grounds of theoretical knowledge.

These issues are prevailing in case of big cities, including Polish ones, which allocate new areas for development (such as post-industrial, railway areas). This leads to a rapid densification in their spatial structure. Cities that fail to foresee the consequences of such major spatial changes in time lose control over the quality of urban space.