The majority of world population is concentrated in urban areas and it is expected that half of the earth’s population will live in urban settlements in a few years. Urbanization is changing city activities, which modify the urban environment causing serious problems like air quality. Air quality is a very important and complicated issue in modern cities and all around the globe, city councils try to address it as top priority. This issue has quite a few different aspects that need to be considered in the process of urban design ranging from air pollution to visibility and various other issues affecting the usability of urban spaces. One of the most important aspects of the latter is the wind funnel effect that is a result of the creation of the urban canyons.
It is essential to understand the wind behavior in macroscale and microscale and analyze the problems that are associated with it before investigating the wind funnel effect. The general pattern of wind movement in outdoor spaces is that it decreases its speed more in urban areas at the level of the urban canopy than in suburban or rural areas but it shows higher speed variations and turbulence. This is because as wind is flowing over an open area and approaches a built-up area, it is deflected from the buildings and the higher roughness of the surface. (Givoni B. et al, 2003) Generally the wind speed near ground decreases with the increase of ground roughness. The problematic area of research is the transitional zone between earth’s surface and urban air dome and it is called “urban boundary layer”(Fig_11).
Analyzing more endoscopically, in the scale of an urban area, the parameters that affect urban canopies wind are the following:
1. The overall density of the urban area
2. Ration of width and height of the individual buildings
3. Orientation of the streets related to the wind
4. Size distribution, and design details of open spaces
According to these parameters, there are three different categories for air movement in outdoor spaces concerning the building density, the ‘isolated roughness flow regime’, the wake interference regime‘ and the ‘skimming flow’. (Oke, 1987) As it is clear from the diagram below (Fig_12) the most common situation in big cities is well represented with the third category where the buildings act as an entity because of the small distance between them. The air starts to skim over the buildings creating a strong vortex at pedestrian level. Also in terms of building layout this dissertation is focusing around the passage between two parallel buildings, as it is believed to be the more indicative and easier to accurately bring into simulation environment. (Blocken and Carmeliet, 2004) (Fig_13)
Provided that we investigate solutions in order to mitigate wind funnel effect improving pedestrian wind comfort, it is essential to analyze wind movement near the ground, at human’s level. Buildings affect winds due to their size, shape, and spacing. When the wind is deflected from the buildings because of the friction turbulence is increased and causes changes in the direction and speed. The average wind velocity increases with height but on the contrary its turbulence level decreases with height as the friction between the wind and earth surface steadily decreases with height. The windward side of the building is the side that faces the wind directly and has higher pressure than the leeward side. In the windward side of the building a stagnation point is created in the center at about ¾ of the building height. (Blocken and Carmeliet 2004) (Fig_14) Most of the air goes down causing localized pressure gradient that lead to strong wind’s vortices. According to Blocken and Carmeliet (2004) there are six types of flow at pedestrian level because of the existence of tall buildings (Fig_15):
Even though there is some significant research in the field of buildings aerodynamics that proposes design solutions for the future of built environment, there is very little work done on the development of systems that try to solve wind problems within an existing build environment. There are some researches that are referred to windscreens but they are only suggestions or experimental solutions and not scientifically proven. On the other hand, windbreak technology in rural areas is a mature research field and a lot of work has been developed and this thesis after a thorough studying upon these researches trying to keep the key points of windbreaks and translating their properties in urban environment requirements.
It is essential to understand the wind behavior in macroscale and microscale and analyze the problems that are associated with it before investigating the wind funnel effect. The general pattern of wind movement in outdoor spaces is that it decreases its speed more in urban areas at the level of the urban canopy than in suburban or rural areas but it shows higher speed variations and turbulence. This is because as wind is flowing over an open area and approaches a built-up area, it is deflected from the buildings and the higher roughness of the surface. (Givoni B. et al, 2003) Generally the wind speed near ground decreases with the increase of ground roughness. The problematic area of research is the transitional zone between earth’s surface and urban air dome and it is called “urban boundary layer”(Fig_11).
Analyzing more endoscopically, in the scale of an urban area, the parameters that affect urban canopies wind are the following:
1. The overall density of the urban area
2. Ration of width and height of the individual buildings
3. Orientation of the streets related to the wind
4. Size distribution, and design details of open spaces
According to these parameters, there are three different categories for air movement in outdoor spaces concerning the building density, the ‘isolated roughness flow regime’, the wake interference regime‘ and the ‘skimming flow’. (Oke, 1987) As it is clear from the diagram below (Fig_12) the most common situation in big cities is well represented with the third category where the buildings act as an entity because of the small distance between them. The air starts to skim over the buildings creating a strong vortex at pedestrian level. Also in terms of building layout this dissertation is focusing around the passage between two parallel buildings, as it is believed to be the more indicative and easier to accurately bring into simulation environment. (Blocken and Carmeliet, 2004) (Fig_13)
Provided that we investigate solutions in order to mitigate wind funnel effect improving pedestrian wind comfort, it is essential to analyze wind movement near the ground, at human’s level. Buildings affect winds due to their size, shape, and spacing. When the wind is deflected from the buildings because of the friction turbulence is increased and causes changes in the direction and speed. The average wind velocity increases with height but on the contrary its turbulence level decreases with height as the friction between the wind and earth surface steadily decreases with height. The windward side of the building is the side that faces the wind directly and has higher pressure than the leeward side. In the windward side of the building a stagnation point is created in the center at about ¾ of the building height. (Blocken and Carmeliet 2004) (Fig_14) Most of the air goes down causing localized pressure gradient that lead to strong wind’s vortices. According to Blocken and Carmeliet (2004) there are six types of flow at pedestrian level because of the existence of tall buildings (Fig_15):
Even though there is some significant research in the field of buildings aerodynamics that proposes design solutions for the future of built environment, there is very little work done on the development of systems that try to solve wind problems within an existing build environment. There are some researches that are referred to windscreens but they are only suggestions or experimental solutions and not scientifically proven. On the other hand, windbreak technology in rural areas is a mature research field and a lot of work has been developed and this thesis after a thorough studying upon these researches trying to keep the key points of windbreaks and translating their properties in urban environment requirements.
Fig_11
Urban Boundary layer - Urban canopy
Urban Boundary layer - Urban canopy
Fig_12
Oke, T.R., 1987
Oke, T.R., 1987
Fig_13
Fig_14
Fig_15
Blocken and Carmeliet, 2004
Blocken and Carmeliet, 2004