Water potential

4.7. Assessment Approach of Outdoor Pedestrian Microclimates in a Street Canyon

4.8. Previous Numerical Simulation Studies on Urban Microclimate and Thermal Comfort Using CFD Tool

CHAPTER 4:

Literature Review on the Effects of

Urban Street Canyon’s Configurations on

Wind Flow

4.1. Introduction

This chapter presents a literature review pertinent to studies on effects of urban street canyon‘s configuration on urban pedestrian microclimate and thermal comfort. It focuses on best practice for planning urban route configuration, limited to the following two aspects: the effect of urban street canyon‘s aspect ratio on microclimatic conditions at the pedestrian level and the effects of orientation of urban canyons on pedestrian thermal comfort level. It also discusses the previous CFD numerical simulation studies on urban microclimate and thermal comfort. The knowledge gaps based on the literature review are also identified and summarised at the end of this chapter.

4.2. Importance of Urban Canyon Design with Microclimatic Awareness

The first part of the literature review (Chapter 3) highlighted that thermal comfort is an important factor that needs to be considered when designing outdoor urban pedestrian spaces in either hot or cold microclimates, as it forms an important factor for the usability and attractiveness of public places (e.g. Lenzhölzer and Koh, 2010). Thermal comfort in outdoor pedestrian microclimates is a factor that encourages people to visit these places and/or to prolong their usage of the space, which in turn provides increased opportunities for sociability and economic activity as well as other benefits (Eben Saleh, 1997; Aljawabra and Nikolopoulou, 2010; Nasir et al., 2012). Givoni et al. (2003:77) pointed that “thermal comfort of persons staying outdoors is one

of the factors influencing outdoor activities in streets, plazas, playgrounds, urban parks, etc.” Therefore, Carmona et al. (2003:185) have argued that “if spaces are not comfortable, they are unlikely to be used.” However, despite the fact that urban

designers must create comfortable urban spaces for the users, there is a lack of understanding of the appropriate design process for the enhancement of outdoor thermal comfort.

There is an interrelationship between urban street canyon design (e.g. urban aspect ratios and street orientations), microclimatic parameters (e.g. air temperature, solar radiation, air speed and humidity), and adaptive personal factors (e.g. activity level and clothing insulation level). This is shown to have direct impact on pedestrians‘ thermal comfort conditions in outdoor urban spaces (Ahmed-Ouameur and Potvin, 2007; Krüger, et al., 2011; Nikolopoulou et al., 2004; Shishegar, 2013; Martins et al., 2012; Yahia, 2012).

Therefore, Ahmed-Ouameur and Potvin (2007) stated that urban configuration is an independent variable of urban microclimate and human‘s thermal comfort factors (Figure 4.1); while microclimate can be influenced by urban morphology and climatology. On the other hand, human‘s thermal comfort factors depend on both attributes of the urban morphology and urban microclimate, as well as on the adaptive behaviour opportunity for enhancing thermal comfort levels (Givoni, 2010; Emmanuel and Johansson, 2006; Vasilikou and Nikolopoulou, 2014; Yahia, 2012; Thani et al., 2013).

Fig. 4. 1: Interaction between urban morphology, microclimate and pedestrian‟s thermal comfort. Source: adapted by the author, source: Ahmed-Ouameur and Potvin (2007).

4.3. Effect of Urban Street Aspect Ratio on Pedestrian Wind Flow Rate

The wind flow within an urban street is strongly affected by the urban street canyon geometry (H: height, L: length, W: width) and the street orientation (Ali- Toudert, 2005; Ahranjani, 2010; Oke, 1988). Therefore, one can develop the relationship between the street canyon parameters and the airflow pattern. The street canyon can be called deep, regular or avenue canyons when the aspect ratio (H/W) is greater than two, one, or half, respectively (Ahranjani, 2010). An urban canyon is called symmetric when the height of buildings at the sides of a street is equal in a cross-section, and an asymmetric canyon is called when one building height at one side of the street width is taller than the opposite building on the other side of the street (Ahranjani,

2010). However, in urban numerical studies simplification of urban aspect ratio is often recommended, due to the limitation of computational power and turbulence issues. Therefore, Nakamura and Oke (1988) recommend assumption of averaging buildings heights and performing roughly symmetrical types of urban canyons. However, many studies have conducted symmetrical aspect ratio (e.g. Ali-Toudert and Mayer, 2006; Georgakis and Santamouris, 2006), while limited number of studies have performed on asymmetrical aspect ratio, particularly for pedestrian wind field studies.

Many researchers studied the effects of street aspect ratio (H/W) with the degree of exposure to solar radiation to control the amount of radiation from reaching the ground and different solar orientations in relation to street level radiant temperatures, particularly during extreme hot conditions. However, most of these studies have focused on symmetrical building heights to width ratios, but did not consider the effect of multi- asymmetrical street aspect ratios on pedestrian thermal comfort. Ali-Toudert and Mayer (2006) conducted a numerical study in a hot and dry climate of Algeria on the effects of a symmetrical urban street height-to-width ratio (H/W =0.5, 1, 2, and 4; with a constant street width of 8m) and orientations with solar radiation (E-W, N-S, NE-SW and NW- SE) on outdoor thermal comfort. It was found that thermal sensation at street level depends strongly on street orientation and building heights and street width. N-S streets showed a trend to be slightly cooler than E-W streets, particularly as the aspect ratio increases.

However, the simulated wind speed in their study was perpendicular incidence on the street axis, which led to a skimming flow regime and found to be strongly reduced at pedestrian level. Nevertheless, their study mainly focused on the orientation in relation to solar radiation rather than with prevailing wind directions, as well as the aspect ratio was symmetrical and focused on exposure to solar angle instead of wind flow rate. Considering wind flow perpendicular to different urban street aspect ratios, for example, three regimes of wind flow over buildings arrays can occur based on the aspect ratios (H/W). These are isolated roughness flow (H/W<0.3), wake interference flow (0.3<H/W<0.7) and skimming flow (H/W>0.7) (Oke, 1988), as illustrated in Figure 4.2.

Fig. 4. 2: Three wind flow regimes over arrays of buildings for different aspect ratios. Source: Oke (1988).

Hussain and Lee (1980) have also found similar results but in the gap between two buildings that are arranged along the prevailing wind direction. According to Ali- Toudert (2005) and Oke (1988), the isolated roughness flow regime occurs between well-spaced buildings when there is no interaction at the windward and leeward flows. The wake interference flow occurs when H/W increases, as a result of disturbed wakes. The skimming flow regime occurs with further increase in H/W, which is the case in many dense urban contexts, as the urban deep street becomes isolated from the above circulating air and a stable circulatory vortex is established in-canyon, thus influencing the microclimate conditions at street and pedestrian levels (Oke, 1988).

Moreover, Jeong and Andrews (2002) suggested that for increasing from a one- circulation regime to a two counter-rotating circulation, a threshold aspect ratio of 1.60<H/W<2.86 is observed. This would generally move the main circulation to the upper part of the urban street, while a secondary weak circulation can generate in the bottom part (Fig. 4.3a and 4.3b respectively). For the evolution of the airflow to a three- circulation regime in the canyon, Kim and Baik (2001) reported the threshold aspect ratio of 3.40-3.60 (see Fig. 4.3c). In addition, the researchers found that by heating street-bottom (e.g. temperature difference of 4K), the characteristic of the latter mentioned flow regime changes to two counter-rotating vortices in the lower layer and a mechanically induced upper vortex (Fig. 4.3d).

Fig. 4. 3: Streamlines of flow vortices within different aspect ratios of; (a) 1.6, (b) 2.86 with two circulations (source: adapted from Jeong and Andrews, 2002), and aspect ratio of 3.6 and potential temperature differences of (c) 0K, (d) 4K, with three-circulation regime (source: adapted from Kim and Baik, 2001).

However, in the current research, only flow speed of vortices at pedestrian level will be analysed, due to the nature of the investigation seeking for enhancing pedestrian level wind comfort, thus improving the outdoor pedestrian thermal comfort conditions. Nevertheless, increasing circulation regime within the urban street canyon may lead to pedestrian wind discomfort, as a result of increased wind speed. For example, Al-Sallal and Al-Rais (2011) found that when wind speed is higher than 5m/s wind can reach deeper inside the narrow streets. Although this case improved the circulation of wind flow in some places maximising thermal comfort, while creating vortices at the building corners causing wind discomfort to some extent at pedestrian level. However, in the current research the yearly average maximum wind speed in Madinah city was found to be 3.5m/s at the airport (Iowa State University of Science and Technology, 2015), while urban areas suffer from very low wind speed. Therefore, methods for increasing wind speeds should be investigated for such regions.

Jeong and Andrews (2002:1137) stated that ―compared with the better

ventilation characteristics associated with better parallel flow, cross-flow produces partial isocirculation formed by a cascade of vortices that descends from the rooftop to the street‖. However, DePaul and Sheih (1986) argue that these type of vortices can

limit air flow movement to funnel within the urban canyon to the crossing free stream, leading to trapped pollutants concentration. Nevertheless, this limitation may occur when parallel wind speed with street canyon is lower than perpendicular wind flow (e.g. Al-Sallal and Al-Rais, 2012).

4.4. Best Practice for Urban Heat Stress Mitigation Techniques in Urban Canyons

The living conditions of people are very difficult in hot arid climates (Ali- Toudert et al., 2005), because of the extreme climatic conditions. However, the microclimate of an urban street in hot arid regions can be enhanced by using appropriate urban configuration strategies (e.g. urban ventilation, shading, urban materials alteration, anthropogenic heat release reduction, etc.), which can be regulated by adjusting urban design and planning policies (Ahranjani, 2010). However, the main limitation of these mitigation strategies is that their implementation is not always economically practical in the existing unplanned urban areas. Moreover, even after adapting proper mitigation strategies for different locations, the outcome could vary from one part of a city to another, which makes the mitigation strategies ineffective.

Many studies on different urban design strategies have been performed for outdoor thermal environments, which provide understanding of their effects on the thermal sensation of a person at street and pedestrian levels (as discussed in Chapter 3, also see Table 4.1). However, only a few have studied the effect of asymmetrical urban street canyon‘s aspect ratios (different heights to width ratio, H/W) in relation to prevailing wind flow directions under different climatic conditions (e.g. Qaid and Ossen, 2014), while very little research on asymmetrical urban canyons‘ aspect ratio has been conducted in hot arid regions. Appropriate urban street geometry design can have a positive effect on airflow regime in hot climates, thus increased wind speed can reduce the urban heat stress and enhance pedestrian thermal comfort levels (Qaid and Ossen, 2014).

Various studies were conducted on the configuration of urban geometries (e.g. H/W height to width aspect ratios and street orientations) with urban microclimates but in relation to exposure to solar radiation (Xi et al., 2012; Johansson, 2006; Santamouris et al., 1999; Ali-Toudert et al., 2005; Ali-Toudert and Mayer, 2006; Coronel and Álvarez, 2001; Swaid et al., 1993). However, most of the previous studies have focused on symmetrical aspect ratios rather than asymmetrical ones, which limit the available knowledge on asymmetrical canyon studies. Very limited research has been conducted in hot arid climates on asymmetry urban street, particularly in a form of two building blocks (Ali-Toudert and Mayer, 2007; Todhunter, 1990) rather than rows of multiple buildings. However, the study of multi asymmetrical urban streets aspect ratios, i.e. diverse height-to-width (H1/W – H2/W – H3/W) ratios for at least three rows of multiple

buildings was not discussed in the context of urban and pedestrian microclimates in hot arid regions.

This represents a fundamental difference between the aim of the present research and the previous studies of Ali-Toudert and Mayer (2007) and Qaid and Ossen (2014). Further, Ali-Toudert and Mayer‘s study (2007) followed a different method of investigating the effects of asymmetrical urban canyons on thermal comfort, which is by manipulating an asymmetrical urban canyon with a wide opening to the sky and comparing an asymmetrical urban canyon with overhanging facades (i.e. smaller opening to the sky). The simulation outcomes revealed that the larger the openness of the canyon to the sky, the greater the heat stress. This study was conducted under hot arid climatic conditions of Ghardaia in the Algerian Sahara, using Env-met model and PET index for the simulation and calculation of thermal comfort, respectively. However, the study did not consider testing different aspect ratios (H/W) of the asymmetrical canyon in terms of altering buildings height. Instead, they have considered the openness to the sky, which seriously lacks in information on the effect of different asymmetrical aspect ratios on pedestrian wind environment, thus on pedestrian thermal comfort.

Other researchers studied the effects of both building shapes and arrangement of buildings in a chess-board design on airflow around multiple rows of buildings (particularly, three rows) in hot arid climates of Aswan and Farafra in Egypt (Figure 4.4), using CFD numerical simulation method (Rizk and Henze, 2010). They focused on the effect of narrow passages between buildings, canyons between rows of buildings and shapes of windward elevations on wind speed ratio. It was found that by creating a narrow passages of 6m between buildings at the same row and wider canyon (10m) between the buildings rows, the Aswan model, which uses rectangular shapes that have slope exterior wall in two directions at the windward elevations and wide distances between passages of buildings at the same row, can achieve 60-80% of wind velocity at the windward side of buildings at the second and third rows.

Fig. 4. 4: Velocity magnitude for Aswan and Farafra Models in hot arid Egyptian climate. Decreasing the passage between buildings while using wider street canyons. Source: Rizk and Henze (2010).

However, the model‘s results were based on high wind velocity input values of between about 12 to 13m/s, which may not be applicable for the current research of Madinah where the yearly average maximum wind speed was found to be 3.5m/s (refer to Chapter 2 for more detail). Nevertheless, the researchers found that a chess-board design arrangement for buildings rows can also increase wind velocity at both windward surfaces and narrow passages of the next row by increasing street canyon width to as much as 20m (Rizk and Henze, 2010). Similarly, it was found that in the Farafra model, which uses narrow distances between passages of buildings (i.e. 6m) at the same row and rectangular shapes that have trapezoid courtyards that face the wind, it is possible to achieve 33-41% of wind velocity at the windward surfaces and 38-70% at the second and third row, by using the chess-board design arrangement and increasing the canyons width. This evidence emphasises the importance of narrow passages which can increase wind velocity by venturi effect (channel effect, i.e. the effect where wind speed increases as it passes through a smaller openings) (e.g. Blocken and Carmeliet, 2004b), as increasing wind speeds is demanding in areas that suffer from very low wind to achieve thermal comfort (Qaid and Ossen, 2014). However, these case studies of Egypt of multiple rows of buildings did not consider the effect of increasing the urban street H/W aspect ratios (particularly increasing the height of buildings) on urban pedestrian microclimate or pedestrian thermal comfort conditions.

Other researchers studied the effects of building materials on outdoor urban thermal microclimates to control the reflectivity and absorption levels of solar radiation (e.g. Oke, 1982; Oke et al., 1991; Tan and Fwa, 1992; Ramadhan and Al-Abdul Wahhab, 1997; Priyadarsini et al., 2008; Taha, 1997; Akbari et al., 2009). For instance, Priyadarsini et al. (2008) have studied the effects of urban surface materials on air

temperature within an urban street canyon in a hot humid region of Singapore. It was found that air temperature in the canyon was significantly different when using high reflectance and low reflectance building surface materials. The results revealed that the high reflectance values will reduce the air temperature within the canyons. Hence, they have concluded that the type of surface materials and their colour are among the most important factors that can control the thermal microclimatic conditions. Surfaces of concrete, asphalt, bricks and other dark urban structures absorb more heat than lighter colour materials during the daytime, increase more heat in the air by convection, and re- radiate the absorbed heat back into the urban environment during the night (Oke, 1982; Oke et al., 1991). Therefore, in the current study, the effect of changing materials on urban microclimate will be considered, to improve the thermal comfort conditions in Madinah.

Fig. 4. 5: Surface albedo values. Surfaces with white paint usually have higher albedo than dark materials, and reflect more solar radiation and are generally cooler. Source: Akbari et al. (1992).

According to Ahranjani (2010), urban heat stress mitigation strategy of urban materials alteration may include using green elements, greening exterior walls and rooftops of buildings, reducing the asphalt-paved area, and using higher solar reflectance (albedo) materials on buildings. The latter two will be considered in the present study. However, increasing the number of vegetation in hot and arid regions is clearly resource intensive, as high evaporation rates may occur, and it requires water and maintenance. Akbari et al. (1992) have studies the albedo range in urban areas, as

highlighted in Figure 4.5, with higher albedo level is found in white paint with value range of 0.5 to 0.9.

Summaries of the findings from previous studies on urban microclimate design strategies and assessment of thermal comfort are listed in Table 4.1, and discussed in this chapter.

Table 4. 1: Previous Studies on Urban Street Geometry (i.e. aspect ratio and orientation of an urban street canyon).

Researcher Climate

Region Main Research Findings Comments

Mayer & Höppe (1987) Munich, Germany (Hot Summer season).

Significant heat stress was found in the urban street canyon that is exposed to south. In contrast, highest amount of shading were found in the "trunk space of the tall spruce forest" exposed to north.

The research did not consider the effect of street orientation and aspect ratio.

Nakamura & Oke (1988) Japan (Hot Humid)

This paper demonstrated that a number of useful relationships exist between the urban geometry and the urban

microclimate of street canyons.

In terms of H/W: a lower limit of about 0.4 is set by the need to provide some degree of shelter and to retain a reasonable proportion of the heat island warmth. An upper limit of 0.60-0.65 ensures that both atmospheric dispersion and solar access is maintained within the street canyons.

Hypothetical average of building heights can be based on this paper, which suggests for simplification of urban geometry.

Gut & Ackerknecht (1993)

Hot Arid The straight streets with parallel buildings can maximise solar radiation, but by orienting the urban pattern diagonally to the east-west axis can provide both potential shade on the street and support the dynamic movement of air.

This study did not consider the effects urban street orientations with prevailing wind direction. It also did not consider the effect of aspect ratio.

Golany (1996)

Hot Dry Straight and parallel urban streets with prevailing wind direction is better than perpendicular urban pattern to reduce the