Discussion on the Applicability of Urban Morphology Index System for Block Natural Ventilation Research Academic research paper on "Civil engineering"

Procedia Engineering

www.elsevier.com/locate/procedia

4th International Conference on Countermeasures to Urban Heat Island (UHI) 2016

Discussion on the Applicability of Urban Morphology Index System for Block Natural Ventilation Research

Yuelang Gana, Hong Chena*

a Architecture and Urban Planning School, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China,

Abstract

Studies are still required to understand the relationship between the wind situation and the block form. Yet prior to this kind of study, selecting an urban spatial morphology indicator system that could not only describe the change of block form precisely but also have a good relativity with the changing mean wind speed caused by the former, which is a reasonable breakthrough point for exploring the relationship between the block form and the wind environment, is necessary. Consequently, this paper intends to find a series of proper urban spatial morphology indicator systems, and then and to discuss the applicability of these systems for block ventilation research. Through extensive literature survey, two groups of urban spatial morphology indicator systems are selected. One of the systems consists of rugosity, porosity, and sinuosity, which is proposed by Luc Adolphe. Another system is obstruction ratio, which is more popular in China, and appears in urban residential thermal environment design standards. The method of CFD is introduced to quantitatively analyze the link between the changing wind condition and the changing block form. Finally, we establish the relationship between urban spatial morphology indicator systems and the wind condition to select the proper indicator system for block ventilation research.

© 2016 The Authors.Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of the 4th IC2UHI2016 Keywords: natural ventilation; urban morphology; index system

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Procedia Engineering 169 (2016) 240 - 247

1. Introduction

Artificialized city underlying surface and disorganized building plans become more popular. This phenomenon has negative impact upon natural ventilation that could ease the urban heat island effect. Yet prior to this kind of study, selecting an urban spatial morphology indicator system that could not only describe the change of block form precisely but also have a good relativity with the changing mean wind speed caused by the former, which is a reasonable breakthrough point for exploring the relationship between the block form and the wind environment, is necessary. In

* Corresponding author. Tel.: +86-180-8668-0653; fax: +86-027-8755-6714. E-mail address: chhwh@hust.edu.cn

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of the 4th IC2UHI2016 doi: 10. 1016/j .proeng .2016.10.029

the present study, there are two authoritative indicator systems. One system consists of rugosity, porosity, sinuosity, which is proposed by Luc Adolphe [1,2]. Another one is obstruction ratio, which is more popular in China, appears in urban residential thermal environment design standards. This research compares these two indicator systems comprehensively by means of correlation analysis, and then select one which is more suitable for block natural ventilation research.

2. Methodology

Firstly, setting different kinds of idealized block form, the numerical statement of its urban morphology index system is established. And next, the method of CFD is introduced to quantitatively analyze the link between the changing wind condition and the changing block form. Finally, we establish the relationship between urban spatial morphology indicator systems and the wind condition to select the proper indicator system for block ventilation research, which could not only describe the change of block form precisely but also have a good relativity with the changing mean wind speed caused by the former, which is also a reasonable breakthrough point for exploring the relationship between the block form and the wind environment.

2.1. Urban morphology index system

Urban morphology index system is a tool to quantitatively describe the urban form. This paper compares two indicator systems, i.e. Luc Adolphe's system and urban residential thermal environment design standards' system for the applicability of block natural ventilation research.

2.1.1. Luc Adolphe' s system

Luc Adolphe proposes using rugosity, porosity and sinuosity as a set of evaluation criteria for urban morphology description in the articles [1] and [2] in which he established a simple geometric city model with these urban morphology system.

2.1.1.1. Definition of rugosity and calculation methods

Rugosity could be divided into absolute rugosity and relative rugosity. Absolute rugosity represents the average height of urban canopy. Relative rugosity describes the variance of average height of urban canopy (including construction and non-construction elements) from the given direction, and is determined by the weight of every related elements' width on the canopy cross section.

Absolute rugosity and relative rugosity can be calculated respectively as follows:

^ _ _£builtAjhj__(1)

Sbuilt Ai+2non built Aj

Hm - absolute rugosity A; - area of building element i hj - height of building element i Aj - area of non-building element j

n _ [£i(hi-hq)2li2]1/2

- ^ " (2)

Ra - absolute rugosity

ha - average height of urban canopy from the direction of a

hj - height of urban canopy (including construction and non-construction elements) lj - average height of urban canopy from the direction of a X li - equivalent diameter of urban canopy

2.1.1.2. Definition of Porosity and calculation methods

Porosity refers to a ratio between the open volume and the total volume of a certain area. Undoubtedly, in the research of block ventilation, the open volume means the volume of non-buildings elements, and the total volume

means the volume of entire area. Because of the air flow direction is not necessarily parallel to the streets direction, and the wind direction is a very important factor, comprehensive porosity is selected in this paper. Comprehensive

porosity can be calculated using Equation 3 to Equation 6. lh

= Ï7h ........... (3) . .

rh - equivalent radius, a radius of the illusion circular that with the same wind speed and wind volume to a

given rectangular area 1 - average width of urban canopy

h - height of urban canopy (including construction and non-construction elements)

Au = urh2 (4)

Au- area of the urban canopy

p / _ Zopen spaces (5)

0 Sopenspaces Vj+^built^i

P0' - Porosity

rhi - equivalent radius of the urban canopy, the non-building area of which is i Lj - length of the non-building area of the urban canopy Vj - average volume of the urban canopy

p0 = Vp'o2 + P;'O2 (6)

P0 - comprehensive porosity P'0 - porosity from x direction P"0 - porosity from y direction a - wind direction

2.1.2. Urban residential thermal environment design standards' system

Urban residential thermal environment design standards put forward obstruction ratio (including volume obstruction ratio and ventilation obstruction ratio) as the index system to quantitatively describe the urban form, and also suggest useful design strategies to shape good urban environment by controlling urban form with obstruction ratio according to different climate zones.

2.1.2.1. Definition of volume obstruction ratio and calculation methods

Volume obstruction ratio, which is the ratio between building elements volume and the total area volume, can be calculated using Equation 7. It is equal to, in numeral, the building density of the area, which is the ratio between volume fraction and building stories.

7 _ £built Vi (7)

Sv - v (/)

- the volume obstruction ratio Vj - the volume of building element i V - the total volume of the area

2.1.2.2. Definition of ventilation obstruction ratio and calculation methods

In certain area, the factors that hindering ventilation are decided by two aspects, i.e. one is volume obstruction ratio, and the other is windward area ratio. Consequently, ventilation obstruction ratio is equal to, in numeral, volume obstruction ratio multiplied by building windward area ratio, taking the two factors into consideration. Windward area ratio represents the proportion between the area of the building surface slowing down the wind from a given wind direction and the largest possible area of windward building surface. The windward area ration can be calculated using Equation 8.

<^=7^ (8)

ryfmax

Windward area ratio

Fyf- the area of the building surface from a given wind direction V- the largest possible area of windward building surface

Consequently, ventilation obstruction ratio can be calculated using Equation 9.

Ç = ^s ■ Çv (9)

Ç- the largest possible area of windward building surface

- Windward area ratio

- the volume obstruction ratio

2.2. Parameter setting of CFD Table 1. Parameter setting of CFD

Turbulent model

Sky, side

Parameter Setting

Standard k-e model

U = U0 ■ (Z/Zo)1/4 U0 = 2m/s, Z0 = 10m k= 1.5(IxU0)2,I = 0.1

e = Cvk3/2/\ 1 = 4(Cuk)1/2Z0Z3/4/U0

Free slip

Generalized logarithmic law

This paper uses CFD technology to simulate the outdoor wind environment of different block form [3,4]. Fluent, as a famous software, is selected. A secondary development is applied to this software by the Building and Environment Research Centre of Huazhong University of Science and Technology to ensure its accuracy. Parameter setting of Fluent is as shown in Table 1 [5]. Wuhan's (Hubei, China) climate information is used as basic research date and background. Wind direction is south and the initial wind speed is 2.0 m/s [6].

3. Case Setting

3.1. Benchmark model setting

Fig. 1. The schematic of point-type benchmark models Fig. 2. The schematic of plank-type benchmark models

Point-type building and plank-type building are very common in Wuhan or other city. We select two models as benchmark model, i.e. one is point-type building model and the other is plank-type building model (see Fig. 1 and Fig. 2) [7, 8]. Based on statistical data, two idealized benchmark block models' specific data are as follows:

• Point-type building benchmark models

Case B-1:10 buildings, which are 35 meters long, 35 meters wide and 54 meters high, are equally arranged in an area with 300 meters long and 300 meters wide (see Fig. 1) [9].

• Plank-type building benchmark models

Case B-2:25 buildings, which are 18 meters long, 40 meters wide and 30 meters high, are equally arranged in an area with 300 meters long and 300 meters wide (see Fig. 2).

When other cases of block form changing based on these two benchmark block models, there is one rule must be followed, i.e. the construction area must be the same. That means plot ratio is kept consistent in the different planning schemes, which is very common when designers compare various schemes in planning phase. Two basic ways of block shape changing should be taken into consideration. One is height form changing and the other is plan form changing.

3.2. Height form changing model setting

• Point-type building models

Series 1: Building height increases from south to north with the same construction area and plane form as benchmark model (Fig. 3). Specific height differences are shown in Table 2.

Series 2: Building height decreases from south to north with the same construction area and plane form as benchmark model. Specific height differences are shown in Table 2.

Fig.3. The schematic of Series 1 Fig. 4. The schematic of Series 5

• Plank-type building models

Series 4: Building height increases from south to north with the same construction area and plane form as benchmark model. Specific height differences are shown in Table 2.

Series 5: Building height decreases from south to north with the same construction area and plane form as benchmark model (Fig. 4). Specific height differences are shown in Table 2.

Table 2. Specific height differences of Series 2 - 4

Block type Case number Height difference Comprehensive porosity Relative rugosity Absolute rugosity Ventilation obstruction Remarks

benchmark Case B-1 0.00 1.51 0.00 7.35 1.73 benchmark model

Case 1-1 6.00 1.51 1.57 7.35 1.73

Case 1-2 12.00 1.51 2.92 7.35 1.73 Series 1

Case 1-3 18.00 1.51 4.09 7.35 1.73

point-type

Case 2-1 6.00 1.51 1.57 7.35 1.73

Case 2-2 12.00 1.51 2.92 7.35 1.73 Series 2

Case 2-3 18.00 1.51 4.09 7.35 1.73

benchmark Case B-2 0.00 0.40 0.00 6.00 1.82 benchmark model

Case 3-1 3.00 0.40 1.21 6.00 1.82

Case 3-2 6.00 0.40 2.43 6.00 1.82 Series 3

Case 3-3 9.00 0.40 3.65 6.00 1.82

point-type Case 4-1 3.00 0.40 1.21 6.00 1.82

Case 4-2 6.00 0.40 2.43 6.00 1.82 Series 4

Case 4-3 9.00 0.40 3.65 6.00 1.82

3.3. Plan form changing model setting

• Point-type building models

Series 5: Buildings spacing decreases with the same construction area and building form as benchmark model (see Fig. 5). Specific spacing differences are shown in Table 3.

Series 6: Buildings spacing increases with the same construction area and building form as benchmark model.

Specific spacing differences are shown in Table 3.

Fig. 5. The schematic of Series 5

Fig. 6. The schematic of Series 8

• Plank-type building models

Series 7: Buildings spacing decreases with the same construction area and building form as benchmark model. Specific spacing differences are shown in Table 3.

Series 8: Buildings spacing increases with the same construction area and building form as benchmark model (see Fig. 6). Specific spacing differences are shown in Table 3.

Table 3. Specific spacing differences of series 5 - 8.

Block type Case number Spacing differences Comprehensive porosity Relative rugosity Absolute rugosity Ventilation obstruction Remarks

benchmark Case B-1 0.00 1.51 0.00 7.35 1.73 benchmark model

Case 5-1 6.00 1.36 0.00 7.35 1.73

Case 5-2 12.00 1.20 0.00 7.35 1.73 Series 5

point-type Case 5-3 18.00 1.06 0.00 7.35 1.73

Case 6-1 6.00 1.97 0.00 7.35 1.73

Case 6-2 12.00 1.81 0.00 7.35 1.73 Series 6

Case 6-3 18.00 1.66 0.00 7.35 1.73

benchmark Case B-2 0.00 0.40 0.00 6.00 1.82 benchmark model

Case 7-1 33.00 0.37 0.00 6.00 1.82

Case 7-2 30.00 0.34 0.00 6.00 1.82 Series 7

Case 7-3 27.00 0.30 0.00 6.00 1.82

point-type -

Case 8-1 39.00 0.44 0.00 6.00 1.82

Case 8-2 42.00 0.47 0.00 6.00 1.82 Series 8

Case 8-3 45.00 0.50 0.00 6.00 1.82

4. Result and Discussion

All cases' wind speed at pedestrian level is shown in Table 4. Some of CFD results are shown in Fig. 7 to Fig. 9. In Series 1, it is evident that with the increase of building height difference, the number of relative rugosity increases

correspondingly, and the wind speed at the pedestrian level (1.5 m) also increases. It means there is positive correlation between them. However, in Series 2, with the increase of building height difference, the number of relative rugosity also increases correspondingly, but the wind speed at pedestrian level (1.5 m) decreases. Series 3 and Series 4 show similar behavior with Series 1 and Series 2 respectively. This indicates that the wind direction is very important to the results. When the building height increases in the same direction as the incoming wind, the number of relative rugosity is positively correlated with the wind speed. Conversely, they are negatively correlated. So the concept of effective rugosity is put forward as an updating indicator of relative rugosity. Their relationship can be calculated with Equation 10. The number of effective rugosity ranges from -6.702 to 6.702 in this paper.

Ra* = Ra ■ cos p (10)

Ra* - effective rugosity

P - angle between the wind direction and the normal direction of the building getting higher

By suggesting the concept of effective rugosity, wind speed changing rate is strongly correlated with effective rugosity changing rate. Their relationship could be described as a linear function (Equation 11). y = 1.4705x- 0.0769 (11)

x -effective rugosity y - wind speed

Comment: the scope of effective rugosity's applicability needs further research. The study mainly focuses on several cases so the results could only be valid in this study.

Velocity

MagniMg 0 0,2040,60.8 1 1.2 1.41,6 1.8 2 2.2

Fig. 7. Wind speed of case B-1

Fig. 8. Wind speed of case 3-3

Fig. 9. Wind speed of case 4-3

Table 4. Results of all cases

Block type Case number Wind speed (m/s) Relative rugosity Case number Wind speed (m/s) Comprehensive porosity

benchmark Case B-1 0.54 0.00 Case B-1 0.54 1.51

Case1-1 0.54 1.57 Case 5-1 0.52 1.36

Case 1-2 0.55 2.92 Case 5-2 0.49 1.20

point-type Case 1-3 Case 2-1 0.54 0.53 4.09 1.57 Case 5-3 Case 6-1 0.47 0.52 1.06 1.97

Case 2-2 0.52 2.92 Case 6-2 0.52 1.81

Case 2-3 0.52 4.09 Case 6-3 0.52 1.66

benchmark Case B-2 0.49 0.00 Case B-2 0.49 0.40

Case 3-1 0.49 1.21 Case 7-1 0.45 0.37

Case 3-2 0.50 2.43 Case 7-2 0.41 0.34

point-type Case 3-3 Case 4-1 0.53 0.48 3.65 1.21 Case 7-3 Case 8-1 0.38 0.61 0.30 0.44

Case 4-2 0.46 2.43 Case 8-2 0.63 0.47

Case 4-3 0.46 3.65 Case 8-3 0.67 0.50

In Series 5 to 8, it is clearly shown that comprehensive porosity is strongly correlated and positively correlated with wind speed at pedestrian level (1.5 m).

In all series, the number of ventilation obstruction has hardly any change, nonetheless, the wind speed at pedestrian level changes greatly. They are poorly correlated. Therefore, it is inappropriate to select ventilation obstruction as a breakthrough point for exploring the relationship between the block form and the wind environment.

5. Conclusion

When discussing about the block plane morphology change, comprehensive porosity could clearly show this change. The number of comprehensive porosity ranges from 0.098 to 0.512 in this paper. Wind speed changing rate is strongly correlated with comprehensive porosity changing rate, and their coefficient is 0.9788.

Relative rugosity could clearly describe the block vertical morphology changes. The number of relative rugosity ranges from 0 to 6.702 in this paper. But when we do a detailed study of the relationship with wind speed and relative rugosity, it is necessary to take wind direction into account. So the concept of effective rugosity is put forward as an updating indicator of relative rugosity. Their relationship can be calculated with Equation 10. The number of effective rugosity ranges from -6.702 to 6.702 in this paper.

By suggesting the concept of effective rugosity, wind speed changing rate is strongly correlated with effective rugosity changing rate. Their relationship could be described as a linear function (Equation 11).

Comment: the scope of effective rugosity's applicability needs further research. The study mainly focuses on several cases so the results could only be valid in this study. Ventilation obstruction proposed by urban residential thermal environment design standards (Chinese standards) could not show the changes by the block shape changing in neither plan nor height ways. To sum up, we suggest that comprehensive porosity and effective rugosity might be the proper indicator system for block ventilation research because it could not only describe the change of block form precisely but also have a good relativity with the changing mean wind speed caused by the former.

Acknowledgements

This study was financially supported by China Clean Development Mechanism Foundation (2013049). References

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