Improving Natural Ventilation Performance in a High-Density Urban District: A Building Morphology Method Academic research paper on "Civil engineering"

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Procedía

Procedía Engineering 205 (2017) 952-958

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10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1922 October 2017, Jinan, China

Improving Natural Ventilation Performance in a High-Density Urban District: A Building Morphology Method

Fei Guoa*, Peisheng Zhua, Shiyuan Wanga, Dongwen Duana and Yun Jina

The urban expansion of Dalian city by a high-density downtown morphological pattern cause the wind speed per year trended downward. Therefore improving urban ventilation performance is becoming more important for urban planners. To evaluate natural ventilation performance in the high-density downtown area of Dalian, an in situ wind environmental survey was conducted. The natural ventilation performance of different building morphologies was further evaluated via CFD simulations tools PHOENICS 2012. The results indicate that urban edifices, such as enclosed city blocks, strip apartments in rows, high-rise buildings with large

podium bulk, are unfavorable to natural ventilation, and strategies as using ventilation channels and increasing building height

while decreasing their land coverage could improve the urban ventilation performance. The quantitative relevance between building morphology and wind environment is examined, according to the result of which an ameliorate morphology proposal is presented and assessed by PHOENICS 2012. This research could provide a method to create a livable urban wind environment.

© 2017 The Aiithors. Published by Elsevier Ltd.

peer-review undler responsibility of the stientific commfaee of the 10th International Symposmm on ^atin^ Ventilation and Air Conditioning.

Keywords: CFD; Wind velocity; Urban morphology; Quantitative relevance

1. Introduction

Urban climate issues, such as the urban heat island (UHI) effect, wind speed decline and air pollution, have gained attention in the context of global climate change, and many researchers at home and abroad have published relevant studies. T. R. Oke [1] is among those who have studied the relation between urban morphology parameters and climate.

* Corresponding author. E-mail address: guofei@dlut.edu.cn

1877-7058 © 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and

Air Conditioning.

10.1016/j.proeng.2017.10.149

aSchool of Architecture and Fine Art, Dalian University of Technology, Dalian 116023, China

Abstract

He summarized the relationship between urban population scale, wind speed and UHI, proposing that UHI intensity is approximately inversely proportional to the square root of local wind speed. W. Ding et al. [2] performed a correlation study on urban morphology and urban microclimate, deriving concepts such as urban morphology and the urban texture unit. Their studies indicated that there is a correlation between measures of "wind comfort" in urban public spaces and the mechanical effects of wind speed. Other studies indicated that wind speed is in an evident inverse relationship with air pollution and the UHI, which could be dramatically alleviated if the wind speed reach above a certain value. Therefore, promoting urban ventilation is an important strategy to be considered in urban planning for high-density cities. However, the quantitative relation between urban form and wind environment of Dalian city remains unclear, making it difficult to effectively carry out ventilation promotion strategies for the local urban planning system, particularly at the level of urban overall planning and detailed planning.

The CFD model (Computational Fluid Dynamics) is a useful tool for micro-scale research, being widely used in detailed planning, urban design, climate assessment and research at the building scale. Its advantages include: (1) it can describe architectural geometry, building and pavement materials, street form and direction, impermeable surfaces, greening, and the effects of anthropogenic heat and pollutants on climate in detail. (2) It is good at simulating a transient climate. (3) By calculating micro scale (0.1-1000 m) climate parameters such as wind speed, temperature, humidity and radiation, and assuming the clothing and human activity level, a comprehensive evaluation of thermal comfort (such as PMV, SET*, PET) can be obtained.

Dalian is a typical coastal city. The city area has gradually expanded over the past 60 years, from 50 km2 (1950) to 396 km2 (2015) [3]. Because of the urban expansion, and high intensity of land use, the city center features high-level and high-density morphological characteristics. With more than 2,200 km of coastline, Dalian has a high average annual wind speed and rich land sea breeze resource. If fully utilized in urban design phase, it will be of great importance for improving the high-density urban thermal comfort. In this paper, by using a typical urban area as the research object, the comparative and simulative analysis on the wind environment was performed before and after optimization through CFD simulation.

121.6E

Fig. 1. Schematic of study regions (a) the satellite image; (b) the 3-d model.

2. Urban MORPHOLOGY OPTIMIZING STRATEGY BASED ON WIND ENVIRONMENT

The study areas include Zhongshan Square, Democracy Plaza, Harbour Plaza, Erqi Plaza and other famous landmarks. It is about 1100 meters from north to south, 1400 meters from east to west. The total geographical area is approximately 1.54 km2 (Fig. 1). Since Dalian Colonial Period, planning and construction of the area has passed 100 years. The area presented high urban road density and complete supporting. With the progress of economic development, studied area gradually appears with typical urban characteristics as high floor area ratio, high-rise and high density.

We conducted field investigation and thermal environment test etc. in advance. Aiming at wind environment problems occurring within the area, morphology-optimizing strategies were proposed such as constructing urban ventilation channel and green space system, adjusting building layout etc. for the studied area.

2.1. Constructing urban ventilation channel

Urban ventilation channel is band-form interspace with lower urban internal surface roughness (Z0<0.5 m). Possessing certain length and width, it can relieve heat island effect and atmospheric contamination in the weather with weaker wind speed [4]. For coastal city Dalian, it should give priority to considering making cool and clean airflow like sea wind, etc. penetrate into core urban area, and avoid building to form folding screen in harbour area and block sea wind to penetrate.

2.2. Constructing green space system

Green space system possesses the role as beautifying environment, purifying air, lowering down the noise, reducing environmental pollution, increasing urban impounding and drainage functions as well as relieving heat island effect. If green land system correlates through the forms like gallery, etc. and shapes network, it can effectively avoid extension and overlaying of urban heat islands, play a role of protecting urban natural environment and biodiversity, forming ventilating duct to promote wind flowage and improve thermal environment [5].

2.3. Adjusting building combination morphology

Determinant building combination pattern has longer wind shadow and it is very unfavorable to urban wind and thermal environment. If urban planners can properly adjust the height of different buildings or adopt stagger layout to form scattered high-low, it is beneficial for the wind to flow among building complex. Meanwhile, large-size high-rise and super-high-rise building has greater hindrance and stronger influence on wind environment. One should also avoid excessively large-size building, but to properly increase development intensity, raise construction height and reduce land cover area, and control the distance between high-rises.

3. NUMERICAL SIMULATION ANALYSIS OF MORPHOLOGY OPTIMIZING

3.1. Setup of simulation conditions

In order to evaluate the effect of the optimization strategies, simulation analysis was carried out the wind environment before and after optimization using PHOENICS 2012. Since the complex urban environment existed around the simulation domain, a five-day continuous site observation was carried out on the thermal environment in the study area before simulation, and the measurement results were used as the reference for setting the boundary condition (Table 1).

Meanwhile, grid irrelevance should also be verified. In theory, the more intense the grid is, the more accurate calculating result can be. However, with increasingly dense grid, round-off error caused by floating point arithmetic

of the computer also magnifies. Therefore, by changing grid density, the change of calculation results was observed to ensure proper scope.

By setting up grid spacing as 5 m, 10 m and 20 m in the X and Y axis of central area to make independence verification, average wind velocities at the height of 1.5 m within the area were respectively 0.79 m-s-1, 0.79 m-s-1 and 0.81 m-s-1, distribution tendency of wind velocity vectors was basically consistent. Therefore, 10 m grid spacing between the grids at the central area was selected to calculate and analyse.

3.2. Correlation analysis of unary linear regression

Average wind speed ratio means the ratio between the actual wind speed of a certain point or area and the inflow speed on the pedestrian height (on 1.5 m height). The study area is divided into 48 pieces of 200 x200 m square grid (Fig. 2), the building density and floor area ratio is calculated for each grid. The relevance between numerical results and urban morphology parameters with building density and floor area ratio as respective independent variables, average wind speed ratio as dependent variables are analyzed. The unary linear regression model between both of them is established.

Table 1. Boundary condition of PHOENICS 2012

Boundary condition

Setting

Computational domain

Central meshing Turbulence model

Incoming flow speed Incoming flow direction Calculation rule Convergence condition

Triple nested, 4800 x3600 x600 m in marginal area, 1800 x1400 x300 m in transition area, 1600 x1200 x10 m in central area

Grid interval of 10 m in X-Y axis, 2 m in Z axis

Modified k-e turbulence model by Kim and Chen [6]

1.8 m-s-1 on the height of 10 m South

SIMPLEST model

The maximum permissible residual error is 10-4

Fig. 2. Urban morphology parameters of the study area (a) building density; (b) floor area ratio.

BuiUing Density Floor Area Ratio

Fig. 3. Regression analysis of wind speed ratio and the urban morphologies (a) building density; (b) floor area ratio

Results show (Fig. 3) wind speed ratio as one of evaluation indices of microclimate has negative relevancy both with building density and floor area ratio, and keeps highest relevancy with building density, which R2 is up to 0.411. Therefore, when improving urban wind environment of Dalian city, building density should be firstly taken into consideration.

3.3. Optimizing plan and verification of grid independence

On the premise of ensuring area development intensity (floor area ratio) invariable, combined with conclusion of above-mentioned regression analysis, a building morphology proposal is presented for the study area (Fig. 4). The building density of the study area is lowered while the floor area ratio maintains the same.

Fig. 4. Urban form contrast before and after optimization (a) and (c) before optimization, the yellow part is the main measured area; (b) and (d)

after optimization, the red part is the adjusted building.

Fig. 5. Wind velocity distribution contrast (a) before optimization; (b) after optimization.

3.4. Simulation result and evaluation

From wind field results (Fig. 5a), it could be intuitively found that urban ventilation is greatly affected by building layout form. For example, ventilation effect with degenerative phenomenon occurred at the intensive place of building layout; for building layout with same intensity, building with larger frontal area, wind velocity weakening is more obvious; compared high-rise building to building with large podium, resistance to wind velocity by building with large podium is more obvious. It could also be found that internal wind velocity distribution within the whole Renmin Road street canyon is smaller, hardly meeting people's requirements of comfortable average wind velocity. In addition, there is apparent vortex occurring at the leeward side of high-rise buildings and the space between the buildings with semi-enclosed layout. Because of sheltering role of the surrounding buildings against the wind, it causes airflow velocity apparently lower down and undesirable circulates.

After simple adjustment for layout mode of large-size building and building unfavorable to ventilation, like proper increasing of building height, reducing low-rise and large-size podium, using streamline building shape, decreasing frontal area of building, hindrance factors of the wind largely are reduced (Fig. 5b). According to comparative results, it could be found that when maintaining a similar building density, air ventilation velocity at the level of pedestrian would be apparently increased and urban heat island effect alleviated if valid planning and measures like open space, ventilation channel, and reasonable layout of building distribution, etc. are effectively utilized.

4. CONCLUSIONS

High-density urban wind environment, on one hand, is strongly influenced by the surrounding complex topography, and the relationship of land and water spatial location, and on the other hand, the intensive internal layout of the buildings makes unique characteristics of wind environment. By giving full considerations to the wind, water, greenery and urban morphology and other factors, the suggestions on the planning of urban wind environment are summarized as follows:

(1) The urban ventilation path, scattered morphology and green space system have remarkable effect on promoting ventilation and alleviating the urban heat island effect.

(2) The wall effect formed by high-rise buildings with large podium is very unfavorable to the ventilation. It is required to strictly control the bulk and appropriately increase the height.

(3) Strip apartments in rows and enclosed city blocks are extremely unfavorable to the ventilation, which should be avoided.

This wind environment simulation and morphology optimization work is still in its infancy. It is necessary to formulate the needs using professional wind and thermal environment assessment tool with the urban planning program. Overall, it is required to integrate the numerous professional and technical information including urban planning, geography, climate, and promote comprehensive cooperation in the government management, regulations, policies and the public participation, to achieve satisfactory effect.

Acknowledgements

This article is sponsored by National Natural Science Foundation of China (Grant NO. 51308087, 51278078). The article is also sponsored by the Dalian High Level Talent Innovation Project (Grant NO. 2016RQ016), and the Fundamental Research Funds for the Central Universities (Grant NO. DUT17RW204).

References

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[2] W. Ding, Y. Hu, and P. Dou. Study on Interrelationship between Urban Pattern and Urban Microclimate. Architectural Journal (2012): 16-21

[3] Dalian Municipal Bureau of Statistics, National Bureau of Statistics survey corps of Dalian, Dalian Statistical Yearbook 2016, China Statistic

Press, 2016

[4] L.P. Hong, Z. Yu, K. Li. Urban ventilation channel planning analysis in hot summer and cold winter region- Take Wuhan Sixin area urban

design as an example. Chinese Garden, 02(2011):39-43.

[5] H. Tong, H.Z. Liu, Y.M. Li, et al. Actuality of summer urban heat island and the impact of urban planning "wedge-shaped Greenland" to reducing the intensity of urban heat island in Beijing. Journal of Applied Meteorological Science, 03(2005):357-366.

[6] S.W. Kim, and C.P. Chen. A multiple-time-scale turbulence model based on variable partitioning of the turbulent kinetic energy spectrum. Numerical Heat Transfer Part B Fundamentals An International Journal of Computation & Methodology 16.2(1989):193-211.