Numerical Simulation of Effect of Urban Geometry Layouts of Wind and Natural Ventilation Under Mediterranean Climate

Abstract

The use of the method "simulation" of the microclimate for an urban site presents much of interest; because this can serve as us observation and analysis of the consequences of various scenarios relating to the existence and the importance of the constituent elements in urban space. Wind in outdoor urban space is among the most difficult parameters to identify and control field given its instability. Currently, in the field of the ventilation, there are some outdoor spaces simulation tools, used to assess the flow of the wind at different spatial scales. The aim of this research is to demonstrate the effect of the urban geometry of the layout on the wind movement and the outdoor natural ventilation. However, this study investigated the effect on outdoor thermal comfort of a building layouts in a planned residential area situated in the city of Jijel humid Mediterranean region of Algeria. In order to improve outside comfort in this open space, a 3D numerical simulation tool ENVI-met 3.1 beta 4 was used to simulate the urban thermal climate taking into account various scenarios. Thus, simulation's results are discussed in this paper.

Full text

NUMERICAL SIMULATION OF EFFECT OF URBAN GEOMETRY
LAYOUTS ON WIND AND NATURAL VENTILATION UNDER
MEDITERRANEAN CLIMATE
BOUCHAHM YASMINA1, FATIHA BOURBIA2 AND BOUKETTA
SAMIRA3
1 Laboratory bioclimatic architecture and environment A.B.E,
Institute of architecture and urbanism, University of Constantine,
Algeria
Email address: ybouchahm2@gmail.com
2 Department of architecture, University of Jijel, Algeria
Email address:sb.archit@yahoo.fr
3 Laboratoire ABE, université Mentouri constantine, Algeria
Email: sb.archit@yahoo.fr
Abstract: The use of the method "simulation" of the microclimate for
an urban site presents much of interest; because this can serve as us
observation and analysis of the consequences of various scenarios
relating to the existence and the importance of the constituent elements
in urban space. Wind in outdoor urban space is among the most
difficult parameters to identify and control field given its instability.
Currently, in the field of the ventilation, there are some outdoor spaces
simulation tools, used to assess the flow of the wind at different spatial
scales. The aim of this research is to demonstrate the effect of the
urban geometry of the layout on the wind movement and the outdoor
natural ventilation. However, this study investigated the effect on
outdoor thermal comfort of a building layouts in a planned residential
area situated in the city of Jijel humid Mediterranean region of
Algeria. In order to improve outside comfort in this open space, a 3D
numerical simulation tool ENVI-met 3.1 beta 4 was used to simulate
the urban thermal climate taking into account various scenarios. Thus,
simulation‘s results are discussed in this paper.
1. Introduction
Wind is the phenomenon of great complexity. It is the clearest manifestation
of the atmospheric circulation. Therefore, know and understand the wind is
an important issue. However, various experimental and theoretical analyses
have been performed in order to study wind flow. This leads us to make a
state of the art for presenting the variables and their interactions in different
facets. The research review was conducted to evaluate the effect of the

NUMERICAL SIMULATION OF EFFECT OF URBAN GEOMETRY LAYOUTS
ON WIND AND NATURAL VENTILATION UNDER MEDITERRANEAN
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wind, taking into account several parameters, namely the orientation, shape
of buildings, their spatial arrangement, the angle of incidence of the wind,
vegetation.
(Zhang A. et al.,2005) studied by simulating, the effect of the
arrangement of buildings on the wind flow for different configurations in
southern China. By comparing different building schemes, it is found that
the wind field depends strongly on the building layout and the wind
direction. The influence of the arrangement of buildings on increasing the
speed of the wind and its effect on pedestrian comfort has been examined by
(Katarzym K.et al., 2004) in Warsaw region. A numerical study of the wind
speed conditions in passages between parallel buildings has been conducted
for a wide range of passage widths with CFD code Fluent 6.1.22 by (Bocken
B. et al.,2007). The results indicate that, at least for the cases studied, the
increase of wind speed in passages is only pronounced at the pedestrian
level and that the flow rate through the passage is at most only 8% higher
than the free-field flow rate, indicating that the so-called Venturi-effect is
rather weak. (Gomes et al., 2006) examines the effects of wind on and
around buildings with irregular shapes, in particular L-form and U, by using
experimental and numerical data. It was found that the pressure distribution
can change substantially with the shape of the building and the angle of
incidence of wind. A study that concerning the effect of a group of building
on wind flow and pedestrian comfort in outdoor spaces was conducted by
(Mazouz et al., 2008) in arid and semi arid areas. A numerical evaluation of
the wind in hot and humid climates has been realized by (Bonneaud and
al.,2001), examining how urban morphology and urban geometry influence
ventilation. In order to determinate the optimal design of complex buildings,
for better thermal and aerodynamic comfort in China, (Chen Q., 2007)
proposes a numerical simulation with CFD.
Different studies results‘ showed that natural ventilation in a humid
urban city depends on good organization of the layout to create air
movement, open spaces, permeable to wind during the warm season, and
also to protect it from strong winds in winter.
The aim of this research is to demonstrate the effect of the urban geometry of
the layout on the wind movement and the outdoor natural ventilation, under
Mediterranean climate.
2. Case study
2.1. PRESENTATION OF JIJEL
Jijel, a coastal city, located in the north-east of Algeria, between the
meridians 5° and 6° 25 East Greenwich, and between 10 and 36° 50, North
Hemisphere (Figure 1). This region is characterized by Mediterranean
maritime climate. It is considered one of the most humid and rainy town of

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BOUCHAHM YASMINA, FATIHA BOURBIA AND BOUKETTA SAMIRA
Algeria. It belongs to the Mediterranean climate, rainy and mild in winter,
hot and humid in summer.
Figure 1.1: Location of Jijel and limits
The maximum wind speed is between 3.20 m/s in the month of
December and 17.8 m/s in March. For the summer season, the maximum
wind speed varies between 14.9 m/s in the month of August and 18.2 m/s in
September. According to the compass rose made by the specialized services
of the meteorological station of Jijel (Figure 2.2) during the period of
January 1rst 1999 to December 31th, 2008, we can deduce:
• The prevailing winds of winter are direction N to O.
• The frequency of wind speeds is divided by classes as follows:
- 1 to 3 m / s: 11.1 %.
- 3-6 m / s: 30.5 %.
- 6 to 10 m / s: 11.6 %.
- 10 to 16 m/s: 1.4 %. To the north class 3-6 m/s the frequency is 4.76 %.
• The frequency of calm winds is 45.45 %.
• The frequency of strong winds is 1.4 %.
Figure 1.2: The annual wind rose 1999-2008
recorded at the meteorological
station of Jijel
2.2. CASE STUDY:
This study investigated the effect on outdoor thermal comfort of a building
layout in a planned residential area (figure 2.2) situated in the city of Jijel,
humid Mediterranean region of Algeria (table 2.1).
TABLE 2.1. features of case study

NUMERICAL SIMULATION OF EFFECT OF URBAN GEOMETRY LAYOUTS
ON WIND AND NATURAL VENTILATION UNDER MEDITERRANEAN
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Figure 2.1: Location of the case study
Figure 2.2: Layout of case study; plan, sections and views
3. Methodology and numerical simulation:
3.1. CHOICE OF SIMULATION SOFTWARE ENVI-MET
ENVI-met is a free software developed as a research project in 1998 by Dr.
Michael Bruse from the Institute of Geography at the University of Bochum
in Germany. It is a three-dimensional non-hydrostatic model used for a
dynamic time, and is designed including analysis by simulation the
interactions between the elements of design urban (buildings, surfaces and
plants) and microclimates on a small scale that which concerns the
Case study Localisation
Features
City center
(colonial core )
Altitude = 16m
(Figure2.1)
-Ratio H/W= 0.44 ( clear form)
- Walkways holes in buildings
- Central area cleared.
- Presence of dense vegetation.
- Near the sea (700m )
Case
study

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BOUCHAHM YASMINA, FATIHA BOURBIA AND BOUKETTA SAMIRA
fragments of a city. The function of the software is done by solving the
basic equations for the physical power of wind, thermodynamics and
equilibrium radiation surfaces. This freeware whose name is derived from
the term "Environmental Meteorology" for various application areas:
Geography and Spatial Dynamics, Economics and Management
Urban, Urban Morphology and Urban Physics (Figure 3.1).
Figure 3.1: structure of the software
3.2. NUMERICAL SIMULATION
This simulation was performed using the software ENVI -met 3.1 Beta 4 in
order to improve the conditions of the external ventilation of case study -
which was based on the investigation - the most efficient (figure 3.1). It
covers both winter and summer periods chosen for the investigation. The
physical parameters studied are the air temperature, relative humidity and
wind speed. In this presentation we will show last parameter results. Three
(03) scenarios were simulated taking into account arrangement of buildings
on the layout
Figure 3.2: Layout of case study

NUMERICAL SIMULATION OF EFFECT OF URBAN GEOMETRY LAYOUTS
ON WIND AND NATURAL VENTILATION UNDER MEDITERRANEAN
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Figure 3.2: proposed scenario
4. Results and discussion
Using the freeware ENVI-met, we were able to assess the effect of geometry
layout on wind and natural ventilation. It retained that the arrangement of
buildings on ground plane has an influence on wind flow. It is said our
assumptions, where the geometry of buildings and their provisions on the
ground plane affects the wind flow in humid climates (Figure 4.1). In this
aspect, our results are consistent with several studies such as that of Zhang.
L, al.,(2005), Katarzym. K and al.(2005), Blocken B., and al. (2004), which
showed by simulation that the wind field is highly dependent on the
arrangement of buildings on the ground plane.
In addition, the influence of the arrangement of buildings on the increase
in wind speed and comfort on the outside has been demonstrated in their
research (Figure 4.1). Also, it can be concluded, according to the specific
context of the study, a ground plane exploded like this case can be generated
for a location on a low altitude in a humid climate, introducing changes vis-
à-vis external works to improve the microclimate.

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BOUCHAHM YASMINA, FATIHA BOURBIA AND BOUKETTA SAMIRA
Figure 4.1: Graphic wind results
Figure 4.2: Data wind results

NUMERICAL SIMULATION OF EFFECT OF URBAN GEOMETRY LAYOUTS
ON WIND AND NATURAL VENTILATION UNDER MEDITERRANEAN
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Acknowledgements
We conducted several simulation tests, and during this experience, we identified
some constraints to the model Envi-met, which could limit our work, namely:
The software considers a single wind direction for the duration of the simulation,
when in fact; the wind direction is constantly changing, even if there is a dominant
direction especially in coastal areas. The simulation speed at a location becomes
difficult when the wind is strongly influenced by local climate: a coastal area, for
example, is strongly influenced by sea breezes that result in significant changes in
speed and direction. It does not take into account the effect of altitude; our case
studies have different altitudes. Jijel‘s coastal humidity is greatly influenced by the
proximity of the sea; this factor of "local weather" is not at all taken into account by
envi -met.
References
ALI-TOUDERT FAZIA, 2005, Dependence of outdoor thermal comfort on street design in
hot and dry climate, Thèse de doctorat, Université de Freiburg, Freiburg.
BLOCKEN B., CARMELIET J., STATHOPOULOS T., 2007, CFD evaluation of wind speed
conditions in passages between parallel buildings — effet of wall – function roughness
modifications for the atmospheric boundary layer flow In: Journal of Wind Engineering
and Industrial Aerodynamics, , Vol 95, 941–962.
BLOCKEN B., CARMELIET J., 2004, Pedestrian wind environment around buildings:
Literature review and practical examples In: Journal of Thermal Envelope and Building
Science, , Vol. 28, N°2, 107-159
BONNEAUD F., MUSY M., DEPECKER P., 2001, In: Simuation Building, Seventh
international IBPSA Conference, Rio De Janeiro, Brazil, August 13-15, Simulation of the
wind in hot and humide climates cities: evaluation of the natural ventilation potential of
the housing in urban blocks.
BRUSE MICHAEL, HERIBERT FLEER, 1998, Simulating of surface–plant–air interactions
inside urban environments with a three dimensional numerical model, University of
Bochum.
CHEN Q., 2007, Sustainable urban housing in China, Edité par L. R. Glicksman and J. Lin,
Springer.
GIVONI BARUCH, 1998, Climate considerations in building and urban design, edition
John Wiley and sons, Inc, New York.
GOMES M. G., RODRIGUES A. M., MENDES P., 2006, Wind effects on and around L- and
U-shaped buildings, University of Lisbon, Lisbon, Portugal.
GOUIZI YAMINA, MAZOUZ SAÏD, 2008, In: The third architecture and sustainability
conference in Biskra (BASC 2008) : Strategies and perspectives, Effet d‘un groupe de
bâtiments sur l‘écoulement de l‘air (le vent) et le confort des piétons dans les espaces
extérieurs. Cas d‘étude : ensemble de bâtiments collectifs des Z.H.U.N à Biskra, 261-
271.
KATARZYM K., MAREK J., 2004, In: PLEA 2004, Wind speed at pedestrian level in a
residential building complex, The 21th Conference on Passive and Low Energy
Architecture, Eindhoven, The Netherlands.
ZHANG L., GAO C., ZHANG A., 2005, Numerical simulation of the wind field around
different buildings arrangements In: Journal of Wind Engineering and Industrial
Aerodynamics, Vol. 93, 891–904.