Energy modeling and air flow simulation of an ancient wind catcher in Yazd

Abstract

Wind catchers (wind towers) are one component of the traditional sustainable buildings with a central courtyard in the hot regions of Iran. In this research a most common type of the wind tower – the four sided wind catcher-was evaluated both experimentally and numerically. The tower of the Mortaz house in the city center of Yazd was equipped with temperature, wind, air velocity and solar sensors. Monitoring took place over a three months winter period. A series of 3D steady CFD simulations was carried out using OpenFOAM. The airflow pattern through the wind catcher model was simulated to predict and monitor the air flow behavior in the four equipped shafts for prevailing wind directions. Finally the validation of the CFD numerical simulation results with the onsite measurements data is discussed. The goal of this research is developing a method of informing the design decision for new wind catchers and the renovation of traditional wind catchers in early design stage using CFD simulation tools and energy modeling in sustainable buildings.

Full text

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
Energy modeling and air flow simulation of an
ancient wind catcher in Yazd
Zhaleh.HEDAYAT 1 , Nima.SAMKHANIANI 2, Bert.BELMANS1
M.Hossein.AYATOLLAHI 3, Ine.WOUTERS 1, Filip.DESCAMPS 1
1. Dept. of Architectural Engineering , Vrije Universiteit Brussel, Belgium , Zhaleh.Hedayat@vub.ac.be ,
bbelmans@vub.ac.be , Ine.Wouters@vub.ac.be ,Filip.Descamps@vub.ac.be
2. Dept. of Mechanical Engineering, Tarbiat modares University, Iran , Nima.samkhaniani@modares.ac.ir
3. Dept. of Architecture,Yazd University, Iran , hayatollahi@yazd.ac.ir
Abstract
Wind catchers (wind towers) are one component of the traditional sustainable buildings with a central
courtyard in the hot regions of Iran. In this research a most common type of the wind tower – the four
sided wind catcher- was evaluated both experimentally and numerically. The tower of the Mortaz
house in the city center of Yazd was equipped with temperature, wind, air velocity and solar sensors.
Monitoring took place over a three months winter period. A series of 3D steady CFD simulations was
carried out using OpenFOAM. The airflow pattern through the wind catcher model was simulated to
predict and monitor the air flow behavior in the four equipped shafts for prevailing wind directions.
Finally the validation of the CFD numerical simulation results with the onsite measurements data is
discussed. The goal of this research is developing a method of informing the design decision for new
wind catchers and the renovation of traditional wind catchers in early design stage using CFD
simulation tools and energy modeling in sustainable buildings.
Key words: Traditional sustainable buildings, Wind catcher, CFD simulation, Energy modeling,
Experimental measurements
1. Introduction
Wind catchers as natural ventilation components were widely used in Iranian traditional
housing to provide acceptable thermal comfort in central courtyard houses. The major
advantage of wind catchers is that they work with the renewable energy of the wind, requiring
no other energy sources for operation. These sustainable ventilation systems have become an
increasingly attractive method for reducing energy consumption and energy costs in building
sector.Many researchers have been studied the functional behavior of the wind catchers in
Iran. Bahadori [1,4] , performed full analysis of the design of wind catchers in several
locations such as Yazd city in summer time and presented two new designs of wind
catchers.Saffari et al. [9] studied a numerical simulation of evaporative cooling in a wind
tower. They used the partitions as surfaces that inject water droplet in very low speed normal
to the surface for simulating evaporative cooling. The interaction between the airflow – as
continues phase – and water droplets – as discrete phase – was described. Several parameters
such as the diameter of the droplet, the injection rate per square meter of the surface, and the
speed of the injection were separately studied. The results were compared with some analytic

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
data and showed a good agreement with them. In addition it was suggested that using
evaporative cooling has a grate increase in wind catchers performance. Montazeri et al. [6-8]
also investigated the effects of the numbers of openings by modeling a circular cross section
wind tower that has several openings at equal angels. The results show that the number of
openings is a main factor in the performance of wind tower systems. It also shows that the
sensitivity of the performance of different wind catchers to the wind angle decreases by
increasing the number of openings. Moreover, when it is compared with a circular wind
tower,a rectangular system provides a higher efficiency.
The present work studied on a full scale four- sided wind catcher to model the air flow pattern
through the tower in the four tower shafts at prevailing wind direction and is validated by
comparison with the onsite measurements data. The air velocity values were calculated and
analysis in the tower shafts to find the hydrodynamic performance of the wind – induced
natural ventilation system . [6-8]
2. EXPERIMENTAL ONSITE MEASUREMENTS
The experimental house - Mortaz house- at the city center of Yazd [ latitude 31°53’50”N ] in
Iran was equipped with temperature, wind, air velocity and solar sensors. Monitoring took
place over a three month winter period [14]. Fig.1 shows the plan of the experimental house
and the wind tower including four equipped shafts . A Wireless Ultrasonic Wind Sensor with
USB interface consisting of a receiver and a cable with a USB plug was installed on a mast
head as a local weather station (fig.2) .The local weather station is installed on the roof of the
Mortaz house which is located around 20 meter far away from the wind tower and is free to
the wind (fig. 3).
Fig1.Plan of the experimental house and the wind tower including four equipped shafts
To study the airflow pattern through the tower of Mortaz house the data from the wind sensor
and air velocity sensors on the warmest day of the measurement period (on 03Nov. 2014
between 14:00 PM -16:00 PM ) are selected to analyze [14]. Fig.4 shows a view of the air
velocity sensor inside shaft B. The data obtained from the wind sensor with the wind speed
range of 0.2-40 m/s , wind direction range of 0-359 ° and the resolution of one degree .Based
on the wind data collection and analysis of the daily wind diagram (fig.5) the prevailing wind
is seen as blowing from the North direction. The mean wind velocity of 1.5 m/s is recorded by
the wind sensor at the height of 11 m during the warmest hours of the day on 03 Nov.2014 .

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
Fig2. Local weather station Fig 3.location of air velocity sensors and local weather station
A1:Air velocity sensor in shaft B
A2: Air velocity sensor in shaft D
Fig 5.Wind rose diagram
Fig.6 shows the orientation of the experimental house in relation to the prevailing wind.Four
air velocity sensors (A4 – A1 – A2 – A3 ) with the working range of 0- 20 m/s was also
installed in the middle of four tower shafts (shaft A – B – D – E) in height of 2.00 m above
the outlet of the tower (5.35 m from the ground level) respectively .
Fig 6.Orientation of the Experimental house
Fig4.Air velocity sensor in shaft B

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
NNUMERICAL SIMULATIO 3.
According to the range of wind velocity, the flow can be considered incompressible and the
temperature variations are considered constant in this simulation. The 3D steady RANS
equations were solved in combination with the realizable
kε
−
turbulence model. The
governing equations are:
(1)
.( ) 0
U
∇=

(2)
() .( ) .(( ) )
t
UUU U p
tµµ
∂+ ∇ − ∇ + ∇ = −∇
∂
   
where
t
µ
is turbulent viscosity. It is determined from kinetic energy (
k
) and dissipation
energy (
ε
) based on Boussinesq eddy viscosity assumptions [15].
The Open source Field Operation and Manipulation (Open FOAM) C++ libraries are used for
numerical simulation [13].It is supplied with numerous pre-configured solvers, utilities, and
libraries. It is open, not only in terms of source code, but also in its structure and hierarchical
design, so that its solvers, utilities and libraries are fully extensible. A full scale model (50 ×
25 × 7 m3) of the experimental house with wind-driven natural ventilation tower (6.05 × 3.45
× 14 m3) was built and used in simpleFoam solver in OpenFOAM 2.3.0 for simulation. The
dimensions of the computational domain were chosen based on the CFD guidelines by Franke
et al. [10] and Tominaga et al. [11] .The resulting dimensions of the domain were 500 × 500 ×
80 m3. The computational grid was created using snappyHexMesh and blockMeshDict as
mesh generation tools in OpenFOAM. Hexahedral cells are dominant in the present grid. The
surface mesh view has shown in Fig.7. For wind direction
0
o
θ
=
, the inlet boundary conditions
(mean velocity U, turbulent kinetic energy k and turbulence dissipation rate) were initialized
based on the measured mean wind speed of 1.5 m/s from the local weather station. The
turbulent intensity set to 0.02. For the ground surface, the standard wall functions were used.
The simulation results are used to validate against the onsite measurement results of air
velocity fields. The governing equations are discretized with the finite volume method.
Second order upwind discretization schemes were imposed on all the transport equations
solved and the SIMPLE is adopted as pressure-velocity solution algorithm.
Fig 7.Surface mesh view generated using snappyHexMesh

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
4. Discussion and results
1-4- Experimental Results
Fig.8 shows the average air velocity variations in four equipped shafts of A, B, D and E on 03
Nov.2014.The graphs follow the sinusoidal pattern which occurs in nature phenomenon (like
breathing system). The indoor air velocity fluctuates between 0.25 m/s and 2.8 m/s . Fig.8
also reveals that the indoor average air velocity is low between midnight and early morning
and increases in the afternoon. The maximum average air velocity of 2.8 m/s is recorded in
shaft A with the maximum fluctuation in the afternoon. As mentioned before, the difference
between the CFD results and experimental data in shaft A can be explained due to the
fluctuations in this shaft in the afternoon. The high air velocity is caused by lighter air density
due to higher air temperature in the afternoon.
Fig8.Average air velocity in four equipped shafts
2-4- Numerical Results
Fig.9 shows the streamlines of the turbulent airflow through and around the tower for an inlet
velocity of 1.5 m/s at zero direction. The velocity field in four equipped shafts presented in
Fig.10 .The results show that the air flow direction in shaft A and B ( inlet shafts ) is
downward with the negative values while the velocity fields is upward in outlet shafts of E
and D.

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
Fig9. CFD simulation of the air flow around and inside the wind tower
Fig10.Velocity field in four tower shafts
3-4-Validation of CFD simulations against experimental data analysis
Fig.11 shows the comparison between the empirical mean air velocity and CFD results.The
root mean square deviation (RMSD) between empirical data and CFD simulation results is
10.8 %. The tabulation of the RMSD is shown in Table1.The RMSD reveals that CFD

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
simulation has a good agreement with the empirical results in three tower shafts B , D and E
with the value of 3.9%. It can be explain by the hydrodynamic behavior of shaft A which
explained in Experimental results section .Table 1 shows that the average air velocity in four
equipped shafts is 0.55 m/s for the CFD results while it is 0.50 m/s for the experimental
analysis.
Sensor Location
CFD
Experimental
data
Absolute
deviation X (%)
X2 (%)
A1 - Shaft B
0.37
0.37
0.0
0.0
A2 - Shaft D
0.70
0.75
6.6
43.5
A3 - Shaft E
0.50
0.51
1.9
3.6
A4 - Shaft A
0.46
0.58
20.6
424
Root mean square deviation
10.8
Table 1.The root mean square deviation between experimental data and CFD simulation
Fig11. Comparison between the experimental mean air velocity and
OpenFOAM CFD simulation

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
5. CONCLUSION
Energy modeling and CFD simulation is an important and interesting method to study and
research on performance analysis of wind catchers as sustainable architecture components ,
providing valuable information in a much shorter time than when carrying out onsite
measurements .The present work has shown the possibility of simulating a real case with the
wind-driven natural ventilation tower in Yazd. The simulated values of air velocity has shown
the functional behavior of air flow in four shafts of the wind catcher .Although the
comparison between the under controlled conditions for CFD simulation and un- controlled
onsite measurements should be considered. The validation results are more accurate since
appropriated information is given as boundary conditions, simplification hypothesis and
geometric mesh .This paper aims to investigate the use of building performance simulation
tools as a method of informing the design decision of sustainable buildings.
References
[1] Bahadori M.N, Mazidi M, Dehghani A.R. Experimental investigation of new design of wind
towers, Renewable Energy; 2273–2281, 2008 .
[2] Blocken B, Stathopoulos T, Saathoff P, Wang X. Numerical evaluation of pollutant dispersion in
the built environment: Comparisons between models and experiments. J Wind Eng Ind
Aerodyn;96:1817, 2008 .
[3] Kalantar V. Numerical simulation of cooling performance of wind tower (Baud-Geer) in hot and
arid region. Renewable Energy;34:246e54 , 2009.
[4] Karakatsanis C, Bahadori MN, Vickery BJ .Evaluation of pressure coefficients and estimation of
air flow rates in buildings employing wind towers. Solar Energy;37(5):363e74,1986.
[5] Saadatian O, Haw LC, Sopian K, Sulaiman MY. .Review of windcatcher technologies. Renewable
and Sustainable Energy Reviews ; 16: 1477–1495, 2012 .
[6] Montazeri H.Experimental and numerical study on natural ventilation performance of various
multi-opening wind catchers, Building and Environment;370–378, 2011.
[7] Montazeri H , Montazeri F, Azizian R, Mostafavi S , 2010. Two-sided wind catcher performance
evaluation using experimental, numerical and analytical modeling, Renewable Energy; 1424–1435.
[8] Montazeri H , Azizian R , 2008 . Experimental study on natural ventilation performance of one-
sided wind catcher, Building and Environment ;2193–2202.
[9] Saffari H, Hosseinnia S.M , 2009 . Two-phase Euler-Lagrange CFD simulation of evaporative
cooling in a wind tower, Energy and Buildings; 991–1000.
[10] Franke J, Hellsten A, Schlünzen H, Carissimo B. 2007 .Best practice guideline for the CFD
simulation of flows in the urban environment. Brussels: COST Office.
[11] Tominaga Y, Mochida A, Yoshie R, Kataoka H, Nozu T, Yoshikawa M, et al. AIJ guidelines for
practical applications of CFD to pedestrian wind environment around buildings. J Wind Eng Ind
Aerodyn;96:1749-61, 2008.

3.th International Congress on Civil Engineering , Architecture and Urban Development
29-31 December 2015, Shahid Beheshti University , Tehran , Iran
[12] van Hooff T, Blocken B, Aanen L, Bronsema B. A venturi-shaped roof for wind-induced natural
ventilation of buildings: Wind tunnel and CFD evaluation of different design configurations. Build
Environ;46:1797-807, 2011.
[13] OpenFOAM project web site, URL: http://www.OpenFOAM.org, OpenCFD Ltd.
[14] Hedayat ZH et al., IBPC 2015 (6th ) .Italy : Performance assessment of ancient wind catchers - an
experimental and analytical study; 2015
[15] Orszag, Steven A., et al. "Renormalization group modeling and turbulence simulations." Near-
wall turbulent flows: 1031-1046, 1993.