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QUALITATIVE STUDY OF AIRFLOW STRUCTURE ACROSS
WOODEN LOUVRED WINDOW PANELS FOR NATURAL
VENTILATION APPLICATIONS
Nur Baitul Izati Rasli
PhD degree in the elds of Indoor Air Quality and Thermal Comfort from Universiti Sains Malaysia.
Research Assistant, School of Civil Engineering, Universiti Sains Malaysia. Nibong Tebal, (Malaysia).
E-mail: nurbaitulizati@gmail.com ORCID: https://orcid.org/0000-0001-5454-8539
Nor Azam Ramli
PhD degree in Environmental Engineering from University of Wales, United Kingdom.
Professor, School of Civil Engineering, Universiti Sains Malaysia. Nibong Tebal, (Malaysia).
E-mail: ceazam@usm.my ORCID: https://orcid.org/0000-0001-6328-0183
Mohd Rodzi Ismail
PhD degree in Building Engineering from University of Liverpool, United Kingdom.
Associate Professor, School of Housing, Building and Planning, Universiti Sains Malaysia. Minden, (Malaysia).
E-mail: rodzi@usm.my ORCID: https://orcid.org/0000-0002-1020-5398
Noorfazreena Mohammad Kamaruddin
PhD degree in Aerospace Engineering from University of Manchester, United Kingdom.
Senior Lecturer, School of Aerospace Engineering. Nibong Tebal, (Malaysia).
E-mail: fazreena@usm.my ORCID: https://orcid.org/0000-0002-5897-8728
Recepción:
25/05/2021
Aceptación:
04/08/2021
Publicación:
14/09/2021
Citación sugerida:
Rasli, N. B. I., Ramli, N. A., Ismail, M. R., y Kamaruddin, N. M. (2021). Qualitative study of airow structure across
wooden louvred window panels for natural ventilation applications. 3C Tecnología. Glosas de innovación aplicadas a la pyme,
10(3), 73-99. https://doi.org/10.17993/3ctecno/2021.v10n3e39.73-99
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ABSTRACT
Window ventilation usually used in energy-ecient buildings as an eective natural ventilation system
to provide an adequate opening for fresh air to ow into the interior space. It could help to reduce
the indoor air temperature and limit the contaminants in buildings. This study aimed to visualise the
airow structure across a wooden louvred window panel using the smoke ow visualisation technique at
dierent airow speeds of 0.5, 1.0, 2.0, 3.0 and 5.0 m/s in a closed-loop wind tunnel. Two Sony 1920
× 1080i cameras captured the airow structure, which took the side view, rear view, front view and back
view images. The wooden louvred window panel promotes optimum outdoor airow and facilitates
continuous air exchange to replace the indoor air. Results showed that smoke lled the space quickly at
the highest airow speed of 5 m/s. This study used an inclination angle of 75° for the wooden louvred
window panel to avoid rain splatter. Besides, wood material could be used as a façade shading device.
Therefore, installing the wooden louvred window panels could enhance natural ventilation, ensure
indoor thermal comfort and reduce indoor air contaminants.
KEYWORDS
Passive Design, Natural Ventilation, Airow Structure, Indoor Air Quality, Thermal Comfort, Sustainable
Development.
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1. INTRODUCTION
Malaysia is a tropical country that experiences a hot and humid climate throughout the year. It is near
the equator, whereas approximately one-third of the world population experiences hot–dry or warm
and humid climates (Jamaludin et al., 2015; Laurini et al., 2017). High humidity and temperature levels
increase the risk of thermal discomfort and moisture problems in indoor settings (Hamimah et al., 2010).
A study by Jamaludin et al. (2015) found that the indoor air temperatures of a residential building in
Malaysia at dierent microclimates exceed the acceptable limit of thermal comfort suggested in the
Malaysian Standard (MS 1525:2007) (23 °C–26 °C), with the highest indoor temperature being 32.6 °C
under the Kuala Lumpur climate.
In response to the eects of high indoor thermal conditions, people install air-conditioning systems
in their indoor environment for cooling purposes (Jamaludin et al., 2015). The practice may increase
the energy demand and energy cost of residential and commercial buildings and thereby challenge
the sustainable cities' eort (Jamaludin et al., 2015; Kubota & Toe, 2015; Kassim et al., 2016; Cui et
al., 2017; Laurini et al., 2017). Modern technology recirculates the indoor air instead of refreshing it,
contributing to poor indoor air quality (Spiru & Simona, 2017). Moreover, air conditioning usage adds to
the imperfect removal of indoor air contaminants (Cui et al., 2017), including carbon monoxide, carbon
dioxide, formaldehyde and biological contaminants (Kaunelien et al., 2016; Li et al., 2017; Cheung &
Jim, 2019). Indoor air quality needs to be maintained within acceptable limits as it can aect human
health (Sun et al., 2015; de Robles & Kramer, 2017; Steinemann et al., 2017; Amoatey et al., 2018;
Krawczyk & Wadolowska, 2018).
As a solution to this problem, the wind from outside can be used as natural ventilation to eliminate the
need for air-conditioning systems. A natural ventilation system is a passive design strategy for buildings,
applied for cost-eective electricity consumption and fossil fuel usage (Allocca et al., 2003; Gratia & De
Herde, 2007; Ahmed & Wongpanyathaworn, 2012; Zhong et al. 2012; Aaki et al., 2014). The adoption
of such systems is aimed at achieving sustainable development goals and is ideal as they do not incur
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any energy cost (Kassim et al., 2016). Fresh air is fundamentally required in the interior spaces to provide
enough oxygen for breathing, reduce excessive heat, limit indoor air contaminants, decrease CO
2
concentrations and dilute and remove odours (Bayoumi, 2017).
In natural ventilation, fresh outside air is induced naturally by the temperature and pressure dierences
between spaces to replace the indoor air continuously through openings (vents, windows, doors and so
forth) (Bangalee et al., 2014). Natural ventilation can either be wind-driven natural ventilation (cross
ventilation) or buoyancy-driven natural ventilation (stack ventilation); the former attributed to pressure
dierences generated by the wind while the latter is caused by buoyancy forces (Mohammadmirzaei,
2018). Cross and stack ventilation may also co-occur, in which wind and stack eects could reinforce or
oppose one another (Allocca et al., 2003). Figure 1 illustrates the cross and stack ventilation in a building.
Cross ventilation supplies and extracts the air owing in the same building level that passes through
vertical openings, whereas stack ventilation concerns the upper and lower openings (Ohba & Lun, 2011).
(a) (b)
Figure 1. (a) Cross ventilation and (b) stack ventilation.
Source: (Ohba & Lun, 2011).
Besides, the opening location can enhance the eectiveness of natural ventilation through cross
ventilation. For single-sided walls, leeward walls are better opening locations than the windward walls
(Ma et al., 2017). Extensive recirculation is produced at the centre of the building with single-sided walls.
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The direction of airow in windward ventilation is anticlockwise as opposed to leeward ventilation,
and it is more robust due to the vortices behind the buildings. Kassim et al. (2016) reported that the
best opening location is at the upper part of the windward façade as it allows a large amount of air to
penetrate a building; however, the ventilation rate decreases as the opening position shifts towards the
lower part of the building façade.
The current study is conducted to develop and improve the previous research by Rasli et al. (2019) by
applying a wooden louvred window panel to enhance the outdoor air inow. In that study, Rasli et al.
(2019) visualised the airow structure of outdoor air that permeates through the window panels with
apertures using the smoke ow visualisation technique in a wind tunnel laboratory. The results suggested
that double apertures with a 2.4% opening on window panels could promote optimum outdoor airow
indoors relative to single apertures (1.2%) and no aperture (0%). Double apertures also provide security
by which windows be kept closed for 24 h.
Hence, this wind tunnel study's objective is to visualise the airow structure across the wooden louvred
window panel using the smoke generator technique at dierent airow speeds of 0.5, 1.0, 2.0, 3.0 and
5.0 m/s. The proposed wooden louvred window panel can also provide protection from the rain splatter
while allowing the air outside to come inside. Besides, wood material could be used as a façade shading
system, in which due to its hygroscopic properties, it could be advantageous for occupants as it could lead
to decreasing in energy cost, enhance energy eciency and improve indoor comfort in buildings (Vailati
et al., 2018; El-Dabaa et al., 2020). When wood is applied, it gives a passive motion technique stimulated
by the variation of relative humidity which can be ideal in a tropical climate.
1.1. LOUVRED WINDOWS
Generally, louvres are apertures with angled slots either horizontally or vertically that permit air and light
to pass through but at the same time block direct sunlight, rain and noise from penetrating the interior
areas. Accordingly, a louvred window consists of integrated angled slats within a frame to perform such
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functions. For this study, a louvred window with horizontal slats was used. The eectiveness of airow
permeating through a louvred window is controlled by the input, geometry and output variables, as
shown in Figure 2.
Geometry variables:
• Width of wall: width of window
• Height of the opening
• Size of inlet
• Size of outlet
• Louver blade angle
Input variables:
• Wind speed
• Wind direction
• Temperature
Output variables:
• Average air velocity
• Distribution of air at a reference plane
• Pressure differential at the opening
• Average air change per hour
Figure 2. Variables that affect the airow through a louvred window.
Source: (Chandrashekaran, 2010).
1.2. WIND TUNNEL
The experiment of smoke ow visualisation can be performed to visualise the airow structure in and
around the test model by using the illumination of smoke laments in the test section of a wind tunnel.
The use of wind tunnel experiment on the natural ventilation study was adopted in several works (Ohba
et al., 2001; Karava et al., 2007; Chu et al. 2009; Chu et al. 2010; Ji et al., 2011), while Elmualim (2006),
Montazeri & Azizian (2009), Chandrashekaran (2010) and Esfeh et al. (2012) have included the smoke
visualisation test in their natural ventilation studies using wind tunnels.
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The airow in a wind tunnel has three dierent characteristics depending on the condition: laminar ow
(i.e. streamline ow), transitional ow (i.e. between laminar and turbulent ow) and turbulent ow (i.e.
chaotic ow) (Chandrashekaran, 2010). In laminar ow, the air moves in parallel at low velocities caused
by the viscous air forces. In transitional ow, some of the air moves in parallel whilst some disperses.
In turbulent ow, air moves at high velocities, and pressure varies irregularly in time and position. The
Reynolds number (Re) characterises the airow characteristics, in which the airow may be laminar,
transitional or turbulent when the Re is below 2,000, between 2,000 and 4,000 and more than 4,000,
respectively (Figure 3).
Laminar
Re < 2000
Transitional
2000 < Re > 4000
Turbulent
Re > 4000
Figure 3. Characteristics of laminar, transitional and turbulent airow.
Source: (Chandrashekaran, 2010).
2. MATERIALS AND METHODS
The ventilation openings of louvred windows from vernacular architecture techniques could improve
cross-ventilation by increasing the indoor air pressure. They promote the outdoor airow towards
indoors to lower indoor air temperature and reduce indoor air contaminants. The variables used in this
study for the model testing are the airow speeds (i.e. 0.5, 1.0, 2.0, 3.0 and 5.0 m/s). The resulting airow
characteristics are subsequently analysed.
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2.1. DIMENSIONS OF WOODEN LOUVRED WINDOW PANEL MODEL
The louvred window used in this work was made of wood and had a smooth nish. The dimension of
the slotted-up wooden louvred window panel was 36 cm (width) × 57 cm (height), inserted at the centre
of a clear acrylic measuring 90 cm (width) × 60 cm (height), which served as the window base. A wooden
frame holder measuring 100 cm (width) × 80 cm (height) was used to hold the wooden louvred window
panel in a vertical position in the middle of the test section (Figure 4).
Figure 4. Conguration of the wooden louvred window panel.
Source: own elaboration.
2.2. THE ANGLE OF THE WOODEN LOUVRED WINDOW PANEL MODEL
Studies on the relationship between the louvre angles and natural ventilation performance were conducted
by various researchers (Yakubu & Sharples, 1991; Hughes & Ghani, 2010; Chandrashekaran, 2010; Lee
et al., 2016). This study focused on a wooden louvred window panel model with 22 slots inclined at 75°,
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in which the gap between each slot was 2 cm (Figure 5). The free area's opening between the slots had
a vital function as it inuenced the velocity and pressure dierences across the wooden louvred window.
Figure 5. Dimension of the wooden louvred window panel with the slot’s gap size and angle.
Source: own elaboration.
2.3. CLOSED-CIRCUIT WIND TUNNEL
The smoke ow visualisation testing was carried out at the Wind Tunnel Laboratory, Science and
Engineering Research Centre, Universiti Sains Malaysia. The closed-loop wind tunnel measuring
2,052.6 cm (length) × 818.8 cm (width) × 350.0 cm (height) produces an airstream for the study of the
eects of airow moving in and around the wooden louvred window panel model. The model was then
placed in the test section between the settling chamber and the wind tunnel diuser. The wind tunnel
had a rectangular test section measuring 1 m (width) × 0.80 m (height) × 1.80 m (length), a contraction
ratio of 10:1 and a turbulence level of 0.1% for the ow speed of up to 80 m/s. The axial fan, with the
aid of the diuser downstream in the test section, drove the airow to the test section. The airow speeds
were controlled using the wind tunnel’s control panel and were veried using a hot wire anemometer.
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2.4. SMOKE FLOW VISUALISATION TECHNIQUE
The testing for smoke ow visualisation was conducted using the smoke generator technique to evaluate
the airow structure of the wooden louvred window panel model at dierent airow speeds as illustrated
in Figure 6. The wooden louvred window panel was placed at the centre of the wind tunnel test section
while the smoke sources (smoke generator) were positioned vertically at 25 cm from the wooden window
panel's midsection. The optimal distance contributed signicantly to the best-illustrated streamlines of
the smoke, which represented the airow structure. The smoke generator was at the top of the wind
tunnel test section while the smoke rake was positioned inside the test section.
Air flow
direction
Front shot
camera
Side view
camera
Back shot
camera
Rear view
camera
Slotted window
panel with
wooden frame
Smoke
rake
Test section
walls covered
with black paper
Test section
floor
25 cm
5 cm
Figure 6. Illustration of wooden louvred window panel using the smoke generator technique at ve different airow speeds of 0.5,
1.0, 2.0, 3.0 and 5.0 m/s and different camera positions (i.e. side view, rear view, front view and back view) inside the wind tunnel
test section.
Source: own elaboration.
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In the smoke generator ow visualisation technique, a smoke generator (SAFEX Fog Generator FOG
2010 by Dantec Dynamics) produces smoke particles at a rate of approximately 600 m
3
/min. The smoke
intensity was controlled using a handheld remote control with a rotary knob scaled at number 5 for a
favourable amount (600 m
3
) of smoke. The setup of the smoke generator is shown in Figure 7. The
generated smoke particles were pumped through the smoke rake located at the inlet of the test section.
Meanwhile, a exible delivery tube was used to connect the smoke generator to the smoke rake.
Figure 7. Flow visualisation setup using the smoke generator technique on the top and inside the wind tunnel test section.
Source: own elaboration.
Two Sony 1920 × 1080i cameras were mounted outside at the centre (side view) and inside (rear view)
of the test section to record the airow structure. The testing was repeated with both cameras mounted
outside and positioned at the front and backside to capture these areas' ow structure. The test section’s
inner wall was installed with a black paper cover to minimise any reection that could aect the results
and to increase the clarity of the streamlines. Two white halogen light bulbs installed on top and at the
bottom of the test section illuminated the smoke ow structure released from the smoke rake.
3. RESULTS
The wooden louvred window panel introduced in this study was aimed at promoting the optimum ow
of fresh outdoor air into indoor spaces for passive ventilation and enhancing natural ventilation towards
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the development of sustainable buildings. Improved indoor air quality and thermal discomfort can be
achieved (i.e. decrease the space's temperature) by increasing the natural ventilation area (Noman et al.,
2016; Lei et al., 2017). Thus, the objective is to solve thermal comfort issues and improve air quality in
indoor spaces (Ocak et al., 2012; Hameed & Habeeballah 2013; Alananbeh 2017; Yu et al., 2017; Nahar
& Mahyudin 2018; Rasli et al., 2019; Azuma et al., 2020).
The side view, rear view, front view and back view images obtained in the wind tunnel testing are shown
in Figures 8–12. Specically, the gures show the airow structure across the wooden louvred window
panel based on the smoke generator technique at ve dierent airow speeds. The smoke streamlines
from the smoke generator in the wind tunnel represented the outdoor airow structure toward the
indoor space.
Figure 8. Visualised airow structure across the tested wooden louvred window panel using the smoke generator technique at
0.5 m/s inside the wind tunnel test section.
Source: own elaboration.
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Figure 9. Visualised airow structure across the tested wooden louvred window panel using the smoke generator technique at
1.0 m/s inside the wind tunnel test section.
Source: own elaboration.
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Figure 10. Visualised airow structure across the tested wooden louvred window panel using the smoke generator technique at
2.0 m/s inside the wind tunnel test section.
Source: own elaboration.
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Figure 11. Visualised airow structure across the tested wooden louvred window panel using the smoke generator technique at
3.0 m/s inside the wind tunnel test section.
Source: own elaboration.
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Figure 12. Visualised airow structure across the tested wooden louvred window panel using the smoke generator technique at
5.0 m/s inside the wind tunnel test section.
Source: own elaboration.
In Figure 8, at 0.5 m/s, the airow had quite a sharp upward curve pattern; it reached the louvres, then
continued to concentrate towards the upper part of the space as it passed through the louvres, upon
which the airow spread slowly to the other amount of the space. A similar pattern could be seen for
the speed of airow at 1.0 m/s in Figure 9. However, less upward curve pattern of airow was observed
between the smoke rake and the wooden louvred window panel, and the airow seemed to be spread
faster to the other part of the space. At 2.0 m/s (Figure 10), a much lesser upward curve of airow could
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be noticed before it entered the space and most of the airow seemed to spread to the other part of the
space as little concentration of airow to the upper part of the space could be observed. Figure 11 shows
that at 3.0 m/s, there was still some airow that moves upwards after crossing the louvres even though
simultaneously the airow spread to the other part of the space. At 5.0 m/s, as shown in Figure 12, the
upward movement of airow after crossing the louvred panel was almost unnoticeable as the airow
spread quickly throughout the space. Also observed that at lower airow speeds of 0.5, 1.0, 2.0 and 3.0
m/s, some air at the upper part of the wooden louvred panel did not pass through the louvre slots.
The airow structure from outdoors to indoors could contribute to the cross and stack ventilation within
an indoor space of a building. According to Malaysian Standard (MS 1525:2014) (“MS 2014. Malaysian
Standard. MS 1525”, 2014), cross ventilation functions by enhancing airow structure ow through
a building caused by a wind-generated pressure drop across it. The warmer air within the building is
discharged through the opposite louvred window opening while the cooler outside air enters the building
and continuously replaces the warmer air.
Meanwhile, stack ventilation functions by enhancing the ow of airow structure across space due to
air density dierences. The warmer air at the upper levels is discharged through the opening near the
ceiling. Then, the cooler outside air enters the building through the lower opening (door or window).
Stack ventilation is more advantageous in reducing energy than mechanical and air conditioning (Lomas,
2007).
Louvre angle plays a vital role in determining airow volume and direction into indoor spaces (Aaki et
al., 2015). Chandrashekaran (2010) reported that the volume of airow passing through indoor spaces
is aected by louvre opening angles of 0°, 15° and 30° and that the direction of airow is aected by
a louvre opening angle of 45°. Louvred openings are also the best ventilation for night-time ushing
in a tropical climate as they could increase force ventilation (Kubota et al., 2009). A perpendicular
louvre window is recommended for natural ventilation systems to increase the pressure inside buildings
(Sahabuddin, 2012).
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The results herein indicated that the wooden louvred window panel could continuously replace the
indoor air with outdoor air to improve natural ventilation. The proposed panel eliminates the need to
open doors and windows for 24 h, thereby oering security. Rasli et al. (2019) proposed apertures on
window panels to increase the maximum amount of fresh outdoor air moving towards indoor spaces
and thereby lower the indoor temperatures and realise proper natural ventilation. The continuation
of natural ventilation is vital to reduce the indoor air contaminants that may remain present for years
because it aids in diluting the outdoor air that enters buildings (Moreau-Guigon et al., 2016).
At a low airow speed, the airow was observed to permeate the window panel slowly, and the airow
structure at the leeward side of the window panel was circulating and broad. As the airow speed
increased, the airow permeated the wooden louvred window panel extremely fast, and the airow
structure at the leeward side straightened and showed small air motions. The results showed that an
increase in airow speed contributed to the high airow velocity passing through the wooden window
panel and that the airow lled the indoor space quickly. Ji et al. (2018) found that the ow rate of smoke
mass increases with the elevated ambient pressure because of the increase in air density and enhanced
air entrainment. By contrast, the rate of smoke mass ow decreases with the reduction in ambient
pressure, air density and air entrainment, and the heat gain within a building reduced with an increase
in air velocity (Sunakorn & Yimprayoon, 2011). With maximum airow facilitating the air exchange, it
can enhance adequate natural ventilation for improved thermal comfort and indoor air quality.
The wooden louvred window panel can protect against the rain splatter. An inclination angle of 75°
of the wooden window panel models can help prevent rain splatter from entering the indoor space.
As mentioned by Recatala et al. (2018), the characteristics of the materials used, the geometry of the
external cladding element and the edge prole of joints inuence the degree of water tightness of the
ventilated façades. This combination can avoid the dampness problem, which causes microbial growth.
Rasli et al. (2019) reported that microbial growth is strongly dependent on the indoor temperature and
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relative humidity; thus, indoor air quality needs to be maintained at the suggested acceptable limit to
protect users from health risks.
4. CONCLUSIONS
Air-conditioning systems typically installed to overcome problems in indoor thermal comfort. However,
the systems' air recirculation could contribute to poor indoor air quality and high energy demand and
energy cost. Hence, this study aimed to visualise the airow structure from the outdoors to indoor
spaces by using the smoke ow visualisation technique (i.e. smoke generator technique) in a wind
tunnel laboratory. The proposed wooden louvred window panel was tested using the smoke generator
technique at dierent airow speeds of 0.5, 1.0, 2.0, 3.0 and 5.0 m/s. Two Sony 1920 × 1080i cameras
were used to capture the side view, rear view, front view and back view images of the airow structure.
The use of wooden louvred window panels improves natural ventilation because it promotes lateral
movement of fresh outdoor air continuously. At the highest airow speed (5 m/s), the proposed wooden
louvred window panel can contribute to the optimum replacement of indoor air with outdoor airow
and increase the air exchange rate. The inclination angle of 75° of the wooden louvred window panel
can help avoid the rain splatter, and the wood material can be used for façade shading purposes. This
work could help address the problems of thermal comfort and indoor air quality and facilitate the
development of sustainable buildings. There is also a limitation in this study in which the window panel
was not tested for dierent slot angles, which could determine the best angle for the optimum outdoor
air due to the high cost of wind tunnel testing.
ACKNOWLEDGMENT
The Universiti Sains Malaysia supported this work under BRIDGING GRANT (304/PAWAM/6316537).
Special thanks to Mr Badrul and Mr Tarek for their contribution to this work.
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