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OSLO BY STEINERSKOLE
An approach to natural ventilation through sustainable building design
Is it possible to create a natural system of ventilation that is solely based on the building’s shape?
Pedro Sjøblom Tavares
2009
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Contents
Introduction ................................................................................................................................ 4
Background ................................................................................................................................. 6
Ventilation in a historic perspective ....................................................................................... 7
Arab housing technologies ..................................................................................................... 9
An example from nature: termites ......................................................................................... 9
The science of fluids ............................................................................................................. 10
Bernoulli ............................................................................................................................ 11
Aerodynamics ....................................................................................................................... 12
Sustainable building design .................................................................................................. 14
Understanding a designs potential ....................................................................................... 16
The British Standards method .......................................................................................... 17
The ASHRAE method ......................................................................................................... 17
The Aynsley method ......................................................................................................... 18
Experiment design .................................................................................................................... 20
Making the model ................................................................................................................. 21
Making wind artificially......................................................................................................... 22
The experiment ........................................................................................................................ 25
Measuring wind speeds ........................................................................................................ 25
Measuring force on walls ..................................................................................................... 25
Measuring pressure with embedded manometers .............................................................. 26
An empirical approach to the buildings ventilation potential ............................................. 28
Discussion ................................................................................................................................. 32
Margins of error .................................................................................................................... 32
Measured wind speeds ......................................................................................................... 33
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Smoke tests ........................................................................................................................... 33
Executing the project ............................................................................................................ 34
Conclusion ................................................................................................................................ 36
Acknowledgements .................................................................................................................. 37
Litterature ................................................................................................................................ 38
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Introduction
Man has always tended to mold and manipulate his environment. Already tens of thousands
of years ago, our ancestors took habitat in caves as shelter. They lighted a fire in the
entrance to keep predators away and to produce a stable climate inside. The technology
used to produce this stable environment has changed a lot through the years, and in special
throughout the twentieth century with the introduction of mechanic ventilation and
mechanic climate control, but the mission has always stayed the same.
«Twentieth century buildings have contributed massively to global
carbon emissions. Can buildings in the 21th century help to eliminate
this problem? » (Libeskind 2008)
I have asked myself the same question architect Daniel Libeskind made in Time magazine. In
the 20th century, buildings gained new dimensions, and the construction time was drastically
cut down. This much due to prefabricated construction parts and standardized layouts of
buildings. However, the “personalized building” is again gaining new grounds. Enterprises
don’t want their offices in standard mass-produced sky scrapers; they want something
original, a platform and ideal for their aspiring business. In our century climate and ecology
matters more than ever, much due to the partly man made changes in weather and seasons
we all can observe. These two factors put together, the need for personalizing and the
climate change, have given birth to what we call sustainable building design.
Throughout this paper I will present ventilation and sustainable building design, both first in
a historic perspective. I will also try to put focus on some of the laws of physics that lay
behind a phenomenon such as natural ventilation. Since moving air is the main concern in
ventilation, one has to always consider “the science of fluids”, which embrace both liquids
and gases. In my work there is also a special focus on aerodynamics, since this is what lies in
the basis of my ventilation design.
Experimenting on my self-designed ventilation system makes the main body of this paper.
This is a personal view of natural ventilation and, as far as I know, an original one. I will
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describe the design process, the tests and their results, and how an implementation of the
system in real life would be executed.
The success of any design can be measured by its ability to combine form and function.
Considering ventilation as a basic function in any building, how can a building’s form
interfere and eventually solve the need for ventilation? Is it possible to create a natural
system of ventilation that is solely based on the building’s shape?
These questions have awakened my curiosity and have served as point of departure for this
paper.
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Background
Throughout history, human beings have had the tendency to overuse those resources that
have been of abundance and easily accessible, without being concerned about the
consequences that these actions would lead to. Early humans were already responsible for
the extinction of various pre-historic species, and today we are overusing almost all of our
natural resources. Our ability to foresee the result of our actions has been hamstrung by cost
efficient thinking and greed. Recent studies have been shown to us through the mass media,
and these tell us about an uncertain future for our planet and, consequently, for our society.
Depletion of natural resources and the increased emission of carbon dioxide, among other
types of greenhouse gases, are pointed out as important factors for this recent pessimism
about the future. Many believe that the climatic process our planet is now entering will be
irreversible. Most people also accept the need for drastic change in our societies, regardless
if they are to confront a new reality or trying to reverse an already occurring process. Drastic
changes need drastic actions, as well as pioneering and extreme ideas. There is never only
one solution to a problem, and in most cases, the combination of many solutions gives the
best results.
Our cities are in constant growth, especially those in young and expanding economies, such
as China and India. When cities grow they consume immense amounts of natural resources.
China’s construction alone stands for one third of our planet’s steel consumption and half
our planet’s concrete consume per year (Mega Structures, 08.01.09), and this is only for the
construction process. This country’s growing cities are also in need of tremendous amounts
of energy to function, and this energy comes from every time bigger dams and coal driven
power plants. Seen in a greater perspective, the results of these actions are easy to track.
Larger dams means larger false lakes, lakes that cover larger fertile areas, which gives less
food production capacity, which makes the urge for import larger, which again makes the
world economy unstable, and so the spiral goes. Such chains of effects of an action can show
us a bigger truth, and often a shocking one. (To this last idea, the Aswan Dam in Egypt
would be a really interesting case study. (Steele, p. 27))
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All these factors and many more have helped the concept of sustainable building design and
energy efficient housing to grow through the last 25 years. All the time, more people open
their eyes to the power of the often ancient technologies used in sustainable building
design. An example of that is the mechanic ventilation of buildings which can often be taken
away without necessarily leaving the inhabitants in an uncomfortable indoor climate. As a
matter of fact, in many cases the contrary happens. At this point, it is important to note that
sustainable building design does not only consist of the ventilation factor, but also of many
others (see AIA and Agenda 21 recommendations, page 9). Throughout this chapter I will
present the main principles of sustainable building design and some of the theories behind
it. My work, however, will focus on the ventilation aspect. I will also put it all into a historical
context.
Ventilation in a historic perspective
As soon as man began making shelters, he recognized the need for constant air renewal. It
was not necessary to understand why the body always needed fresh air; he could easily feel
it. Early humans inhabited open caves that where ventilated by nature, and in these the
main concern was maintaining a stable temperature. These people were nomads, constantly
on the move, and their habitats were chosen by the accessibility of alimentation, not by
comfort. Food was mostly obtained through hunting and gathering, and in order to survive,
they had to move after their food. This made it hard to predict how long they would stay in
one place, and if there would be any caves in the next area.
For this lifestyle they developed movable tents covered with animal skin or matting, similar
to the “tipi” used by the Native Americans, the ”lavvo” used by the Lapps, and the “ma’dam”
used by Mongolian nomads for over 6000 years. This traditional tent design is easy to carry,
as its structure only consists of stiff wooden poles and a covering membrane, and is a very
efficient shelter. The membrane covers the whole structure, except for an opening on the
top where smoke is allowed out. This opening is partly covered by an adjustable flap that
allows the inhabitants to control the smoke exhaustion, as well as sealing the shelter to keep
water out on rainy days. To maintain a comfortable inner climate on hot days, the lower part
of the covering can be raised, allowing a bigger airflow through the interior. When it is very
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cold, an extra covering layer is added, making a dead air space around the interior,
preventing heat loss.
Figur 1
When man began manipulating the soil through agricultural practice around 10 000 years
ago, it lead to a new lifestyle and the development of larger societies. It was no longer
necessary to constantly move after the food, as it now was up to the people themselves to
produce it. This showed the way to the development of more permanent and efficient
housing facilities where the need for air renewal and indoor comfort was necessary.
The early agricultural societies were settled in hot climate zones. Therefore, when these
societies developed and larger building masses where built, more advanced permanent
ventilation systems were also created. In Iran for example, “curved-roof” air ventilation
systems were incorporated as early as 3000 BC. Ancient Egyptians used wind catchers, also
called “malkafs”, in the city of Tal Al-Amarna more than 1000 years BC. This system is
represented in several wall paintings in the tombs of Thebes, among which is the tomb of
the Pharao Neb-Amun from ca. 1300BC. (Allard 1998)
Figur 2
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Arab housing technologies
Our planet is divided into several global climate zones. We have the tropical, the dry, the
temperate the cold and the polar climates. All these areas are inhabited by humans with
similar basic needs, among these the need for fresh air and a stable indoor climate. The main
concerns in the polar and cold areas are with maintaining a stable and warm temperature,
while in hot and dry areas the concern is with lowering the temperature and renewing the
air.
Most people manage to light a fire for heat, but there are few that understand the factors
necessary to cool a building. The dry and extremely hot areas of the Arabic peninsula is one
of the places we can find such expertise.
The Arabs understood early that the main heating source was the sun. They constructed
their housing with large shaded areas because of this. If we walk through a traditional Arab
town, we will notice that most parts of the city’s public areas are shaded. For the same
purpose the streets are narrow with high walls on both sides. These dry desert areas are also
windy most of the time. The shade and the wind help maintaining a stable temperature that
is much lower than one would expect. Arab housing also focuses on keeping all the leisure
areas shaded as much of the day as possible. Bigger housing complexes are usually
constructed around a shaded inner garden with a fountain in the middle. This flowing water
absorbs the heat from its surroundings, lowering the temperature. The roofs of these houses
are equipped with wind catchers similar to the ones used by ancient Egyptians. The wind
catchers transfer the slightest movement of air into the building, renewing it as often as
possible. Without such technologies the dry and hot climate zones would have been
inhabitable.
An example from nature: termites
In nature many creatures build housing structures similar to the ones we construct. All living
beings above sea level need fresh air to survive. Because of this, all structures made for
housing purposes must necessarily have a system for constant air renewal. Termites of the
subfamily Macrotermitinae in South Africa live below the ground to protect themselves from
the heat. However, 2 million individuals living under the surface rapidly feels the urge for
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fresh air. To this purpose, they have construct a type of high raising wind catcher above the
ground, which efficiently renews the inside air. These termite mounds can rise up to 8
meters high. The hot and “used” air from the inside flows up a giant inner chimney which is
sealed on the top. Around this central chimney there are several smaller vertical ventilation
channels that are both connected with the exterior of the structure and the central chimney.
When wind passes around the structure, air is allowed in trough the walls and into these
smaller channels. From here, some of this air is sent into the central chimney and mixes with
the hot stale air. On the opposite side of the structure, the outside wind creates a puff that
pushes the air out of the ventilation channels. The hot air is therefore pushed out from the
structure, while the fresh air is allowed down the channels to replace this old air. This system
is very similar to the Iranian wind catchers; however, this is nature’s own solution which has
been used for millions of years. It is not unthinkable that such systems found in nature, has
served as inspiration for similar ones built by humans.
Figur 3: mound (background) in courtesy of BBC’s “Life in the Undergrowth”
The science of fluids
A fluid can be defined as a substance that constantly deforms (flows) under an applied
shared stress. All liquids and all gasses are therefore fluids.
The study of fluids has been conducted ever since the Greek philosophers started
questioning the physical properties of water and the different qualities of the air.
Archimedes was one of the first, studying fluid statics and buoyancy. For centuries the
Greeks studies were forgotten in the western world, mostly due to the church’s censorship
of any scientific study that did not explain things in accordance with the Bible. Several of the
Greek philosophy schools were closed by the Roman emperor Justinian in 527. Copies of the
work of the Greek philosophers, however, were kept at libraries such as the one in
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Alexandria. With Islam’s later expansion, Arab scholars became acquainted with these
works. Several new findings on the area were made, most significant the fluid dynamics. This
was a new area within the science of fluids that would be severely studied in newer time.
Moving away from the science’s roots and into its practical appliance one can state that the
fluid statics and fluid dynamics together give us fluid mechanics, also known as the science
of fluids. Jeg ser det ikke nødvendig å regne på liquid og gas som forskjellig siden jeg
behandler lave hastigheter.
Bernoulli
One of the ground rules in physics is the conservation of energy. The mechanical energy of
an object will always be the same, since this is the sum of kinetic and potential energy. In
fluid mechanics one can apply the same rule, but here to a steady flow. The sum of potential
and kinetic energy will be the same at all parts of the flow. The first one to make a formula
for this was the Swiss-Dutch mathematician Daniel Bernoulli. His study was on liquids, which
are non-compressible flows. Later a version of the equation for compressible flows was
developed.
He saw that as a fluid travels horizontally it accelerates when moving from a higher
pressured area to a lower pressured area, and decelerates when doing the opposite.
Knowing this he stated that the highest speed occurs in the lowest pressured region and the
lowest speed occurs where there is high pressure. Based on rules of the conservation of
energy he made the following equation for non-compressible fluids.
𝑣𝑣2
2+ 𝑔𝑔𝑔𝑔 +
𝑝𝑝𝜌𝜌
= 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
Where z is stated as a point elevated over the base line of the flow. We can therefore also
refer to it as h. One can easily see the similarity to the general conservation of energy
equation, when Bernoulli is multiplied by ρ.
General equation: 12𝑚𝑚𝑣𝑣² + 𝑚𝑚𝑔𝑔ℎ = 𝐸𝐸𝑚𝑚𝑐𝑐𝑚𝑚
Bernoulli’s equation: 12𝜌𝜌𝑣𝑣² + 𝜌𝜌𝑔𝑔ℎ + 𝑝𝑝 = 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
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The fluids stated as non-compressible are what we call liquids. It is not that they are not
compressible, but their compressibility is much lower than that of gases. For example an
increase of pressure by 1psi (6892.7Pa) would increase the air density by about 7%. Water
would only have a minimal increase in density under the same conditions. The two cases
differ roughly by a factor close to 200. This easily shows us why you cannot apply the same
equations to both.
For compressible flows Bernoulli’s equation states the following:
𝑣𝑣2
2+ 𝑔𝑔𝑔𝑔 + ( 𝛾𝛾
𝛾𝛾−1) 𝑝𝑝𝜌𝜌
= 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐.
Here the ratio of the specific heats is taken into account trough γ. This is because all fluids
alter their volume under different temperatures.
Aerodynamics
Air, as well as other fluids, consists of enormous amounts of small molecules. If you took out
a 25μm³ sample of the air, it would contain around 6·10⁸ molecules under normal
conditions. Each one of these small sample areas, containing millions of molecules, acts
differently, just as us humans. However, seen in a bigger perspective, they have similar
reactions to situations, and therefore operate in larger patterns. These patterns are what we
attempt to predict, however, without taking the individual areas into account, they will
never be perfect. Since air is a gas it is also compressible, a quality that further complicates
the prediction. However, under Mach 0.3 one can omit the compressibility factor. (Anderson
& Eberhardt 2000)
Most people have a vague idea of how an airplane wing works. The most common
explanation is the one that relates difference in pressure to difference in velocity. The air
over the wing has to travel longer than the air underneath it, and therefore it will have a
greater speed. Greater speed means smaller pressure, so the higher pressure underneath
the wing tends to regulate the difference by lifting the wing up. This picture is commonly
accepted as an explanation for the wing’s lift, but unfortunately not correct. There are
several important factors omitted in this picture, and it is therefore impossible to try to
calculate a wing’s lift in that way. Therefore the most common mathematical prediction
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tools used by airplane designers are the Navier-Stokes equations, Bernoulli’s equation and
the Euler equations, but always first considering Newton’s laws of motion. What is important
to state about these equations, except Navier-Stokes, is that they, like all equations on
fluids, omit many factors. They are, however, applicable with satisfying accuracy.
But how is it possible to explain the wing’s lift in physical terms? The common theory based
on equal transit time fails at the moment an airplane turns upside-down under a flight show.
How could one then describe the wings opposite lift? The answer lays in Newton’s laws of
motion and the phenomenon of viscosity. When the air passes the top of the wing, it
produces its lift by diverting the air down. Why this happens can be explained by something
called the Coanda effect. The book “Understanding Flight” shows this effect with a simple
example:
“Run a small stream of water from a faucet and bring a horizontal water glass over
to it until it touches the water[…]the water will wrap partway around the glass.”
(Anderson & Ebenhardt 2000)
Through Newton’s first and third law, we know that a force acts in the direction the water is
bended, and that an opposite force also acts on the glass. The explanation for why this effect
happens is the viscosity that makes the inner layer of fluid “stick” to the objects surface.
How the rest of the fluid reacts to this is explained later on.
“A small distance from the surface the fluid has a small velocity with respect to the
object. The farther out one looks from the surface, the faster the fluid is flowing,
until it eventually comes to the speed of the uniform flow some distance away
from the object.” (Anderson & Ebenhardt 2000)
This difference in speed in the different layers is what makes the bend. The faster layers
tend to wrap around the slower layers and the object to prevent voids (gaps) between them.
It is the same thing that happens over a wing. The air is deflected downwards because of this
Coanda effect. The bending of the air then causes the lowering of pressure, and therefore,
by Bernoulli’s equation, the gain in speed. It is this downwash of the air that gives the
airplane its lift. It can be hard to imagine, but if one observes a helicopter, the rotor in reality
is a set of rotating wings, and these wings create the lift upward by rotation. If the helicopter
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hovers over water, the downwash can be observed as a marked out circular area right under
the blade area.
There are many other important factors to be considered in the lift, such as the vortex, etc.,
but one will already manage to give the wing its lift by these factors.
Sustainable building design
In order to understand sustainable building design, we must first understand the meaning of
the word sustainability. Sustainability is a fairly common word nowadays, and it is often
placed out of context. The overall meaning of the word is the ability to maintain a certain
process or state. In the most common sense, it is applied to processes in nature. After the
first Earth Day in June 1970, and the oil embargo in 1973, a consensus emerged about the
need for changes in the way modern society worked, mostly linked to the consumption of
natural resources and the modern lifestyles enormous impact on nature. The first time the
word “sustainability” was used in connection with the environment was in a report named
“World Conservation Strategy”, published in 1980 by the UCN (Union for the Conservation of
Nature).(Steele 1997) In this paper the word was only linked to development. Seven years
later, in 1987, the Brundtland Commission produced on behalf of the United Nations a
report named “Our Common Future”. In this document the commission defines sustainable
development to involve “those paths of social economic and political progress that meet the
needs of the present without compromising the ability of future generations to meet their
own needs” (World Commission on Environment and Development, document A/42/427)
The Earth Summit-Agenda 21, was held in Rio de Janeiro in 1992 as a continuation of the
Brundtland Report. More than 100 state leaders were present in these meetings, with the
US president notable absence. Together they developed the Agenda 21, a document
containing 40 separate sections concerning different areas of how society works, and 1000
proposals for possible improvements. In the fourth area of concern, “Management of
Human Settlements”, a need for changes in the construction industry and focus on
infrastructure that is environmentally adequate was proposed. It puts focus on the built
environment’s capacity to destroy nature and points it out as a “major source of
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environmental damage” (United Nations Conference on Environment and Development,
Agenda 21, Chapter 7)
The report made twelve specific recommendations to this issue.
1. The use of local materials and indigenous building sources
2. Incentives to promote the continuation of traditional techniques, with
regional resources and self-help strategies
3. Recognition of the toll that natural disasters take on developing countries,
due to unregulated construction and use of inadequate materials, and the need
for improvements both in use and manufacture of materials and in construction
techniques, as well as training programs
4. Regulation of energy-efficient design principles
5. Standards that would discourage construction in ecologically inappropriate
areas
6. The use of labor-intensive rather than energy-intensive construction
techniques
7. The restructuring of credit institutions to allow the poor to buy building
materials and services
8. International information exchange on all aspects of construction related to
the environment, among architects and contractors, particularly about
nonrenewable resources
9. Exploration of methods to encourage and facilitate the recycling and reuse of
building materials, especially those requiring intensive energy consumption in
their manufacture
10. Financial penalties to discourage the use of materials that damage the
environment
11. Decentralization of the construction industry, through the encouragement of
smaller firms
12. The use of “clean technologies”
Shortly after, the AIA (American Institute of Architecture) together with the International
Union of Architects suggested eight additions to the Agenda 21 proposals in the “AIA
Environmental Resource Guide.”
1. An extension of the view of the built environment beyond shelter, to include
“energy harvesting, waste management and reuse, food production and
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distribution, water harvesting and handling, as well as facilities for recreation,
health, education, commerce, etc.”
2. Reduction of construction processes that damage the environment in favor of
those that restore it
3. The strict implementation of reuse and recycling of building materials
4. Encouraging the creation of self-reliant communities to reduce transportation,
energy, and material use
5. A return to well-established methods of design that conserve energy and natural
resources
6. A further examination and exploration of the potential of self-help in the
“making, remaking, and use” of sustainable settlements
7. The encouragement of community participation in the design and construction
process, an idea that was first popularized in the early 1970s in the United States by
Charles Moore
8. Urban energy and harvesting, forestry, food production and hydrology, and
wildlife management supported by the involvement of UN agencies.
(Recommendations as put forth by Steele, 1997)
This paper puts its focus on the 12th recommendation of the Agenda 21 report; the use of
“clean technologies”.
Understanding a designs potential
To understand a designs potential, one must first understand the mechanisms that makes
the proposed design function. These mechanisms are often hard to comprehend, but for the
most they can be explained through a theoretical approach. A design always has specific
tasks, and it is the potential of accomplishing those tasks that one has to predict. The
prediction methods used varies therefore with the designs purpose.
As Mr. Santamouris states in the book “Natural Ventilation in Buildings:”
The physical processes that are involved in natural ventilation are very complex
and the interpretation of their role in ventilation effectiveness is a difficult task.
[…] However, taking into account the chaotic character of the wind
characteristics, a full knowledge of the boundary and initial conditions are almost
impossible. (Allard, 1998)
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Knowing this, there are few prediction methods that are satisfying in their accuracy. The
most common approach is therefore through empirical models, but nowadays there also
exist complex computer programs that solve the Navier-Stokes equation. However, in order
to get a full picture of a design’s potential, the most common method is still making a scaled
model down and testing it in a wind tunnel.
Most empirical models do not fit my case due to their different approach to the wind in the
ventilation process, but there exists several simplified models, partly based on empirical
data. Their accuracy is only sufficient for the early design stages of a given project. I will,
however, present the most common of these methods.
The British Standards method
The British Standards Method can be considered as one of the simplest prediction methods
that are available. The method proposes formulas for the prediction of airflow through a set
of simplified scenarios. There are two main building scenarios, one with cross ventilation and
one with single-sided ventilation. In both, all internal partitions and obstacles are ignored.
The ventilating factors considered are wind and temperature. The scenarios are proposed
with wind or temperature based ventilation, or a combination.
Single-sided ventilation due to wind has the easiest formulae:
𝒬𝒬 = 0.025𝐴𝐴𝐴𝐴 (Where A is the opening surface and V is the wind velocity)
The formulae for cross ventilation due to wind is slightly more complicated:
𝒬𝒬𝑤𝑤 = 𝐶𝐶𝑑𝑑𝐴𝐴�𝛥𝛥𝐶𝐶𝑝𝑝 (Here 𝐶𝐶𝑑𝑑 is the discharge coefficient, 𝐴𝐴𝑤𝑤 is the opening
surface coefficient and 𝛥𝛥𝐶𝐶𝑝𝑝 is (𝐶𝐶𝑝𝑝2 − 𝐶𝐶𝑝𝑝1), which are the pressure coefficients of the
buildings two sides)
The ASHRAE method
This technique depends partly on empirically gathered data. To use the method, you must
first have knowledge of the total effective leakage area of the building. For this method
there also exist tables with data for predefined scenarios.
𝒬𝒬 stands for the bulk airflow rate in this method, which can be described as:
𝒬𝒬 = 𝐴𝐴�𝑐𝑐∆𝑇𝑇 + 𝑏𝑏𝑈𝑈2𝑚𝑚𝑐𝑐𝑐𝑐 (𝑚𝑚3ℎ−1)
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For this formulae:
• A is the total effective leakage area of the building measured in cm²
• a is the stack coefficient(𝑚𝑚6ℎ−2𝑐𝑐𝑚𝑚−4𝐾𝐾−1). A coefficient with given values depending on the
building’s number of storeys.
• b is the wind coefficient(𝑚𝑚4𝑐𝑐2ℎ−2𝑐𝑐𝑚𝑚−4). It is a coefficient that has different values
depending on number of storeys and the surrounding areas shielding of the building, also
with given values.
• ΔT is the average indoor-outdoor temperature difference (K)
• 𝑈𝑈𝑚𝑚𝑐𝑐𝑐𝑐 is the meteorological wind speed (𝑚𝑚𝑐𝑐−1)
The Aynsley method
Richard Aynsley developed a method for global airflow prediction, in the case of cross
ventilation, based on the use of pressure coefficients. His formulae assume that there are
two main openings on opposite sides of the building and that there is a conservation of mass
between these two openings. When first defining the pressure coefficients 𝐶𝐶𝑝𝑝1 and𝐶𝐶𝑝𝑝2, he
proposed the following expression for the global airflow rate:
𝒬𝒬 = �𝐶𝐶𝑝𝑝1−𝐶𝐶𝑝𝑝21
𝐴𝐴12𝐶𝐶𝑑𝑑1
2 + 1𝐴𝐴2
2𝐶𝐶𝑑𝑑22𝐴𝐴𝑔𝑔
Where:
• 𝐶𝐶𝑑𝑑1 and 𝐶𝐶𝑑𝑑2 are the discharge coefficients given as functions of the opening configuration
• 𝐴𝐴1 and 𝐴𝐴2 are the areas of the openings
• 𝐴𝐴𝑔𝑔 is the reference wind velocity
Other and more advanced methods also exist, such as the CSTB methodology, which is based
on wind tunnel data of scaled down architectural models. In this data the data retrieved
form experiments are evaluated upon:
• The characteristics of the site
• The orientation of the building and of the wind
• The exterior characteristics of the building
• The interior architecture and the interior aerodynamics of the building
(All formulas retrieved from Allard, 1998)
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Experiment design
As explained, natural ventilation can be implemented as an important part of sustainable
building design. The recommendations for sustainable design made by Agenda 21 and AIA
clearly state the need for a construction with fundaments in the buildings area and
adoptions made especially for this location. Ventilation is a universal need and can therefore
be studied more easily in a bigger perspective. I chose to develop a building design that
allowed a big air change rate in hot areas with more wind. The coastal areas near to equator
would be the ideal place, since these often have very consistent sea breezes the whole year.
Such phenomena are for example very common in Brazil, where I spent parts of my
childhood, and where I had the opportunity to observe it first-hand.
A sea breeze is caused by the different heat absorbing capacities of water and land. Water
can absorb much more heat than land. Therefore, the land air becomes hot quicker and a
low pressure area develops. The air from the higher pressure sea area rushes in over land to
compensate the difference. This air flow is what we know as sea breeze. In the evening
when the sun is setting, the water maintains its heat longer, and the process is therefore
inversed. In the northern areas of Brazil, the reliability of this phenomenon is extremely
important for the local fishermen who still to a great degree depend on primitive sailboats
for their work.
With these two opposite wind directions in mind, I have created a building design that would
make the internal air move and cause ventilation. I based my initial idea on the study of
termites, but I soon went in another direction; the airplane wing. As explained in the part
about aerodynamics, an airplane gets its lift from the wings design, which has a curved top.
When the wing cuts the air, the part that travels the top of the wing must travel longer than
the air under. This causes a difference in speed, and therefore, as explained by Bernoulli’s
equation, also a difference in pressure. As the sea breeze, the air tends to compensate for
the pressure difference. This compensating process gives the wing its lift. We can then
imagine that if the wing is placed vertically on the ground and the air moves, instead of the
wing, the same process will occur. Instead of letting the wind push the building too much to
one side, I have then imagined to make holes trough the wing-building’s cross section which
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would let the air compensate, and not only the wing. The idea was simple, but its potential
in reality was uncertain. As clearly stated by many authors, a theoretical approach will only
uncover certain aspects of a design’s performance. Therefore, the need for real time
observations is urgent in most scenarios. In my case, I thought that a scaled down model
tested in different conditions would be the most giving way to retrieve data.
Making the model
I based my construction on a regular model airplane wing structure, with molded cross
sections and strong carrier bars. Since my wing was not supposed to fly, my concern was
with the structure’s stability and stiffness, rather than weight. I therefore chose to make the
cross sections with thick cardboard and the bars of two rounded wood planks, all in scale
1:100.
Figur 4: covering the structure with film
This structure I then covered with a heat shrinking wing coat.
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Figur 5: caption of the 3D model used in the design process
In the middle of the building, and through its cross section, I placed a single pipe that would
simulate a ventilation channel. I also implemented two homemade manometers, connected
to each of the side walls. These would hopefully give me an idea of the pressure difference
that was produced by the wind. In addition, one floor was fitted with two small ventilation
entrances on opposite sides. Experiments on this floor would allow me to get an idea of the
real air flow rate.
Figur 6: fitting embedded manometer
Making wind artificially
I now had my model, but one component was missing: the wind. As I did not have access to
any wind tunnels, I would have” to make wind by myself.” My first idea was to use a normal
table fan, but the ones I found were not strong enough and did not manage to give me a
constant and uniform airflow. After thinking of my possibilities, I opted to use an air pressure
pistol. This was not ideal either, since the focus area of its wind was not big enough. On the
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positive side, however, it produced a uniform airflow with constant speed. I was glad to see
that the system worked during the testing process.
Figur 7: my wind source and model ready for tests
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The experiment
Experimenting on the model, I first tried to analyze the outcome theoretically, and
afterwards to gather empirically the information on the model. The data that was of basic
interest to me were airflow rate, wind speed around the building, pressure difference, and
the force applied on the building.
Measuring wind speeds
Velocity is given by the equation v=s/t. My cross section measured 20cm. If the entering
wind was traveling at 10m/s the time used to travel this length would be given by t=s/v (z).
With numbers it would give us 𝑐𝑐 = 0.2𝑚𝑚10𝑚𝑚/𝑐𝑐
= 0.02𝑐𝑐. The travel time is the same for both sides
of the building, however, the distances are not the same. The top side of the wing measures
21.2cm, and the shorter 20.2cm. Put into the speed equation with t as 0.02s, we get a
10.6m/s speed over the wing’s longer side and 10.1m/s on the opposite side. The difference
Δv would then be 0,5m/s, 5% of the entering wind speed. As stated in the chapter about
aerodynamics, this data would not be correct. The speeds over the building sides would be
greater than the ones calculated. I opted to include them because of their value as
guidelines and since the Δv would hopefully be the same.
With these theoretic data in mind I tested it on the model, measuring the speeds with an
anemometer. Placing the wind source 1.7m away from the model, and adjusting the
pressure to 15bar, I got an initial wind speed of 5.2m/s. On the short side the speed was,
5.3m/s and on the long side, 5.6m/s. The calculated Δv as 5% of 5,2m/s was, 0.26m/s. The
measured Δv was, 5.6m/s-5.3m/s=0.3m/s, a pretty good match considering the
anemometers 0.1 accuracy.
Measuring force on walls
The building’s vertical orientation made it difficult to find an efficient method for measuring
force on the model’s walls. I chose to use a wire connected to the short wall of the building
and to a digital force measurer. To let the building move freely, I did not fix the building to its
sole.
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When I let the wind hit the building, I was confronted with a problem. Because of the wind
pistol’s focus, there was only a smooth flow around the building in a small area. This was no
problem earlier, but now, when the building passed a certain angle, it did not get a constant
flow. This caused it to bounce back to its starting position. Data was retrieved, but it was
neither constant nor easy to deduce. The force data retrieved varied between 0.2N and a
max of 0.6N before bouncing back.
I found then the solution to this through NASA’s program Foilsim. This is an easy-to-use
program that measures lift force on different wing designs. Entering the data of my
construction into this program, I got a 0.597N result. Comparing this to my experiment
results, I could see that 0.6N actually was the maximum force on the building’s section, and
that the test had not actually failed.
Figur 8: caption of the program Foilsim used in my force measurment
Measuring pressure with embedded manometers
A manometer is a U-shaped pipe that contains a liquid with a known density.
The entrances to this pipe are connected to different pressure zones, one to
the sealed interior of the building and the other to the exterior of the wall.
These pressures will exert a force on the liquid, which causes it to move. The
difference in height of the liquid will tell us the difference in pressure through
the following conversion of Bernoulli’s equation:
We have to different columns with the same potential energy:
𝜌𝜌𝑔𝑔ℎ₁ + 𝑝𝑝₁ = 𝜌𝜌𝑔𝑔ℎ₂ + 𝑝𝑝₂
Figur 9: Manometer
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When transposed:
𝐻𝐻 = ∆ℎ = ℎ₂ − ℎ₁ =𝑝𝑝₂ − 𝑝𝑝₁𝜌𝜌𝑔𝑔
Which gives us:
𝐻𝐻 =P2 − P1
ρg
By transposing this equation we get: ∆𝑝𝑝 = 𝜌𝜌𝑔𝑔ℎ (where g is the acceleration of
gravity)
My manometers where made out of a silicone fuel tube. Because of this tube’s small
diameter, I opted to use acetone as the liquid inside in order to minimize the effects of
viscosity and surface tension.
Filming the two manometers in action, one connected to each side of the building, I could
analyze the height difference in each picture. This way I managed to calculate the pressure
on each wall by the Bernoulli equation.
Figur 10
Analyzing the pictures showed above, and knowing the height of the normal level columns, I
managed to calculate the liquid’s height. On the short side of the building there was a height
difference of 6mm, and on the long side a 10mm difference in height.
Acetone has a density (ρ) of 0.785g/cm³, and the acceleration of gravity is 9.81m/s². Put into
the equation we get:
For the short side ∆𝑝𝑝₁ = 𝜌𝜌𝑔𝑔ℎ = 785kg/m³· 9.81m/s²· 0.006m = 46.2Pa
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For the long side ∆𝑝𝑝₂ = 𝜌𝜌𝑔𝑔ℎ = 785g/m³· 9.81m/s²· 0.010m = 77.0Pa
The difference in pressure on both sides of the building is given by:
∆𝑝𝑝 = ∆𝑝𝑝₂ − ∆𝑝𝑝₁ = 77.0𝑃𝑃𝑐𝑐 − 46.2𝑃𝑃𝑐𝑐 = 30.8𝑃𝑃𝑐𝑐
As we can see, there is a considerable difference in pressure, and it is this difference that is
the whole ventilation system’s motor in my building design. Already at this point my
assumptions where therefore mostly confirmed.
An empirical approach to the buildings ventilation potential
There have been made many tentative theoretical approaches to a building’s ventilation
potential, most of which are derivatives of data retrieved from experiments made by various
authors. This is mostly due to the chaotic character of the air. These formulas, in most cases,
also involve the need for complementary data from experiments, such as the one I have
done. Some of the most interesting of those approaches are described earlier. But here I will
only do an empirical approach.
My first concern was; does the building have the ability to ventilate? In the cross section
tube, I placed a small plastic ball. When wind was applied to the building, the ball came out
on the long side. A clear and easy way of confirming my theory, there was passing air trough
the building.
Figur 11: ball passing through the ventilation pipe
To observe how the air traveled, I used a smoke marker. When put next to the tube
entrance, it was clear that some of the air entered the tube and passed to the other side.
This was possible to see by the smoke exiting the opposite side.
Figur 12: smoke expelled on the opposite side of the smoke source
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I did the same test next to the ventilation holes on the test floor, and with the same results.
These holes had a 3mm diameter and where on different sides of the building, but clearly
pushed the air with the same facility as the bigger tube.
To check the air flow rate trough the floor I figured out that the easiest way would be filling
the floor with smoke and observing how long it would take to empty it. By applying the
following mathematical calculation of the floors volume I would know how many cm³ air that
was changed per second:
Each floor was composed by two curved shapes. By entering some of the coordinates of
each of these curves into a graphic calculator, they could be shown through the following
quadratic equations:
Figur 13: equations and their respective curves
Integrating these we get the area of each section:
� 𝑓𝑓(𝑥𝑥)𝑑𝑑𝑥𝑥20
0= � −0.03𝑥𝑥² + 0.6𝑥𝑥
20
0𝑑𝑑𝑥𝑥 =
−0.03 · 203
3+
0.6 · 20²2
= 40𝑐𝑐𝑚𝑚²
� 𝑓𝑓(𝑥𝑥)𝑑𝑑𝑥𝑥20
0= � −0.02𝑥𝑥² + 0.4𝑥𝑥
20
0𝑑𝑑𝑥𝑥 =
−0.02 · 203
3+
0.4 · 20²2
= 26.67𝑐𝑐𝑚𝑚²
Adding both together we get the total area of the floor:
𝐴𝐴 = 40𝑐𝑐𝑚𝑚² + 26.67𝑐𝑐𝑚𝑚² = 66.67𝑐𝑐𝑚𝑚²
The volume of the floor is given when multiplying with the floors height:
66.67𝑐𝑐𝑚𝑚² · 6𝑐𝑐𝑚𝑚 = 400𝑐𝑐𝑚𝑚³
Now, knowing the volume of the floor and the time it took to empty it, which was 52s, I
could calculate the air change rate:
400𝑐𝑐𝑚𝑚³52𝑐𝑐
= 7.692𝑐𝑐𝑚𝑚³/𝑐𝑐
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My filming of this test also revealed an interesting phenomenon. When the air entered the
room filled with smoke, it made a clear transparent mark of its path, like a river. This river
went from one side of the room the other, where the exit was, but managed to drag the
smoke out of the extremes of the room first. This way the whole floor was rapidly emptied
of smoke.
Figur 14: frame capture of the ventilation system in action
It is difficult to understand what was happening looking at only one video frame, but to give
an idea I have circled the areas of interest. The next free frames have been captured with a
20s interval.
Figur 15: the ventilation system off
Figur 16: the ventilation system in action (blue lines are attempts to show the fresh air flowing, and red lines are showing the smoke movement)
31 | P a g e
Figur 17: the ventilation system is still on, but the smoke is out
With the video observations of the air flow and the calculations of air change rate, I had
managed to confirm what I hoped for. There was a considerable change of air in the
building.
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Discussion
This work can probably not be called a pioneering one in all its aspects, but the fact that a
new approach to the use of wind as the ventilation source in naturally ventilated buildings
has been used, makes it difficult to compare it to other designer’s works. Through my
empirical approach to the design I have gathered sufficient information to conclude that this
method is efficient enough to have practical appliance in the real world. But since the use of
the system depends on factors that were artificially created for the purpose of the
experiment, a thorough climatic study of a potential construction ground would be
necessary. In other words, under ideal circumstances the system works, but unexpected
conditions must be taken into account in a real planning situation. However, I believe that
the main concept of the design is flexible enough to be accommodated to a wide range of
environments.
Margins of error
In all experiments that are not conducted in a totally “neutral” environment, unexpected
factors can alternate with the wanted results. These factors are difficult to identify,
especially in cases where one rely solely on empirical data, and does not have the
opportunity to compare the findings with good theoretic data.
A factor that was under my control and which could help produce misleading data was the
wind source. The air compressor’s nozzle did not give a large air spread. Focusing the wind in
only one spot could lead to false observations on the building as a whole.
The room where the tests were executed had heating radiators. Air, as explained earlier, has
a variable density depending on the temperature. Since these were turned to the minimum,
I did not consider them as a big enough radiating heat source to interfere in my tests. But of
course they might have altered my results.
In order to get a stable wind, one cannot have any objects in the airflow’s path, since those
interfere with the flow and create turbulence. Considering this, I have always tried to have a
clean as possible surface between model and wind source. In some tests there was no other
solution than placing sensors close to the building’s walls.
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Testing the force, I used a line to connect building and sensor. This lowered the interference
to a minimum. But in the wind speed test, there was no other way than placing the
anemometer next to the building. This could have caused turbulence, but only after passing
the sensor, and therefore also the testing area.
With these possible margins of error in mind throughout the testing, and trying to minimize
the effect or appearance of these, I believe that all data that was retrieved is as accurate as
possible, considering the circumstances.
One can often consider an experiment successful when the results are close to what has
been predicted. Of course this applies only if one is looking for a specific outcome. In my
case, the experiment consisted of several tests that would lead to revealing an idea’s
potential in the real world. Theoretical approaches were used in the creation of the design.
The tests were meant to show its empiric potential.
Measured wind speeds
The measured wind speed was not surprising in any way. This was the easiest test to execute
and the results were only intended to give an idea of the relation between wind speeds on
different parts of the building. As mentioned earlier, speed can alter the air’s density.
Therefore, knowing the speed can give you an indication of the differences in pressure.
Observing the different velocities, especially the fact that it was greater on the long side,
gave me a confirmation of the pressure difference that was intended by the design.
Smoke tests
Since the smoke is, and acts, as a part of the air, its introduction in the stream does not alter
it. This, and the fact that smoke is easy to spot, makes it ideal to help observe the air’s
actions in a certain area. I induced the smoke close to the ventilation pipe and observed its
path (fig 12). The fact that the smoke did not follow the wall and instead passed through the
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pipe clearly showed the design’s success in provoking certain actions. The question was with
which strength and potential.
Wind streams can be very small in diameter, but their impact on the surrounding air is not
necessarily small. From observing the floor full of smoke and observing how the entering air
stream altered the existing picture, I could easily see the system’s effectiveness. As we know
from before, different pressured areas tend to compensate each other. Inside the building
there were two very differently pressured areas: the air stream which had a low pressure,
and the surrounding air that had a normal pressure. The surrounding air rushed into the
stream to compensate for the difference in pressure, while the fresh air slid around this
stream to fill its holes (fig 16). An appropriate question to ask is why the two contrary flows
did not mix with each other. It can be explained by the indestructibility of matter, one of the
main pillars in fluid mechanics. I saw that this was the case, since the fresh air first filled the
extremities of the building. They first mixed when both areas where submerged to the same
conditions, and this only happened in the boundary layer.
I knew the volume of the floor when filled with smoke and let the ventilation system remove
it. With this I showed how the system replaced the used air. This is the whole meaning of a
ventilation system, and my design certainly fulfilled this criteria. It was because of the known
volume and the measured time it took to remove the smoke, that I could calculate an
estimated air change rate.
Executing the project
A question difficult to answer, but which must be considered, is whether or not this would
be a practical design to be built and used?
First of all, I must say that I have never intended to make a perfect real-life building design,
only a concept. Concepts tend to have the quality of not considering all the practical factors
that would be necessary, and my design is no exception. Because of this, before applying it
in the real world, the model would need to go through some adjustments. According to my
measures, there is a considerable force on the building that tries to bend it over. In stronger
winds or hurricanes, these forces could be able to do so. Cutting down on the buildings
height would help, but this could jeopardize its performance. Another solution would be
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splitting the building into smaller modules performing opposite to one other. This would
lead to a stabilizing of the forces on both sides, taking stress away from the structure.
In Dubai there is a module based “green” skyscraper being constructed and which will be self
efficient in terms of energy. This building, designed by the Dynamic Architecture Group and
David Fisher, is also the world’s first building in which each module will be able to rotate
separately around a central ax. Combining my design with this technology would enable the
building to always get the best wind conditions. In addition it would make the building much
more sculptural and interesting.
In my tests I used a plain ground, but in the real world one would have to look at its
potential in urbanized areas. In such areas, the surrounding buildings would alternate and
create turbulence in the wind, compromising the airflow’s strength and direction. Such
constructions could also increase the building’s potential, since studies show that winds
often are stronger inside cities. This is due to the buildings’ heat absorbance and larger built
areas’ ability to direct the wind.
Many of the problems that my design could confront are already being confronted and
solved in existing buildings. New and more efficient building materials are nowadays being
developed at a fast rate. The first buildings made of concrete which is ten times stronger
than its ancestor are already being erected. This concrete also tries to eliminate some of the
toxics found in normal concrete. Smart paints are being currently perfected. It is possible
today to paint an air cleaning coat on any building. Such coats can eliminate nitrogen oxide,
or even produce solar energy. There are also heat sensitive thermochromic paints which
change color during the seasons, always maintaining an indoor climate as stable as possible.
These “chameleon paints” can actually alter the temperature as much as 4.5 degrees Celsius,
eliminating much of the building’s energy usage for heat and cooling. (Hosley 2009)
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Conclusion
I asked myself in the introduction: “Considering ventilation as a basic function in any
building, how can a building’s form interfere and eventually solve the need for ventilation? Is
it possible to create a natural system of ventilation that is solely based on the building’s
shape?” After going through researching, experimenting and the process of writing this
paper, I believe that it is possible to give a positive answer to these questions.
My design provoked a considerable air change rate, and all though the conditions in the
experiments were not ideal, I was surprised by the systems efficiency. The model was only
built with a supporting frame and a thin shell. This shell imitated the buildings shape, and it
was this shape that provoked the air change inside. Both through my theoretical approach to
aerodynamics and my empirical approach to the design in test, I managed to show the
systems efficiency. I was able to succeed in my goal of integrating a specific function in a
shape.
It was very interesting to see how my knowledge of physics, more specifically the science of
fluids and aerodynamics, could serve as basis for a practical consideration of air renewal in a
given building. For me, personally, this work has opened my eyes for the importance of a
building’s shape, not simply as an esthetical matter, but also as a way to accomplish some of
its basic functions. In that sense I can see how the theoretical knowledge of physics can not
only serve as a basis for an architect’s work, but also as a source of inspiration. It is easy to
understand how the physical laws of mechanics restrict a building’s shape, but now I have
also observed how other aspects of physics can add new dimensions to its form and
function.
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Acknowledgements
I would like to recognize the invaluable contribution of Halvor Egner as a mentor throughout
the process of making this project, in special through the early stages of determining the
subject matter of the paper.
I would also like to thank my father, Gustavo Tavares, for the massive contribution on
questioning my ideas and revising my final texts.
I would also like to recognize the assistance given to me by Øyvind Thormodsæter, as my
English teacher, and Hans Marius T. Jacobsen for helping me in the search for relevant
literature. Thanks also to all my fellow students that assisted me in my experiments and
served as good companions at the lab.
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