MODULE 3

Natural ventilation and air movement: Air movement around and through buildings, Orientation for
wind, stack effect, Induced ventilation.

Natural ventilation and Air movement

Air, the atmosphere that surrounds the earth, is a mixture of gases. Air movement is a change in
position of air regardless of cause or degree.

Air flow is the movement of air from one area to another. The primary cause of airflow is the
existence of pressure gradients. Air behaves in a fluid manner, meaning naturally flow from areas of
higher pressure to those where the pressure is lower.

Wind is the natural form of air movement, usually but not always the movement is horizontal.

Ventilation is a method of controlling the environment with air flow. It is the process of supplying
unconditioned or conditioned air to and removing it from a given space by any method. For
example, when wind enters a bedroom, it becomes ventilation, but it is always air movement.

Ventilation is classified by the force acting on the air.

1. Natural ventilation is the process of supplying and removing air through a space by natural
means it can be achieved with openable windows or trickle vents.
2. Induced ventilation depends on influencing natural forces to perform specific tasks as in
thermal chimney (eg: stack ventilation)
3. Artificial ventilation: depends on mechanical methods.

FUNCTIONS OF VENTILATION

It has three distinctly different functions:

1 . SUPPLY OF FRESH AIR

2 . CONVECTIVE COOLING

3. PHYSIOLOGICAL COOLING

1.SUPPLY OF FRESH AIR

• The requirements of fresh air supply are governed by the type of occupancy, number and
activity of the occupants and by the nature of any processes carried out in the space.

• Requirements may be stipulated by building regulations and advisory codes in terms of m

3/h person, or in number of air changes per hour, but these are only applicable to
mechanical installations.

• Nevertheless, they can be taken as useful guides for natural ventilation. The aim of all these
rules is to ensure ventilation, but the rigid application of such rules may often be
inadequate.
 • For natural ventilation usually certain limited solutions are prescribed and not the expected
performance.

• The provision of 'permanent ventilators', i.e. of openings which cannot be closed, may be
compulsory.

• These may be grilles or 'air bricks‘ built into a wall, or may be incorporated with windows.

• The size of openable windows may be stipulated in relation to the floor area or the volume
of the room.

2. CONVECTIVE COOLING

The exchange of indoor air with fresh out-door air can provide cooling, if the latter is at a lower
temperature than the indoor air. The moving air acts as a heat carrying medium

3.PHYSIOLOGICAL COOLING

The movement of air past the skin surface accelerates heat dissipation in two ways:

1. Increasing convective heat loss

2. Accelerating evaporation

Cooling by air movement is most needed where there are no other forms of heat dissipation
available, when the air is as warm as the skin and the surrounding surfaces are also at a similar
temperature.

STACK EFFECT
stack ventilation can be defined as the upward movement of air through openings in a building
fabric due to thermal buoyancy

• As air gets warmer it becomes less dense and so more buoyant. This means that warm air
has a tendency to rise.
• The stack effect relies on thermal forces, set up by density difference (caused by
temperature differences) between the indoor and out-door air.
• It can occur through an open window (when the air is still): the warmer and lighter indoor air
will flow out at the top and the cooler, denser outdoor air will Flow in at the bottom.
• The principle is the same as in Wind generation.
• A positive pressure area is created at the top of a building and negative pressure area at the
bottom. This process can take place without mechanical assistance, simply by introducing
openings at the bottom and the top of buildings. It is known as the stack effect or stack
ventilation.

Generally, the funnel effect describes the phenomenon where winds will tend to increase in speed as they squeeze
through narrow valleys and it can be explained by Bernoulli's theorem. According to this, the pressure is least where
velocity is greatest and in the same way pressure is greatest where velocity is least.
Special provision can be made for it in the form of VENTILATING SHAFTS. The higher the shaft, the
larger the cross-sectional area and the greater the temperature difference: the greater the motive
force therefore, the more air will be moved. Such shafts are often used for the ventilation of
internal, windowless rooms (bathrooms and toilets).
The motive force is the 'stack pressure' multiplied by the cross-sectional area (force in Newtons–
area in m²).

The stack pressure can be calculated from the equation:

describe the strategies in which induced ventilation in courtyards

Stack Effect in Traditional Building helps in providing thermal comfort in warm humid climate
Due to the incident solar radiation in the courtyard, the air in the courtyard becomes warmer and rises
up. To replace it, cool air from the ground level flows through the louvered openings of the room, thus
producing the air flow. During the night the process is reversed. This concept can very well be applied
in a warm and humid climate.

At night, the warm roof surfaces get cooled by convection and radiation.

If the roof surfaces are sloped towards the internal courtyard, the cooled air sinks into the court and
enters the living space through low-level openings, gets warmed up, and leaves the room through
higher-level openings.
Air movement at Meso Climatic Scale
Internally Air movement is driven by buoyancy and pressure differentials and modified by inertia and
friction. Externally, Landforms , vegetation and buildings influence air movement by altering the
velocity and pattern of air flow.

General Principles

The flow of patterns of air movement fall into three categories.

(a) Laminar

(b) Turbulent

(c) separated

As air flows from one location to another , Inertia , friction and differential affect the movement.
THE EFFECT OF LAND FORMS

• Landforms deflect and impede the air flow.

• They changing the pattern and reduce its velocity while influencing its quality and quantity.

• Landform impact varies in Intensity and degree.

• Velocity of air movement is altered by topography.

THE EFFECT OF VEGETATION

• Air movement can be controlled by vegetation properly selected and placed .

• Filtration, reflection, guidance and /or obstruction of air flow can be provided.

• It can be even reduced or accelerate air movement around the buildings.

• Thereby reduce the energy demand of the building

• The effectiveness of controlling air movement depends on the form, density, rigidity,
location or position and other vegetation characteristics.

• Vegetation creates a frictional drag on the air flow.

• The vegetation with optimum density can reduce the velocity along the ground as much as
70%.

THE EFFECT OF BUILDINGS

• Structures deflect, obstruct and guide the movement of air as well as reduce and accelerate
the airflow velocity.

• The intensity and magnitude of the influence of buildings on air movement vary with
structures heights, widths, lengths and forms.

• Air movement deflects over and around buildings, distinct air patterns are established

•
• The leeward calm area is determined by the height, width and windward face of the
building.

Air movement at Micro Climatic Scale

Effect of building form
Effect of Height
Effect of Roofs

Effect of adjacent buildings

Factors affecting indoor air flow (both patterns and velocities):
1. Orientation

2. External features

3. Cross-ventilation

4. Position of openings

5. Size of openings

6. Controls of openings

Orientation

The greatest pressure on the windward side of a building is generated when the elevation is
at right angles to the wind direction, so it seems to be obvious that the greatest indoor air
velocity will be achieved in this case.

•A wind incidence of 45° would reduce the pressure by 50%.

•Thus, the designer must ascertain the prevailing wind direction from wind frequency
charts of wind roses and must orientate his building in such a way that the largest openings
are facing the wind direction.

•It has, however, been found by Givoni that a wind incidence at 45° would increase the
average indoor air velocity and would provide a better distribution of indoor air movement.

Figure a show the outline of air flow at 90 ° and Figure b at 45 °, to a building square in plan.
In the second case a greater velocity is created along the windward faces, therefore the
wind shadow will be much broader, the negative pressure (the suction effect) will be
increased and an increased indoor air flow will result.
EXTERNAL FEATURES

Wind shadows created by obstructions upwind, should be avoided in positioning the

building on the site and in positioning the opening in the building.

Building structures can redirect prevailing winds to cross-ventilation

•External features of the building itself can strongly influence the pressure build-up.

•For example, if the air flow is at 45◦ to an elevation, a Wing Wall at the downwind end or a
projecting wing of an L-shaped building can more than double the positive pressure created.

•A similar funnelling effect can be created by upward projecting eaves. Any extension of the
elevational area facing the wind will increase the pressure build-up

CROSS VENTILATION

When placing ventilation openings, inlets and outlets are placed to optimize the path air
follows through the building. Windows or vents placed on opposite sides of the building give
natural breezes a pathway through the structure. This is called cross-ventilation. Cross-
ventilation is generally the most effective form of wind ventilation.

It is generally best not to place openings exactly across from each other in a space. While
this does give effective ventilation, it can cause some parts of the room to be well-cooled
and ventilated while other parts are not. Placing openings across from, but not directly
opposite, each other causes the room's air to mix, better distributing the cooling and fresh
air.

POSITION OF OPENINGS

To be effective, the air movement must be directed at the body surface. In building terms
this means that air movement must be ensured through the space mostly used by the
occupants: through the 'living zone' (up to 2 m high). As Figure shows, if the opening at the
inlet side is at a high level, regardless of the outlet opening position, the air flow will take
place near the ceiling and not in the living zone.
 Placing inlets low in the room and outlets high in the room can cool spaces more effectively,
because they leverage the natural convection of air. Cooler air sinks lower, while hot air
rises; therefore, locating the opening down low helps push cooler air through the space,
while locating the exhaust up high helps pull warmer air out of the space. This strategy is
covered more on the stack ventilation.

Effect of parapet wall:

SIZE OF OPENINGS

Window or louver size can affect both the amount of air and its speed.

•For an adequate amount of air, one rule of thumb states that the area of operable
windows or louvers should be 20% or more of the floor area, with the area of inlet openings
roughly matching the area of outlets.

•However, to increase cooling effectiveness, a smaller inlet can be paired with a larger
outlet opening.

•With this configuration, inlet air can have a higher velocity.

•Because the same amount of air must pass through both the bigger and smaller openings
in the same period of time, it must pass through the smaller opening more quickly

• With a given elevational area – a given total wind force (pressure x area) – the largest air
velocity will be obtained through a small inlet opening with a large outlet.

• This is partly due to the total force acting on a small area, forcing air through the opening at
a high pressure and partly due to the ‘Venturi Effect’: in the broadening funnel (the
imaginary funnel connecting the small inlet to the large outlet) the sideways expansion of
the air jet further accelerates the particles.

• Such an arrangement may be useful if the air stream is to be directed (as it were focused) at
a given part of the room
CONTROLS OF OPENINGS

• Sashes, canopies, louvres and other elements controlling the openings, also
influence the indoor air flow pattern.
• Sashes can divert the air flow upwards. Only a casement or reversible pivot sash will
channel it downwards into the living zone

Canopies can eliminate the effect of pressure build - up above the window, thus the
pressure below the window will direct the air flow upwards. A gap left between the building
face and the canopy would ensure a downward pressure, thus a flow directed into the living
zone

Louvres and shading devices may also present a problem. The position of blades in a slightly
upward position would still channel the flow into the living zone (up to 20° upwards from
the horizontal)

Fly screens or mosquito nets are an absolute necessity not only in malaria infested areas, but
also if any kind of lamp is used indoors at night. Without it thousands of insects would
gather around the lamp. Such screens and nets can substantially reduce the air flow.

A cotton net can give a reduction of 70% in air velocity. A smooth nylon net is better, with a
reduction factor of only approximately 35%. The reduction is greater with higher wind
velocities and is also increased with the angle of Incidence.

Sashes canopy louvers