CLIMATE AND BUILT ENVIRONMENT

BY MADHUMATHI HINDUSTAN UNIVERSITY

• • • • •

Wind rose Wind shadows Air movement around and through buildings Stack effect Thermally induced Air currents

1/24/2015

UNIT 4 – Air movement

Air Movement and Natural Ventilation Factors influencing

Ventilation • VENTILATION is the process by which fresh air is introduced and used air is removed from an occupied space. It also provides cooling by air movement. • An indoor air speed of 1.5 – 2.0 m/s can cause comfort in warm and humid regions where the outdoor maximum air temperature does not exceed 28 – 32 C. • The primary aim of ventilation is to preserve the qualities of air. • Sometimes, ventilation may also be used to lower the temperature inside an occupied area.

Types of ventilation • Natural ventilation • Mechanical ventilation

Natural ventilation • Natural ventilation is the process of supplying and removing air by means of purpose-provided aperture (such as openable windows, ventilators and shafts) and the natural forces of wind and temperature-difference pressures.

Types of natural ventilation Natural ventilation may be divided into two categories • Controlled natural ventilation is intentional displacement of air through specified openings such as windows, doors, and ventilations by using natural forces (usually by pressures from wind and/or indoor-outdoor temperature differences). It is usually controlled to some extent by the occupant.

Types of natural ventilation • Infiltration is the uncontrolled random flow of air through unintentional openings driven by wind, temperaturedifference pressures and/or appliance-induced pressures across the building envelope. In contrast to controlled natural ventilation, infiltration cannot be so controlled and is less desirable than other ventilation strategies, but it is a main source of ventilation in envelope-dominated buildings.

Mechanical ventilation Mechanical or forced ventilation is the process of supplying and removing air by means of mechanical devices, such as fans. It may be arranged to provide either supply, extract or balanced ventilation for an occupied space.

Purposes of ventilation • Provide sufficient supply of air/oxygen for the physiological needs of human beings (a minimum of 0.2 l/s/person is required for breathing purpose) and/or livestock; • Remove the products of respiration and bodily odour (including those from smoking) of human and/or animal occupants; • Remove contaminants or harmful chemicals generated by processes or from building materials; • Remove heat generated by people, lighting and equipment inside the occupied space; • Create some degree of air movement which is essential for feelings of freshness and comfort (usually a velocity of 0.1 to 0.3 m/s is required). • 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. • Physiological cooling - Cooling the environment of living

Principles of Natural Ventilation For air to move into and out of a building, a pressure difference between the inside and outside of the building is required. The pressure difference is caused by: • Wind (or wind effect); • Difference in air density due to temperature difference between indoor and outdoor air (stack or chimney effect); or • Combination of both wind and stack effects.

Wind shadow • When moving air strikes an obstacle such as a building, this will slow down the air flow but the air flow will exert a pressure on the obstructing surface. • This slowing down process effects a roughly wedge-shaped mass of air on the windward side of the building, which in turn diverts the rest of the air flow upwards and sideways. • A separation layer is formed between the stagnant air and the building on the one hand and the laminar air flow on the other hand. • The laminar air flow tends to maintain a straight path after it has been diverted, • A stagnant mass of air is formed on the leeward side, but this is at a reduced pressure. The movement is light and variable and is often referred to as ‘wind shadow’.

Air movement around buildings

Wind effect

• When air flow is due to wind, air enters through openings in the windward walls, and leaves through openings in the leeward walls

Wind flow Wind pressures are generally high/positive on the windward side of a building and low/negative on the leeward side. The occurrence and change of wind pressures on building surfaces depend on: 1. wind speed and wind direction relative to the building; 2. the location and surrounding environment of the building; and 3. shape of the building.

Wind induced pressure differences • Positive pressure is created on the building sides that face the wind (windward sides) whereas suction regions are formed on the opposite sides (leeward sides) and on the side walls. • This results in negative pressure inside the building, which is sufficient to introduce large flows through the building openings. • In a general case, n airflow of air is induced on the windward side and an outflow on the leeward side. Airflow through an external opening is mainly attributed to a wind induced pressure difference across it.

• When the architect’s task is the design of more than one building, a cluster of buildings or a whole settlement, especially in a warm climate,provision for air movement must be one of the most important considerations • If there are tall blocks in mixed developments air stream separates on the face of a tall block • - part of it moving up and over the roof • - part of it down, to form a large vortex leading to a very high pressure build-up. • An increased velocity is found at ground level at the sides of the tall block. This could serve a useful purpose in hot climates, although if the tall block is not fully closed but is permeable to wind, these effects may be reduced.

Air stream separation at the face of buildings

• If a low building is located in the wind shadow of a tall block, the increase in height of the obstructing block will increase the air flow through the low building in a direction opposite of that wind. • The lower (return-) wing of a large vortex would pass through the building.

Reverse flow behind a tall block

• If in a rural setting in open country, single storey buildings are placed in rows in a grid-iron pattern, stagnant air zones leeward from the first row will overlap the second row. • A spacing of six times the building height is necessary to ensure adequate air movement for the second row. • Thus ‘five times height’ rule for spacing is not quite satisfactory

Air flow grid-iron layout

• In a similar setting, if the buildings are staggered in a checker-board pattern, the flow field is much more uniform, stagnant air zones are almost eliminated.

Air flow checker-board layout

Landscape and wind flow

AIR MOVEMENT Through the buildings

Factors influencing air movement through buildings • • • • •

ORIENTAION CROSS VENTIALTION SIZE OF OPENINGS POSITION OF OPENINGS CONTROL OF OPENINGS

O r i e n t a t i o n

• The designer must ascertain the prevailing wind direction from wind frequency charts of wind rose and must orient his building in such a way that the largest openings are facing the wind direction. • It has, however, been found that a wind incidence at 45⁰ would increase the average indoor air velocity and would provide a better distribution of indoor air movement. See fig below

• It often happens that the optimum solar orientation and the optimum orientation for wind do not coincide. • In equatorial regions a north-south orientation would be preferable for sun exclusion but most often the wind is predominantly easterly. • The usefulness of the above findings is obvious for such a situation – it may resolve the contradictory requirements.

Effect of direction on width of wind shadow

Cross ventilation • Cross-ventilation utilises differential wind pressure. • When the air outside is cooler, windows on opposite sides of the home can be opened. • Cool air enters on the windward side and passes out on the other side, replacing warm inside air with cool outside air.

Causes worse condition

• With a windward opening and no outlet, a pressure similar to that in front of the building will be built up indoors, which can make condition even worse, increasing discomfort. • In some cases oscillating pressure changes, known as ‘buffeting’ can also occur which may also be produced by an opening on the leeward side only, with no inlet.

Size of openings Inlet & Outlet Areas • In both cross and stack ventilation, the amount of heat removed from a building is directly proportional to the inlet and outlet areas. • The larger the inlet and outlet areas, the more air can travel through the building and the more heat can be removed.

• Increasing the inlet and outlet area increases ventilation.

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 2m high).

Position of openings

Larger solid surface creates a larger pressure build-up and this pushes the air stream in an opposite direction, both in plan and in section.

Position of openings • In a two storey building (fig below) the air flow on the ground floor may be satisfactory but on the upper floor it may be directed against the ceiling. • One possible remedy is an increased roof parapet wall.

Control of openings

Control of openings

Control of openings • Sashes can divert the air flow upwards. Only a casement or eversible pivot sash will channel it downwards into the living zone .ref fig

• 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

Control of openings • 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 20⁰ upwards from the horizontal)

• Mosquito 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 when the angle of incidence.

Air movement and rain: • Opening of windows during periods of wind-driven rain would admit rain and spray; while closing the windows would create intolerable conditions indoors. • Verandahs and large roof overhangs are perhaps the best tradition methods of protection. • Only type ‘M’ was found to be capable of keeping out water at wind velocities up to 4m/s and the same time ensuring a horizontal air flow into the building.

Air movement through buildings

Effect of window width on wind speed

Inner obstructions

Internal obstructions • Air must be free to move from inlet to outlet. The two are not very useful if there is a wall between them. In addition, the cross sectional area of the paths from inlet to outlet should be at least the area of the smaller of the inlet and outlet.

• A clear path between inlet and outlet must be maintained.

STACK EFFECT

Stack effect • When air movement is due to temperature difference between the indoor and outdoor, the flow of air is in the vertical direction and is along the path of least resistance. • The temperature difference causes density differentials, and therefore pressure differences, that drive the air to move.

How stack effect occurs • The stack effects relies on thermal forces, setup by density difference (caused by temperature differences) between the indoor and out-door air. • The principle is the same as in wind generation. • The indoor temperature is higher than outdoor temperature; • The warmer air in building then rises up; • The upward air movement produces negative indoor pressure at the bottom; • Positive indoor pressure is created on the top; • Warmer air flows out of the building near the top; and • The air is replaced by colder outside air that enters the building near its base.

• 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. • Therefore, the more air will be moved.

Design strategies for air movement • The following guidelines are important for planning and designing natural ventilation systems in buildings: • A natural ventilation system should be effective regardless of wind direction and there must be adequate ventilation even when the wind does not blow from the prevailing direction; • Inlet and outlet openings should not be obstructed by nearby objects; • Windows should be located in opposing pressure zones since this usually will increase ventilation rate; • A certain vertical distance should be kept between openings for temperature to produce stack effect; • Openings at the same level and near the ceiling should be avoided since much of the air flow may bypass the occupied zone;

• Architectural elements like wing walls, parapets and overhangs may be used to promote air flow into the building; • Topography, landscaping, and surrounding buildings should be used to redirect airflow and give maximum exposure to breezes; • In hot, humid climates, air velocities should be maximised in the occupied zones for bodily cooling; • To admit wind air flow, the long façade of the building and the door and window openings should be oriented with respect to the prevailing wind direction; • If possible, window openings should be accessible to and operable by occupants; • Vertical shafts and open staircases may be used to increase and generate stack effect; • Openings in the vicinity of the neutral pressure level may be reduced since they are less effective for thermally induced ventilation; • If inlet and outlet openings are of nearly equal areas, a balanced and greater ventilation can be obtained.

HUMIDITY CONTROL • Dehumidification is only possible by mechanical means; without this, in warm-humid climates, some relief can be provided by air movement. • In hot-dry climates humidification of the air may be necessary, which can be associated with evaporative cooling. • In these climates the building is normally closed to preserve the cooler air retained within the structure of high thermal capacity, also to exclude sand and dust carried by winds. • However, some form of air supply to the building interior is necessary. • All these functions: 1. Controlled air supply 2. Filtering out sand and dust 3. Evaporative cooling 4. Humidification are served by a device used in some parts of Egypt – the wind scoop.

WIND SCOOP • The large intake opening captures air movement above the roofs in densely built up areas. • The water seeping through the porous pottery jars evaporates, some drips down onto the charcoal placed on a grating, through which air is filtered. • The cooled air assists the downward movement – a reversed stack effect. • This device is very useful for ventilation , but It cannot be expected to create an air movement strong enough for physiological cooling.