Previous - Next
This is the old United Nations University website. Visit the new site at http://unu.edu
5. The wind factor in air movement
Air movement by pressure differential
The
Claustrum
The
wind-escape
The malgaf
The bãdgir
When skin is wet with perspiration and is exposed to the air with a dew point below skin temperature, the perspiration evaporates. The skin temperature is lowered because energy is needed to convert the perspiration into water vapor. However, the air in contact with the skin soon becomes saturated, and evaporation stops. For the evaporation process to continue, this air must be removed either mechanically, using a fan, e.g., or naturally by air movement and drafts.
The architectural design can ensure such natural air movement through two principles. In the first, differences in wind velocity produce a pressure differential which results in air flowing from the higher to the lower air pressure region. In the second, air is warmed, causing convection, with the warm air rising and being replaced by cooler air. A cool draft is created in the space between the warm area and the cool-air intake opening. The rate of airflow caused by convection in buildings is determined by the difference in the level of openings, with greater airflow resulting from a greater difference in the heights of the openings. It is most important when the outside air is still and yet the interior requires ventilation to achieve comfort. Both these principles have been used in architectural design and town planning in many ways using several innovations. Air movement by pressure differential and cooling systems based primarily on this principle will be discussed in the present chapter, while the following chapter will concentrate on the air movement by convection which requires the effect of the sun.
Air movement by pressure differential
An important concept in understanding how wind-generated pressure differentials produce air movement is "Venturi action," which is based on the Bernoulli effect. From Bernoulli's theorem, the pressure of a moving fluid decreases as its velocity increases. Figure 34 shows a funnel-shaped tube that opens to a side tube. When air is channeled into the larger end of the funnel, it accelerates as it passes through, owing to the reduced open area through which the same volume of air must pass in the same period. This increased airspeed lowers the pressure in the airstream at A with respect to the atmospheric pressure at B in the lower part of the side tube. Thus air is drawn up the side tube by the pressure difference which is proportional to the square of the velocity. This concept can be used in a variety of ways to provide steady streams of air through buildings.
For indoor air movement caused by a pressure differential, the airflow is steadier in cases that depend more on the suction resulting from low air pressure than on the high air pressure caused by wind force. Obviously, a window or an opening will not create the desired air movement in a room unless an air outlet of some sort is also provided. Experience has shown that air movement is faster and steadier when the area of the openings on the leeward side of a structure is larger than the inlets on the windward side.
An important example is illustrated by the loggia in a guest house in Gourna village near Luxor, Egypt, shown in figure 35. Even on an uncomfortably hot day, the shaded area of the loggia is provided with a cool and refreshing breeze, a result of intelligent architectural design following the principles of aerodynamics. The loggia opens onto a courtyard on the leeward side and is nearly closed to the prevailing wind by a wall pierced with two rows of small openings. The airflow over and around the building produces a zone of low pressure on the leeward side, and thus inside the loggia as a result of the Bernoulli principle. This ensures steady airflow due to suction through the small openings. Figure 36 shows schematically the airflow and pressure changes for this loggia. Variations of this effective method of climatisation are widely used for many types of buildings in the hot arid regions. This example shows that a detailed analysis of the aerodynamic lines of air movement is essential to a clear understanding of how architectural devices can ensure optimized thermal comfort.
Other applications of this principle can provide valuable practical information. In the region of Al-Hilla in Iraq, the villagers adopted the arrangement for creating air movement by suction shown in figure 37. However, the inlet vents on the windward side are placed low. The reason for this is that the indoor space is used for sleeping when the roof is unsuitable, and the air temperature near the ground drops considerably at night. By placing the door, which is considerably larger in area than the inlet vents, on the leeward side, a draft is created by suction, causing air to flow through the room at the level of the sleepers. In addition, with the top of the inlet vents considerably lower than the top of the door, the hot air escaping through the open door is accelerated by convection and replaced by cooler air drawn in through the inlet vents.
Vents also can be used as outlets for hot air. An example can be seen in the exterior of a traditional building in Najd, Saudi Arabia, shown in figure 38. Here the triangular vents are positioned on the wall just under the roof to evacuate hot air collected in the higher parts of the room by convection. The air passing through these outlet vents is then replaced with air drawn from cooler parts of the building.
Often a multitude of small vents is preferable to a few large openings for purposes of privacy, security, uniform distribution of air flow, blocking of direct solar rays, and decoration. Large openings, used mainly for ventilation and lighting and set at specific places in the building, can then be filled with lattice work, in the form of a pierced screen wall. These lattices, called claustra, were originally used in large openings at high levels in the Roman baths. In vernacular architecture, they generally are made in different decorative patterns of carved plaster plates, unlike the mashrabiya which are wooden. Claustra are mainly used to evacuate the hot air collected in the higher parts of the room, or in parapet walls, the low walls around roof edges, to produce drafts over people sleeping on the roofs in summer. Examples of various claustra designs are shown in figures 39 and 40, from Dubai, United Arab Emirates, and figures 41 and 42, from Oman.
In modern architecture, claustra are sometimes used inappropriately over the entire facade of a building to serve as a brise-soleil. In fact, the claustrum is a screen to be set in an opening of proper size and should not be used as a bearing wall. In extending it beyond its frame and scale to cover an entire facade, the structural scale and aesthetic rules of architecture are disturbed. Furthermore, when claustra are set at eye level, they annoy the eye with dazzling contrasts of light and shade, resulting from the inappropriate relative and absolute sizes of the solid and void lattice components and the lack of graduation caused by the rectangularity of the bars. When a claustrum is used as a brise-soleil, it shares with the latter many defects which are overcome by the mashrabiya. Figure 43 illustrates inappropriate use of a claustrum in a facade in Kuwait. However, the claustrum is effective at eye level in infrequently used indoor spaces, such as in a staircase wall, or in outdoor spaces, like courtyards or roofs, where the play of light and shade does not dazzle the eye when looking outward.
The technique of using the suction caused by low air-pressure zones to generate steady air movement indoors is used in the design of the windescape. The funnel and side tube used to illustrate the Bernoulli effect or Venturi action (see fig. 34) are transposed into the structural elements of an architectural design in order to accelerate air movement and to create drafts in places with no exposure to the outside, such as basements in Iraq.
An interesting example occurred by accident in the design of a pump room for an artesian well in Alexandria, Egypt. The pump room was located about 6 meters below ground level because the underground water level was 12 meters deep. The room had an opening overlooking the well for the passage of piping and for inspection, and it was covered with a slanting-vault roof with the higher end toward the leeward side, as shown in figures 44 and 45. It was feared that the pumpengine exhaust gases would pollute the air in this very small chamber. However, the vaulted-roofing arrangement of the pump room created a strong air current, which drew air through the wellshaft opening at ground level.
This concept can be applied more advantageously in designs for use above ground. The wind-escape can accelerate effective ventilation and air circulation when used with other devices for air movement such as windows, doors, and the malqaf or wind-catch, described in detail below.
In hot arid zones, a difficulty is found in combining the three functions of the ordinary window: light, ventilation, and view. If windows are used to provide for air movement indoors, they must be very small, which reduces room lighting. Increasing the size to permit sufficient lighting and an outside view lets in hot air as well as strong offensive glare. Therefore, it is necessary to satisfy the three functions ascribed to the window separately.
To satisfy the need for ventilation alone, the malqaf or wind-catch was invented. This device is a shaft rising high above the building with an opening facing the prevailing wind. It traps the wind from high above the building where it is cooler and stronger, and channels it down into the interior of the building. The malqaf thus dispenses with the need for ordinary windows to ensure ventilation and air movement. The malqaf is also useful in reducing the sand and dust so prevalent in the winds of hot arid regions. The wind it captures above the building contains less solid material than the wind at lower heights, and much of the sand which does enter is dumped at the bottom of the shaft.
The value of the malqaf is even more obvious in dense cities in warm humid climates, where thermal comfort depends mostly on air movement. Since masses of buildings reduce the wind velocity at street level and screen each other from the wind, the ordinary window is inadequate for ventilation. This situation can be corrected by using the malqaf.
The malqaf is much smaller than the building facade and therefore offers less surface area to screen the malqaf of buildings downwind. The example shown in figure 46 is from Sind, Pakistan, where the malqaf is universally used and can be seen rising above the houses like sails capturing the wind.
In Egypt the malqaf is very developed and has long been a feature of vernacular architecture. The excellent example of the Qã'a of Muhib AdDin Ash-Shãf'i Al-Muwaqqi, known as Othmãn Katkhudã, in Cairo dates from the fourteenth century A.D. The plan and a section of this qã'a are shown in figures 47 and 48.
The qã'a is a central upper-story room for receiving guests, usually a living room in a residence or a meeting room in a formal hall. It is traditionally composed of three connected spaces: a central part called the dur-qã'a, an uncarpeted high-roofed circulation area which provides light and ensures ventilation; and two closed, raised, and carpeted recesses called iwãnãt (singular: iwãn). The walls of the qã'a, being very high, are stiffened by buttresses to provide rigidity with lightness of structure. The spaces between these buttresses are used as sitting alcoves called kunja. The floors of the kunja are usually more elevated than the adjacent spaces, the dur-qã'a and iwãn. Access to the qã'a is through the dur-qã'a, which is in fact a covered courtyard or sahn that has retained the paved floor and marble mosaics characteristic of an open courtyard.
A simplified section through the Qã'a Muhib Ad-Din is shown in figure 49. This example demonstrates the operation of the malqaf as part of a complete climatization system. As shown, the malqaf is a large shaft rising high above the roof of the northern iwãn. If an appreciable amount of air is to flow into the malqaf, a wind-escape must be provided, and, as for the loggia, airflow will be faster if the air can be strongly drawn out through the air escape by suction. The system of climatization developed depends primarily on air movement by pressure differential, but also secondarily on air movement by convection, producing the stack effect (discussed in more detail below). The ceiling of the dur-qã'a rises far above the ceilings of the iwãnãt and is equipped with high clerestory windows in its upper structure which are covered with mashrabiya. In addition to diffused and agreeable lighting, these openings provide the required air escape. Thus the malqaf in the northern iwãn channels the cool breeze from the north down into the qã'a, due to the increased air pressure at the entrance of the malqaf caused by the wind. Once inside the iwãn, the air slows down, flows through the iwãn, rises into the upper part of the dur-qã'a, and escapes through the mashrabiya. Outside wind blowing over the dur-qã'a is accelerated owing to the shape of the durqã'a roof. From the Bernoulli or Venturi-action effect, the air pressure in the outside wind is lower than that in the qã'a. Air from the region of the dur-qã'a escapes into the wind, to be continuously replaced by inside air. Thus, complete circulation through the qã'a is effected.
Figure 49 shows the results of airflow-rate and direction measurements made on 2 April 1973 by scholars from the Architectural Association School of Architecture in London, which substantiate the airflow pattern described. The lengths of the arrows in the figure are proportional to the measured airspeeds, some of which are indicated in units of meters per speed.
But this is not the entire situation. Convection is also important because warm air in the qã'a rises naturally to the upper part of the durqã'a. This air movement is accelerated because the flat upper part of the qã'a is exposed to the sun. The upper air inside it heats even more, rises even faster into the upper part of the dur-qã'a, and thus escapes through its mashrabiya openings. Heating the air in the upper part of the qã'a does not disturb the thermal comfort due to its extremely high ceiling. Air is drawn from below and ultimately from the malqaf, which contributes toward the total air movement. In fact, this arrangement of openings ensures the circulation of air indoors even when the air outside is still. Thus, it is important that the qã'a is placed in the middle of the building and surrounded by rooms that protect the sides from external heat, thus ensuring a maximal temperature difference between the lower and upper parts of the qã'a to promote air circulation.
The idea of the malqaf dates back to very early historical times. It was used by the ancient Egyptians in the houses of Tal Al-Amarna and is represented in wall paintings of the tombs of Thebes. One example, shown in figure 50, is the Pharaonic house of Neb-Amun depicted on his tomb, which dates from the Nineteenth Dynasty (1300 B.C.). It has two openings, one facing windward and the other leeward, to evacuate the air by suction. It is interesting to find the same concept applied in the modern design of the workshop at the University of Science and Technology in Kumasi, Ghana, as shown in figure 51, where a Y-beam system is used for routing the air circulation.
The malqaf can be incorporated into modern buildings aesthetically, as in one of the preliminary designs made by architect Paul Rudolph for the School of Architecture building at Yale University, shown in figure 52. Some of the forms he chose for ventilation can be successfully used as malqaf. Thus some of the traditional functional elements of vernacular architecture may enrich the otherwise bare products of modern architecture.
In planning for a malgaf, it is important to locate and orient its opening in the direction facing the on-coming wind. The surrounding buildings, and indeed even the new building that includes the malqaf, can significantly alter the direction of the prevailing winds. The aerodynamic flow of the new building in its surroundings should be studied to ensure that the malqaf is properly positioned. As shown in figure 53, a malqaf on the left side of the building facing the prevailing wind would be well placed to capture the airflow. But another on the right side, facing the same direction, would become a wind-escape due to the suction caused by the airflow pattern unless its opening was far above the low-pressure zone.
The size of a malqaf is determined by the external air temperature. A larger size is required where the air temperature at the intake is low, and a smaller size where the ambient air temperature is higher than the limit for thermal comfort, provided that the air flowing through the malqaf is cooled before it is allowed to circulate into the interior. In Iraq, where the air temperature in summer rises to 45°C (113°F), the typical malqaf shaft is very narrow. It is placed in the northern wall with a small inlet allowing the air to cool before it flows into the interior, as illustrated in figure 54. This is very similar to the shape of human nostrils, which are narrower in colder countries so that cold air will not reach the lungs before it has been heated by contact with the trachea, which is at body temperature.
In the areas of An-Najf and Al-Kufa in Iraq, where air temperature is very high in summer, people live in basements ventilated by small holes in the ceiling and a malqaf with a very small inlet. Figure 55 shows the plans and the section of a residence with a basement from this region. However, as the airflow is small and the air circulation is insufficient, this design is unhealthy and a possible cause of lung diseases.
In some designs, the drafts from the malqaf outlet are cooled by passing over water in the basement. However, this method is not very effective, and some other device is required to provide air cooling, at increased rates of airflow, sufficient to meet the conditions of both hygiene and thermal comfort.
By increasing the size of the malqaf and suspending wetted matting in its interior, the airflow rate can be increased while providing effective cooling. People in Iraq hang wet mats outside their windows to cool the wind flowing into the room by evaporation. The matting can be replaced by panels of wet charcoal held between sheets of chicken wire. Evaporation can be further accelerated by employing the Bernoulli effect or Venturi action with baffles of charcoal panels placed inside the malqaf, as shown in figure 56. The wind blowing down through the malqaf will decrease the air pressure below the baffle, which increases airflow and thus accelerates evaporation. Metal trays holding wet charcoal can be advantageously used as baffles. As shown in figure 56, air can be directed over a salsabil, a fountain or a basin of still water, to increase air humidity. These components are discussed in Chapter 7. The baffles are also effective in filtering dust and sand from the wind.
Examples of malqaf placed directly over a roof opening and without a shaft to channel the airflow into the room are found in nineteenthcentury Turkish-style houses in Cairo, illustrated in figure 57.
Figures 58 and 59 show the design for a neighborhood in Bans Oasis, Egypt, illustrating how the malqaf principle can be incorporated into new architectural designs. Other modern examples of the use of the malqaf are the villa designed for Saudi Arabia in figure 60 and the Fu'ad Riyad house in Cairo, shown in detail in figures 61-63.
In Iran and the countries of the Gulf, a specific type of malqaf called the bãdgir was developed. It has a shaft with the top opening on four sides (occasionally only two), and with two partitions placed diagonally across each other down the length of the shaft to catch breezes from any direction. This shaft extends down to a level that allows the breeze to reach a seated or sleeping person directly. An example from Dubai, United Arab Emirates, is shown in detail in figures 64-66. The bãdgir is usually treated decoratively as an architectural element, as shown in figure 67. In addition to ventilation, the bãdgir can be used in pairs or four at a time to cool underground water tanks, as shown in figure 68.
A great advantage of the malqaf and the bãdgir is that they solve the problem of screening resulting from the blocking of buildings in an ordinary town plan. Several research centers have been working to develop the best configuration for the implantation of blocks of buildings, while avoiding screening of blocks by those upwind. But after six or seven blocks, no configuration will solve the problem of screening. The malqaf and the bãdgir, however, being smaller in size than the buildings themselves, do provide an effective solution.
When designing the malqaf and the bãdgir, it is important to determine the airflow pattern around the house, following the principles of aerodynamics, and to orient the inlet appropriately in the airflow. Generally, a building placed in the wind will create a zone of compression to the windward side and a low-pressure zone to the leeward side. This low-pressure zone continues a certain distance beyond the build ing, depending on the wind velocity, as illustrated in figure 53. The faster the wind velocity, the shorter the low-pressure zone extends, because of eddies created on the leeward side which disrupt the smooth airflow pattern. For normal wind velocities, the length of the low-pressure zone can be taken to be five times the height of the building.