The architectural design can ensure 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.
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. 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.
Vents also can be used as outlets for hot air. 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.
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.
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 wind-escape
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 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 .
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.
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.
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.
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 building, depending on the wind velocity, 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.
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