Natural Ventilation  »  Wind driven ventilation

When a space is ventilated only through a single opening or multiple openings located on a single surface it is referred to as single sided ventilation. In Cass of multiple opening at various levels, stack ventilation is also induced along with wind driven ventilation. However, single sided ventilation is usually less effective because the opening is only located either on the windward side or the leeward side preventing effective movement of air inside and outside of the space. Single sided ventilation should be avoided if possible or should be confined to small spaces.

Room depth for single sided natural ventilation should be limited to 1.5 times the room height in case of singe opening and limited to 2.5 times the room height in case of multiple openings for effective circulation of air. Given the clear height in typical space is approximately 3m room depth for effective single sided ventilation should not exceed 3 – 7.5m. Single sided ventilation with projections, shades or balconies near or around the opening create enhanced pressure and suction zones on the windward and leeward sides (Givono, 1976).

Internal wind speed as a percent of the entering wind speed in different opening scenarios

In addition to this, temperature difference between the inside and outside can aid ventilation. Through the "Stack Effect” ventilation can also be improved in single sided multiple opening scenarios. This effect is caused by air displacement due to the temperature differences (and thus density differences) in the air. Ventilation through the use of the stack effect can be as simple as a higher window letting warm air out of the building a lower window letting in cooler air. The higher the difference between the indoor and outdoor temperature causes a stronger the stack effect. Studies conducted by Chen & Wang (2012) concluded that effective velocity at the opening increases non-linearly with the opening width and height and also increases with the increase in the elevation of the opening from the ground level. Empirical equations governing the same have also been deduced. The combination of stack effect and wind induced ventilation in single sided openings is complex and hard to predict since the fluctuating and cycling eddies formed by airstreams both entering and leaving through the opening. Heiselberg & Larsen, (2007) studied the combined effects of wind pressure and temperature difference on the ventilate rate in case of single openings. It has been found out that air change rate depends on wind direction besides wind velocity and temperature difference and the dominating force between wind pressure and temperature difference changes as a function of wind direction and also as a function of the ration between them.

Drawbacks of single sided ventilation are that there could be unwanted heat exchange between the air streams entering and exiting the space. In addition, as the wind direction deviates from normal to the aperture, the effectiveness of air circulation within the space is greatly reduced. The internal wind speed is lower for single opening (e.g. 36% and 39% maximum in case of oblique wind direction on the windward and leeward side respectively) compared to multiple openings (e.g. 50% maximum in case of oblique wind direction in the leeward side) (Givoni 1976).

Cross ventilation occurs when there are openings on more than one face and results in higher indoor speeds than single sided ventilation. Compared to single-sided ventilation, cross ventilation can increase the ventilation rate by a factor of 5 (Fordham, 2000). Buildings at right angles to the airflow have the largest pressure differences on either side of the building and thus the largest potential for cross-ventilation.

As a rule of thumb room depth for cross ventilation should be limited to 5 times the room height. It is given by the equation:
Q_wind = K x A x V, where

Q_wind = volume of airflow (m³/h)
A = area of smaller opening (m²)
V = outdoor wind speed (m/h)
K = coefficient of effectiveness

The coefficient of effectiveness (K) is determined by the size of the openings on the windward and leeward side and is in the range of 0 – 1. Value of K for wind entering normal to the opening is approximately 0.9 while the wind entering at an angle of 45° is about 0.4. Effectiveness of ventilation decreases as the wind direction deviates from the normal and is lowest when the wind direction is parallel to the opening.

Internal wind speed as a % of the entering wind speed in different opening scenarios

The optimization of airflow between leeward and windward rooms is necessary for cross ventilation. Buildings at right angles to the airflow have the largest pressure differences on either side of the building and thus the largest potential for cross-ventilation. The use of larger doors and windows on the leeward side of the building can also help to improve cross ventilation through the Venturi effect. Walls and other structural and vegetative elements can also be used to enhance cross-ventilation. Care must however be taken to shade these large doors and windows to reduce/control solar insolation and allow wind flow to the building interior. For example, using louvers which while allowing wind into the building can block direct sunlight from entering and at the same time direct the flow of air (upward or downward) within the building. Upward air flow is also received when a canopy is place over a window while a gap between canopy and walls allow a downward pressure (Watson & Labs, 1983). Where cross ventilation is desired Interior walls should be perforated, have openings or not go completely to the ceiling. Also vertical projections such as wing walls, trees, shrubs positioned at right angles to the wall on the downwind side of windows negate solar gain and can further increase the air flow. In warm climates buildings can also be raised to improve access to cooling breezes. Care must be taken in hot climate zones in open systems to remove the internal load through ventilation. Also buildings should be planned according to their activities whether day or night time and ventilation should be enough to remove excess heat (as well as for air quality) from a building without cooling it below thermal comfort ranges. Zoning of buildings with high thermal loads are at the lee side of the building where the thermal gains cannot influence other parts of the buildings, such as kitchens and bathrooms which bring smells, higher humidity and odours (Givono, 1976).

The relationship of zones to each other
Zoning of buildings so that zones with high thermal loads are at the lee side of the building where the thermal gains cannot influence other parts of the buildings. Ventilation should be enough to remove excess heat (as well as for air quality) from a building without cooling it below thermal comfort ranges. Such as Kitchens and bathrooms benefit smells, higher humidity and odours. Buildings can also be zoned to add passive cooling. The buildings should be planned according to their activities whether day- or night-time. This is both vertically and horizontally. In single side ventilated buildings the air flow can be reduced by up to 5 times, in cross ventilated buildings by up 10 times (Givono, 1976).

Wind-catchers are wind scopes above the building envelope orientated in a direction to catch the prevailing winds. They capture the prevailing winds and funnel them into the building interior. Size of the wind catcher can influence the effectiveness. Wind catchers are also combined with evaporative cooling and double up as passive cooling systems in the form of Passive Down-draught Evaporative Cooling systems (PDEC).

Locations with pre dominant unidirectional wind are designed as one or two directional wind towers. Locations with no pre dominant wind direction are designed as four or eight directional wind catchers to be able to catch wind flowing from all directions. The internal structure of the wind catcher is further divided into smaller shafts for structural purposes and also for channelling the wind (Maleki, 2011). Typically wind catchers are approximately 3 meters high above the roof level and seldom rise above 5 meters high (Maleki, 2011).

Modern variants of wind catchers such as wind cowls are widely used especially as ventilating devices in colder climate. Typical wind cowls come with a fin and rotate about an axis to adjust themselves to catch the prevailing winds. The cowl is divided into two or four partitions and each shaft acts as air inlet or outlet depending on the pressure conditions around it. The fresh air is drawn in in from a high-pressure area created either by wind movement or by stack effect and the stale air from the inside is sucked out from the low-pressure area. Wind cowls come in two configurations. One is a complete passive device and the other one is fan assisted. Some cowls, especially the fan assisted ones also have optional heat exchangers between the air streams.