Cross ventilation

Cross ventilation is a natural phenomenon where

The phenomenon occurs when openings in an environment (including vehicles) or building (houses, factories, sheds, etc) are set on opposite or adjoining walls, which allow air to enter and exit, thus creating a current of air across the interior environment.[3]

There is also a

If the windows on both sides of the buildings are opened, the overpressure on the side facing the wind, and/or low pressure on the adjacent protected side, will make a current of air through the room from the uncovered side towards the sheltered side. If there are windows on both sides in a building, cross ventilation is appropriate where the width of the room is up to five times the floor-to-ceiling height. If openings are only one side then wind-driven ventilation is more suited for structures where the width is around 2.5 times the floor to ceiling height.^[5]

Factors

Cross ventilation relies on many factors, such as the tightness of the establishment, wind direction and how much wind is available, its potential travel through chimneys, vents and other openings in the home. Casement windows can be installed to improve cross-breezes. Air quality may also affect cross ventilation.

Although cross ventilation is generally more direct at its job than stack ventilation, its cons include its effects being unproductive on hot, still days, when it is most necessary. Moreover, cross ventilation is generally only suitable for narrow buildings. The contrasting height of the openings (walls, sill, panels or furniture) ordered by the space also immediately influence the level and velocity of ventilation.^[1]

Effectiveness

Cross ventilation works well in climates with hotter temperatures, where the system allows continual changes of the air within the building, refreshing it and reducing the temperature inside the structure and also when the window on the windward side of the building is not opened as much as the one on the leeward side. Cross ventilation will not be efficacious if the windows are more than 12m apart and if a window is behind a door that is regularly shut.^[6]

An opened window that faces a

Cross ventilation is not recommended for use in disease prevention when air is being moved by the cross ventilation from an unclean area into a clean area.[10]

Types

There are four different types of cross ventilation:[11]

• Single-sided ventilation: This method depends on the pressure contrasts between different openings within the occupied space. For rooms that only feature a single opening, the ventilation is impelled by turbulence, thereby creating a pumping activity on that lone opening, causing small inflows and outflows. It is worth noting that single-sided ventilation has a weak effect. It is preferable when cross ventilation is not achievable, where it uses windows or vents at the other side of the space to control air pressure.
• Cross ventilation (single spaces): Being unsophisticated and efficacious, this type of ventilation is a horizontal process that is driven by pressure differences between the windward and leeward sides of the occupied indoor environment. Ventilation here is generally provided using windows and vents at either side of a building where the variation in pressure draw air in and out.
• Cross ventilation (double-banked spaces): Involving banked rooms, this method features openings in the hallway structure. The openings allow a way for noise to move between spaces. It can provide a much higher air-exchange rate in comparison with single-sided ventilation.
• Stack ventilation: This ventilation is a vertical process and it's beneficiary for taller buildings with central atriums. It draws cooler air in at a lower level, whereby the air rises thereafter due to heat exposure before it is ventilated out at a higher level. Benefits from temperature compartmentalization and related pressure quality of the air, whereby warm air loses density when it rises and the cooler air supplants it.

Equation

For a simple volume with two openings, the cross wind flow rate can be calculated using the following equation:^[12]

${\displaystyle Q=U_{\textrm {wind}}{\sqrt {\frac {C_{\textrm {p1}}-C_{\textrm {p2}}}{1/\left(A_{\textrm {1}}^{2}C_{\textrm {1}}^{2}\right)+1/\left(A_{\textrm {2}}^{2}C_{\textrm {2}}^{2}\right)}}}\qquad {}\left(1\right)}$

where ${\displaystyle U_{\textrm {wind}}}$ is the far-field wind speed; ${\displaystyle C_{\textrm {p1}}}$ is a local pressure drag coefficient for the building, defined at the location of the upstream opening; ${\displaystyle C_{\textrm {p2}}}$ is a local pressure drag coefficient for the building, defined at the location of the downstream opening; ${\displaystyle A_{\textrm {1}}}$ is the cross-sectional area of the upstream opening; ${\displaystyle A_{\textrm {2}}}$ is the cross-sectional area of the downstream opening; ${\displaystyle C_{\textrm {1}}}$ is the discharge coefficient of the upstream opening; and ${\displaystyle C_{\textrm {2}}}$ is the discharge coefficient of the downstream opening.

For rooms with single opening, the calculation of ventilation rate is more complicated than cross ventilation due to the bi-directional flow and strong turbulent effect. The ventilation rate for single-sided ventilation can be accurately predicted by combining different models for mean flow, pulsating flow and eddy penetration.^[13] The mean flow rate for single-sided ventilation is determined by:

${\displaystyle {\bar {Q}}={\frac {C_{d}\;l\;{\sqrt {Cp}}\;\int \limits _{z_{0}}^{h}{\sqrt {-{\frac {2\;\Delta \;P(z)}{\rho }}}}\,\mathrm {d} z}{z_{ref}^{1/7}}}\;{\bar {U}}}$

where

l = width of the window;

h = elevation of the top edge of the window;

z₀ = elevation of neural level (where inside and outside pressure balance);

z_ref = reference elevation where the wind velocity is measured (at 10 m) and

${\displaystyle {\bar {U}}}$ = mean wind velocity at the reference elevation.

As observed in the equation (1), the air exchange depends linearly on the wind speed in the urban place where the architectural project will be built. CFD (

• Windcatcher
• Passive cooling
• Passive ventilation
• Room air distribution
• Thermal comfort
• Stack effect

References