12.4.1 Wind Directions
The wind direction outside the greenhouse is an important factor in defining the
flow of air and climate inside the greenhouse system. The boundary conditions of
wind speed distribution are deduced from experimental data and wind direction
with respect to the longitudinal axis of the greenhouse, which can range from 0 to
90°. Roy and Boulard (2005) simulated the impact of wind at 45 and 90°, showing
the influence of wind direction in the air velocity, temperature, and humidity
distributions inside the greenhouse; a similar result was found by Campen (2008).
Rico-García et al. (2006) also showed that a greenhouse with larger vertical roof
windows works better with a windward condition, whereas the multi-span
greenhouse works better with a leeward condition. Therefore, wind direction
affects the degree of ventilation. In an experiment carried out by Khaoua et al.
(2006), four different openings of roof vents obtained ventilation rates from 9 to
26.5 air exchanges per hour for the windward and 3.7–12.5 on the leeward wind
condition, respectively, which can maintain acceptable and uniform climate conditions for particular cases where the wind is perpendicular to the main axis of the
greenhouse. Overhead ventilation to the windward and leeward directions represents a reduction in the ventilation rate by 25–45 %, compared with only opening
to the windward direction (Bournet and Ould Khaoua 2007). Openings to the
windward direction generate the highest rate of ventilation; however, the greatest
homogeneity of the temperature and wind speed arises from combining windward
and leeward roof vents (Bournet and Ould Khaoua 2007).
Kacira et al. (2008) showed that the air temperature inside the greenhouse was
higher on the windward side than on the leeward side when roof vents were used.
Wind speed had a linear influence on air exchange rates, while the wind direction
did not affect them. Majdoubi et al. (2009) observed a strong wind air current
above a tomato canopy that was fed by a windward side vent and a slow air stream
flowing within the tomato canopy space. The first third of the greenhouse, until the
end of the leeward side, was characterized by a combination of wind and buoyancy
forces, with warmer and more humid inside air that was removed through upper
roof vents. There may be a conflict between increasing ventilation and improving
uniformity because there is little information on air movement affecting the
cooling efficiency and the uniformity of the environment (Sase 2006). According
to Rico-García (2008) the relationship between the thermal gradient and ventilation of gases shows a linear behavior, while the relationship between the combined
effect of temperature and wind greenhouse ventilation presents a piecewise linear
behavior. The wind pattern in a greenhouse is strongly affected not only by the
outside wind velocity but also by the number of greenhouse spans. In case the
greenhouse has three or four span roof windows orientation is independent,
however, when the greenhouse has five or more spans, side ventilation is dominant
over the roof ventilation (Fig. 12.5).
348
G. De la Torre-Gea et al.
The wind direction outside the greenhouse is an important factor in defining the
flow of air and climate inside the greenhouse system. The boundary conditions of
wind speed distribution are deduced from experimental data and wind direction
with respect to the longitudinal axis of the greenhouse, which can range from 0 to
90°. Roy and Boulard (2005) simulated the impact of wind at 45 and 90°, showing
the influence of wind direction in the air velocity, temperature, and humidity
distributions inside the greenhouse; a similar result was found by Campen (2008).
Rico-García et al. (2006) also showed that a greenhouse with larger vertical roof
windows works better with a windward condition, whereas the multi-span
greenhouse works better with a leeward condition. Therefore, wind direction
affects the degree of ventilation. In an experiment carried out by Khaoua et al.
(2006), four different openings of roof vents obtained ventilation rates from 9 to
26.5 air exchanges per hour for the windward and 3.7–12.5 on the leeward wind
condition, respectively, which can maintain acceptable and uniform climate conditions for particular cases where the wind is perpendicular to the main axis of the
greenhouse. Overhead ventilation to the windward and leeward directions represents a reduction in the ventilation rate by 25–45 %, compared with only opening
to the windward direction (Bournet and Ould Khaoua 2007). Openings to the
windward direction generate the highest rate of ventilation; however, the greatest
homogeneity of the temperature and wind speed arises from combining windward
and leeward roof vents (Bournet and Ould Khaoua 2007).
Kacira et al. (2008) showed that the air temperature inside the greenhouse was
higher on the windward side than on the leeward side when roof vents were used.
Wind speed had a linear influence on air exchange rates, while the wind direction
did not affect them. Majdoubi et al. (2009) observed a strong wind air current
above a tomato canopy that was fed by a windward side vent and a slow air stream
flowing within the tomato canopy space. The first third of the greenhouse, until the
end of the leeward side, was characterized by a combination of wind and buoyancy
forces, with warmer and more humid inside air that was removed through upper
roof vents. There may be a conflict between increasing ventilation and improving
uniformity because there is little information on air movement affecting the
cooling efficiency and the uniformity of the environment (Sase 2006). According
to Rico-García (2008) the relationship between the thermal gradient and ventilation of gases shows a linear behavior, while the relationship between the combined
effect of temperature and wind greenhouse ventilation presents a piecewise linear
behavior. The wind pattern in a greenhouse is strongly affected not only by the
outside wind velocity but also by the number of greenhouse spans. In case the
greenhouse has three or four span roof windows orientation is independent,
however, when the greenhouse has five or more spans, side ventilation is dominant
over the roof ventilation (Fig. 12.5).
348
G. De la Torre-Gea et al.