Agriculture Reference
In-Depth Information
values normally exceeded in the Medi-
terranean coastal area during daylight hours.
However, with the use of low porosity anti-
insect screens (nets), ventilation by the
buoyancy effect is gaining importance.
In areas where the density of green-
houses is high, so the structures are very
close, the wind effect is very restricted (see
more details in the Appendix 1).
of the air volume of the greenhouse is pro-
portional to the velocity, varying with:
(i) the number of spans of the greenhouse
(Kozai and Sase, 1978); (ii) the dimension
of the spans (dynamic pressure coefficient,
Fig. 8.3); and (iii) the wind direction.
The wind loads on the greenhouse
structure depend on the dynamic pressure
of the wind, which varies depending on
the effective height of the greenhouse and
the cladding surface affected, mainly (see
Appendix 1).
The pressures generated by the wind
(Fig. 8.3) are positive on the side exposed to
the wind (windward) and negative over the
roof and in the side protected from the
wind (leeward). This pressure distribution
is altered when opening the vents.
With low wind velocities (less than 2 m s
−1
),
ventilation depends mainly of the tempera-
ture differences between the greenhouse
and the exterior. With wind velocities
greater than 2 m s
−1
, the number of exchanges
(a)
+0.4
-0.4
Wind
+0.4
-0.6 -0.6
-0.4
-0.4
+0.7
-0.4
+0.7
+0.4 -0.6
-0.4
-0.4
-0.4
-0.4
-0.4
Wind
+0.4
-0.4
Wind
-0.7
-0.5
-0.2
-0.3
-0.2
-0.5
-0.5
+0.5
+0.6
-0.3
(b)
Plastic film
Wind
Structure
Fig. 8.3.
(a) Wind pressure coefficients over several types of structure. A positive wind pressure
coefficient shows high pressure, whereas a negative value shows suction (adapted from Zabeltitz, 1999).
(b) Scheme of the effects of a perpendicular wind to the ridge on a tunnel covered with a plastic film,
showing the effects of suction on the plastic film, which may break it.
